synodic period, in astronomy, length of time during which a body in the solar system makes one orbit of the sun relative to the earth, i.e., returns to the same elongation. Because the earth moves in its own orbit, the synodic period differs from the sidereal period, which is measured relative to the stars. The synodic period of the moon, which is called the lunar month, or lunation, is 291/2 days long; it is longer than the sidereal month. The moon is full when it is at opposition. One sidereal month later it will not yet be full, since it must travel further in its orbit around the earth to reach the point of opposition, which has moved relative to the stars because of the earth's motion. Since the calendar month is not equal to the lunar month, the full moon does not occur on the same day every month. The length of time between recurrences of the full moon on the same date is 235 lunar months, or 19 years. This period, called the Metonic cycle, was discovered by the Greek astronomer Meton in 433 B.C. It is used in determining the date of Easter in the Gregorian calendar and was used in placing the intercalary month in the ancient Greek calendar. For the inferior planets the synodic period is longer than the sidereal period, but for the superior planets it is shorter.

sidereal period, in astronomy, length of time a body takes to complete an orbit relative to the fixed stars. See sidereal time.

period: see punctuation.

period, in physics: see harmonic motion; wave.

period, unit of time on the geologic timescale. Periods are shorter than an era and longer than an epoch. Periods are of variable length, generally lasting tens of millions of years, with characteristic fossils found preserved in the sediments deposited during the period. It is also used to designate a characteristic of geologic time, such as the glacial period.

Triassic period, first period of the Mesozoic era of geologic time (see Geologic Timescale, table) from 205 to 250 million years ago.

Throughout the Triassic, E North America, as a result of the mountain-building episode that formed the Appalachians in the late Paleozoic era, was elevated above sea level. California and Nevada, however, were submerged. In the Lower Triassic the sea extended E to Idaho, Colorado, and Wyoming; in the Middle Triassic it submerged British Columbia; in the Upper Triassic it extended into Alaska. In Lower and Upper Triassic time the west coast, from Alaska to British Columbia, was disturbed by violent and widespread volcanic activity. The Triassic formations of W North America are chiefly marine shale and limestone, with considerable igneous intrusions.

Near the end of the period, the only Triassic formation of E North America was deposited in downfaulted troughs, parallel to the Appalachians, from Nova Scotia to North Carolina. Composed of shale, conglomerate, and sandstone, this Newark series is comprised of sediments from the Appalachians. It is widely interrupted by so-called traprock—diabase dikes and sills—which forms ridges and cliffs, such as the Palisades of the Hudson near New York City. The end of the Triassic in North America was marked by extensive faulting and tilting of the Newark series, called the Palisade disturbance, and by the emergence of W North America.

The Triassic deposits of Germany form three series. In the Bunter series, the land was emergent, and red sandstone and sandy shale, with some salt and gypsum, were deposited. The Muschelkalk series saw the transgression of the land by the sea and the deposition of marine shale and limestone; the Keuper series saw the land again emergent and shale, sandstone, and gypsum being formed. In England there was no marine phase corresponding to the Muschelkalk; the Triassic of England is commonly called the New Red Sandstone. The Tethys, a great seaway, extended through the Mediterranean region E through the Middle East to the Himalayas and to E India. During the Triassic a subduction complex including an elongate volcanic arc system developed along the N American west coast. N Africa and Europe were still attached to N America as part of the supercontinent Pangaea.

The climate of the Triassic was semiarid to arid. In the plant life, marine algae were abundant, ferns and tree ferns less important than in the Paleozoic, conifers dominant among the trees, and a new group, the cycads, appeared. Many Paleozoic invertebrates appeared for the last time in the Triassic. The ammonites became very important, then were reduced at the end of the Triassic to one species, but were destined to become numerous again in the succeeding Jurassic period. Amphibians were apparently not as numerous as in the Paleozoic, but some types were more highly developed. The dominant animals of the Triassic were the reptiles; although the Triassic reptiles were less specialized than those of the Jurassic, there were already a number of types of dinosaurs, pterosaurs, and marine reptiles. The Triassic rocks also contain the fossils of the earliest known mammals.

Tertiary period, name for the major portion of the Cenozoic era, the most recent of the geologic eras (see Geologic Timescale, table) from around 26 to 66 million years ago. The name Tertiary was first applied about the middle of the 18th cent. to a layer of deposits, largely unconsolidated sediments, geologically younger than, and overlying, certain other deposits then known as Primary and Secondary. Later (c.1830) a fourth division, the Quaternary, was added. Although these divisions of the earth's crust seemed adequate for the region to which the designations were originally applied (parts of the Alps and plains of Italy), when the same system was later extended to other parts of Europe and to America it proved to be inapplicable. It was realized that one scheme of classification could not be applied universally: The names Primary and Secondary were generally abandoned; Tertiary and Quaternary were, and still are, used, but other geologic literature substitutes other names, including the Palaeogene and Neogene. The main divisions of the Tertiary are the Paleocene, Eocene, Oligocene, Miocene, and Pliocene epochs. Sometimes the Paleocene is included in the Eocene. At the beginning of the Tertiary, the outlines of the North American continent were very similar to those of today; by the close of the period, Europe also had emerged substantially in its present form. Marine submergences in Europe were moderately extensive, but in North America they never went beyond the Atlantic, Gulf, and Pacific coasts and the lower Mississippi valley. These inundations took place chiefly in the Eocene, Oligocene, and Miocene epochs, the continents being generally emergent in the Pliocene epoch. The Tertiary formations of either unconsolidated sediments or quite soft rocks are widespread. In the Tertiary, Gondwanaland finally split completely apart, and India collided with the Eurasian plate (see plate tectonics). The previously existing mountain ranges of North America were again elevated, the Alps, Pyrenees, Carpathians, and other ranges were formed in Europe, and in Asia the Himalayas arose. Widespread volcanic activity was prevalent. At the beginning of the period the mammals replaced the reptiles as the dominant animals; each epoch was marked by striking developments in mammalian life. Modern types of birds, reptiles, amphibians, fishes, and invertebrates either were already numerous at the beginning of the period or appeared early in its history.

Silurian period [from the Silures, ancient tribe of S Wales, where the period was first studied; named by the British geologist R. I. Murchison], third period of the Paleozoic era of geologic time (see Geologic Timescale, table) lasting from 405 to 435 million years ago. The continents in the Silurian period remained much as they had been in the preceding Ordovician period, with approximately the same areas being subject to flooding by shallow seas. The earth was relatively tectonically inactive during the Silurian. The Appalachian Mountains, which uplifted during the Ordovician, were being eroded. Large coral reefs and algae were abundant, indicating that warm, shallow seas predominated. Major evaporite basins, including the circular Michigan Basin, showed evidence of subsidence. The transition between the Ordovician and Silurian rocks is not clearly marked in the United States. The Medina sandstone extending from New York to Alabama has been assigned to both periods but is generally considered to be Silurian. Three main series, based on the succession of strata in New York state, are usually distinguished—the lower Silurian (Medinan, or Alexandrian, series), the middle Silurian (Niagaran series), and the upper Silurian (Cayugan series). The early Silurian deposits in the East are commonly sandstone, shale, and conglomerate, comprising erosion products from high-standing mountains; in the West, marine limestone predominates. There were also desert conditions, under which the Salinan "red beds" of the Appalachian area and the salt deposits of New York, Michigan, Ontario, and Ohio were formed. Some areas were later reflooded, depositing Cobleskill and Rondout limestone of New York. The Silurian of the Far West is as yet not well established. In North America, the Silurian ended quietly; however, in the British Isles, Scandinavia, and France, as a result of the Caledonian disturbance, great mountains continued to be thrust up. Economic resources of the Silurian strata, besides salt, are iron ore (near Birmingham, Ala.) and quartz sandstone, used in glass manufacture. Dominating the life of the Silurian were marine invertebrates, including crinoids and cystoids, mollusks, and eurypterids, invertebrates related to crabs and insects. Members of the trilobite family were still numerous; primitive fishes increased in number. Also notable in the Silurian fauna were scorpions, possibly the first animals to live on land and take their oxygen from the air.

Quaternary period, younger of the two geologic periods of the Cenozoic era of geologic time (see Geologic Timescale, table) from 2 millon years ago to the present. Comprising all geologic time from the end of the Tertiary period to the present, it is divided into the Pleistocene and Holocene, or Recent, epochs. It was named (1759) by Giovanni Arduino, an Italian scientist who thought that the biblical great flood was responsible for its deposits. During the early Quaternary, Europe and North America were covered by the glaciers of the Pleistocene epoch. Retreat of the glaciers led to isostatic rebound (see continent) of the crust in the Holocene. In the Quaternary the climate and present physical features of the earth continued to develop. Significant changes in sea level within historic times are demonstrated by the submergence of the temple of Jupiter Serapis near Naples and by the rising of the shores of the Baltic. The life of the Quaternary has been marked by the rise and dominance of humans.

Permian period [from Perm, Russia], sixth and last period of the Paleozoic era (see Geologic Timescale, table) from 250 to 290 million years ago.

Historical Geology of the Period

The Lower Permian

During the Permian period, changes in the earth's surface that had begun in the preceding Carboniferous period reached a climax. At the close of the Carboniferous, large areas of E North America were dry land. In the Lower Permian, sandy shales, sandstones, and thin limestones of the Dunkard formation (formerly called the Upper Barren measures) were deposited in the remaining submerged areas of West Virginia, Pennsylvania, and Ohio, but the continued rising of the land soon put an end to deposition. The Dunkard is the last Paleozoic formation of the E United States. More extensive deposits were formed in the West. Parts of Texas, Oklahoma, Kansas, and Nebraska were covered by an arm of the sea or possibly by one or more salt lakes or lagoons, now represented by masses of salt or gypsum in layers separated and overlaid by red beds. There are important Permian salt mines at Hutchinson and Lyons in Kansas and gypsum mines in Oklahoma, Texas, and Kansas. The longest marine submergence of the Lower Permian in North America was in W Texas and SE New Mexico, where there is a system of marine limestones and sandstones 4,000 to 6,000 ft (1,200-1,800 m) thick. The Cordilleran region was also submerged; here marine beds are more common toward the west, and land sediments, especially red beds, toward the east. The red beds are generally considered to be indicative of increasingly arid conditions in Permian times.

In Europe, the Lower Permian, or Rotliegendes [red layers], was marked principally by erosion from the Paleozoic Alps of the Carboniferous into the low-lying land to the north; the formations are chiefly shale and sandstone, with some conglomerate and breccia. Red is a prominent color for the beds. The Pangaea supercontinent formed from an aggregation of all continents at this time.

The Permian and late Carboniferous of the Southern Hemisphere were radically different from those of the Northern Hemisphere. Australia, S Africa, and South America experienced a series of glacial periods, as is shown by the presence of tillite and of conspicuous striations of the underlying rock formations. This condition prevailed also in India. Paleozoic glaciation in North America is suggested by the Squantum tillite near Boston, Mass. This glaciation and the aridity of which the red beds seem to be the result are the two most strongly marked characteristics of the Permian period.

The Upper Permian

In the Upper Permian practically all of North America was above sea level, and the continent was larger than at present. Toward the close of the Upper Permian the greatest earth disturbance of the Paleozoic era thrust up the Appalachian Mts. In Europe, the Upper Permian was a period of more extensive marine invasion; the Zechstein formation is predominantly limestone, though it includes rich deposits of copper, salt, gypsum, and potash. The Upper Permian beds of Germany were long the chief source of the world's potash.

Evolution of Plant and Animal Life

Many marine animals became extinct during the Permian, but there was at the same time an evolution to more modern types, a marked change in the insects, and a notable increase in numbers and varieties of reptiles mainly because of the continental changes. Among plants, Lepidodendron and Sigillaria became rare, but ferns and conifers persisted. The widely distributed "seed fern," Glossopteris, which was apparently successful in resisting glacial conditions, was the most conspicuous development in the Permian flora. The presence of Glossopteris in South America, Antarctica, Australia, and S Africa is a strong argument favoring the interconnection of these land masses in a large supercontinent during Permian time. The end of the Permian is marked in the fossil record by a mass extinction.

Pennsylvanian period: see Carboniferous period.

Paleolithic period or Old Stone Age, the earliest period of human development and the longest phase of mankind's history. It is approximately coextensive with the Pleistocene geologic epoch, beginning about 2 million years ago and ending in various places between 40,000 and 10,000 years ago, when it was succeeded by the Mesolithic period. By far the most outstanding feature of the Paleolithic period was the evolution of the human species from an apelike creature, or near human, to true Homo sapiens (see human evolution). This development was exceedingly slow and continued through the three successive divisions of the period, the Lower, Middle, and Upper Paleolithic. The most abundant remains of Paleolithic cultures are a variety of stone tools whose distinct characteristics provide the basis for a system of classification containing several toolmaking traditions or industries.

The Lower Paleolithic Period

The oldest recognizable tools made by members of the family of man are simple stone choppers, such as those discovered at Olduvai Gorge in Tanzania. These tools may have been made over 1 million years ago by Australopithecus, ancestor of modern humans. Fractured stones called eoliths have been considered the earliest tools, but it is impossible to distinguish man-made from naturally produced modifications in such stones. Lower Paleolithic stone industries of the early species of humans called Homo erectus include the Choukoutienian of China and the Clactonian, Chellean-Abbevillian, Acheulian and Levalloisian represented at various sites in Europe, Africa, and Asia, from 100,000 to 500,000 years ago. Stone tools of this period are of the core type, made by chipping the stone to form a cutting edge, or of the flake type, fashioned from fragments struck off a stone. Hand axes were the typical tool of these early hunters and food-gatherers.

The Middle Paleolithic Period

The Middle Paleolithic period includes the Mousterian culture, often associated with Neanderthal man, an early form of humans, living between 100,000 and 40,000 years ago. Neanderthal remains are often found in caves with evidence of the use of fire. Neanderthals were hunters of prehistoric mammals, and their cultural remains, though unearthed chiefly in Europe, have been found also in N Africa, Palestine, and Siberia. Stone tools of this period are of the flake tradition, and bone implements, such as needles, indicate that crudely sewn furs and skins were used as body coverings. Since the dead were painted before burial, a kind of primitive religion may have been practiced.

The Upper Paleolithic Period

In the Upper Paleolithic period Neanderthal man disappears and is replaced by a variety of Homo sapiens such as Cro-Magnon man and Grimaldi man. This, the flowering of the Paleolithic period, saw an astonishing number of human cultures, such as the Aurignacian, Gravettian, Perigordian, Solutrean, and Magdalenian, rise and develop in the Old World. The beginnings of communal hunting and extensive fishing are found here, as is the first conclusive evidence of belief systems centering on magic and the supernatural. Pit houses, the first man-made shelters, were built, sewn clothing was worn, and sculpture and painting originated. Tools were of great variety, including flint and obsidian blades and projectile points. It is probable that the people of the Aurignacian culture migrated to Europe after developing their distinctive culture elsewhere, perhaps in Asia. Their stone tools are finely worked, and they made a typical figure eight-shaped blade. They also used bone, horn, and ivory and made necklaces and other personal ornaments. They carved the so-called Venus figures, ritual statuettes of bone, and made outline drawings on cave walls.

The hunters of the Solutrean phase of the Upper Paleolithic entered Europe from the east and ousted many of their Aurignacian predecessors. The Solutrean wrought extremely fine spearheads, shaped like a laurel leaf. The wild horse was their chief quarry. The Solutrean as well as remnants of the Aurignacian were replaced by the Magdalenian, the final, and perhaps most impressive, phase of the Paleolithic period. Here artifacts reflect a society made up of communities of fishermen and reindeer hunters. Surviving Magdalenian tools, which range from tiny microliths to implements of great length and fineness, indicate an advanced technique. Weapons were highly refined and varied, the atlatl first came into use, and along the southern edge of the ice sheet boats and harpoons were developed. However, the crowning achievement of the Magdalenian was its cave paintings, the culmination of Paleolithic art.

Ordovician period [from the Ordovices, ancient tribe of N Wales], second period of the Paleozoic era of geologic time (see Geologic Timescale, table) from 505 to 438 million years ago. It was similar to the preceding Cambrian period, with shallow seas spread for most of the time over the British Isles, Scandinavia, the Baltic region, the Mediterranean region, a large part of Siberia, and much of North America. The Ordovician rocks are chiefly sedimentary. Because of the restricted area and low elevation of the solid land, which set limits to erosion, marine sediments that make up a large part of the Ordovician system consist chiefly of limestone; shale and sandstone are less conspicuous. The Ordovician of North America can best be studied in New York state. In the Early, or Lower, Ordovician epoch, also called the Canadian epoch, the waters spread over the Appalachian area and deposited the Beekmantown limestone, then withdrew generally, to return and deposit the Chazy limestone of the lower Middle Ordovician, also known as the Champlainian epoch. In the interval between Beekmantown time and Chazy time, large areas, chiefly outside New York, were apparently covered with wind-blown sand which became the St. Peter sandstone. In the Middle Ordovician the sea spread over North America to a greater extent than in any other period and laid down the Trenton limestone, which in its eastern section is overlaid or intercalated with the Utica mud shale. In the east, increased erosion of the land subsequently led to the deposition of other shales, which became more and more sandy toward the end of the period. The close of the Ordovician was marked by more general earth disturbances than the close of the Cambrian. The Taconian disturbance created a chain of fold mountains extending from Newfoundland to New Jersey and was accompanied by volcanic activity. The later start of the Acadian-Caledonian uplift may have also been the start of the proto-Atlantic Ocean. Among the economic resources of the Ordovician strata are oil, natural gas, the lead and zinc of Wisconsin, Iowa, and Illinois, the "Portland cement rock" of Pennsylvania and New Jersey, Vermont marble, and the calcium phosphate of the Tennessee limestone. The Ordovician seas were rich in animal life. The most characteristic invertebrates were minute graptolites, other numerous forms being brachiopods, bryozoans, and trilobites. Some cystoids and crinoids appeared; there were a few corals and many cephalopods. Especially noteworthy was the appearance of a few primitive, fishlike vertebrates (jawless fishes) and tiny land plants resembling liverworts.

Neolithic period or New Stone Age. The term neolithic is used, especially in archaeology and anthropology, to designate a stage of cultural evolution or technological development characterized by the use of stone tools, the existence of settled villages largely dependent on domesticated plants and animals, and the presence of such crafts as pottery and weaving. The time period and cultural content indicated by the term varies with the geographic location of the culture considered and with the particular criteria used by the individual scientist. The domestication of plants and animals usually distinguishes Neolithic culture from earlier Paleolithic or Mesolithic hunting, fishing, and food-gathering cultures. The Mesolithic period in several areas shows a gradual transition from a food-collecting to a food-producing culture. The termination of the Neolithic period is marked by such innovations as the rise of urban civilization or the introduction of metal tools or writing. Again, the criteria vary with each case. The earliest known development of Neolithic culture was in SW Asia between 8000 B.C. and 6000 B.C. There the domestication of plants and animals was probably begun by the Mesolithic Natufian peoples, leading to the establishment of settled villages based on the cultivation of cereals, including wheat, barley, and millet, and the raising of cattle, sheep, goats, and pigs. In the Tigris and Euphrates river valleys, the Neolithic culture of the Middle East developed into the urban civilizations of the Bronze Age by 3500 B.C. Between 6000 B.C. and 2000 B.C. Neolithic culture spread through Europe, the Nile valley (Egypt), the Indus valley (India), and the Huang He valley (N China). The formation of Neolithic cultures throughout the Old World resulted from a combination of local cultural developments with innovations diffused from the Middle East. In SE Asia, a distinct type of Neolithic culture involving rice cultivation developed, perhaps independently, before 2000 B.C. In the New World, the domestication of plants and animals occurred independently of Old World developments. By 1500 B.C., Neolithic cultures based on the cultivation of corn, beans, squash, and other plants were present in Mexico and South America, leading to the rise of the Inca and Aztec civilizations and spreading to other parts of the Americas by the time of European contact. The term Neolithic has also been used in anthropology to designate cultures of more contemporary primitive, independent farming communities.

Mississippian period: see Carboniferous period.

Mesolithic period or Middle Stone Age, period in human development between the end of the Paleolithic period and the beginning of the Neolithic period. It began with the end of the last glacial period over 10,000 years ago and evolved into the Neolithic period; this change involved the gradual domestication of plants and animals and the formation of settled communities at various times and places. While Mesolithic cultures lasted in Europe until almost 3000 B.C., Neolithic communities developed in the Middle East between 9000 and 6000 B.C. Mesolithic cultures represent a wide variety of hunting, fishing, and food gathering techniques. This variety may be the result of adaptations to changed ecological conditions associated with the retreat of glaciers, the growth of forests in Europe and deserts in N Africa, and the disappearance of the large game of the Ice Age. Characteristic of the period were hunting and fishing settlements along rivers and on lake shores, where fish and mollusks were abundant. Microliths, the typical stone implements of the Mesolithic period, are smaller and more delicate than those of the late Paleolithic period. Pottery and the use of the bow developed, although their presence in Mesolithic cultures may only indicate contact with early Neolithic peoples. The Azilian culture, which was centered in the Pyrenees region but spread to Switzerland, Belgium, and Scotland, was one of the earliest representatives of Mesolithic culture in Europe. The Azilian was followed by the Tardenoisian culture, which covered much of Europe; most of these settlements are found on dunes or sandy areas. The Maglemosian, named for a site in Denmark, is found in the Baltic region and N England. It occurs in the middle of the Mesolithic period. It is there that hafted axes, an improvement over the Paleolithic hand axe, and bone tools are found. The Ertebolle culture, also named for a site in Denmark, spans most of the late Mesolithic. It is also known as the kitchen-midden culture for the large deposits of mollusk shells found around the settlements. Other late Mesolithic cultures are the Campignian and Asturian, both of which may have had Neolithic contacts. The Mesolithic period in other areas is represented by the Natufian in the Middle East, the Badarian and Gerzean in Egypt, and the Capsian in N Africa. The Natufian culture provides the earliest evidence of an evolution from a Mesolithic to a Neolithic way of life.

Louis period styles, 1610-1793, succession of modes of interior decoration and architecture that established France as a leading influence in the decorative arts.

Louis XIV

During the reign of Louis XIII (1610-43) there was a transition from the baroque style, strongly influenced by Italy, to the classical dignity of the period of Louis XIV (1643-1715). The Louis XIV [Louis Quatorze] style, established after the king took personal control of the government in 1661, was molded by the chief minister, Colbert. He established manufactories of tapestries, textiles, furniture, and ornaments; assembled leading artists and artisans in the royal service; and appointed Charles Le Brun director of the Gobelins manufactory and decorator of the palace of Versailles.

Colbert worked in close cooperation with J. H. Mansart, achieving interiors of great splendor, in which the decoration was closely integrated with the architectural framework. Neutral backgrounds were often used to emphasize the strong, rich colors of Gobelin, Aubusson, and Beauvais tapestries, Savonnerie and Oriental rugs, velvet or brocade upholstery, hangings, and large paintings on walls and ceilings. Such ornaments as scrolls, acanthus leaves, caryatids, busts, and full figures with festoons of flowers and fruit were employed. Large mirrors decorated the walls. Furniture scaled to the huge proportions of the rooms was made of ebony or covered with silver, gilt, or lacquer and decorated with carving and with marquetry in the manner of A. C. Boulle.

Regence

In contrast to the heavy, massive members and curves used in the period of Louis XIV, the régence style, established during the regency of Philippe II, duc d'Orléans (1715-23), began to employ delicate lines and intricate curves. Finely sculptured bronze reliefs became the outstanding mode of furniture decoration under the leadership of the cabinetmaker Charles Cressent.

Louis XV

The Louis XV [Louis Quinze] (1723-74) style was characterized by free curves and the use of rococo ornament and chinoiserie. Rooms were smaller, specialized, and arranged for convenient use. Colors were delicate. Tinted wood, veneer, lacquer panels, marquetry, mounts by Caffieri and Pierre Gouthière, and porcelain plaques of Sèvres ware distinguish the designs. The style in its later phase was more restrained.

Louis XVI

The restraint of the later Louis XV style presaged the strong reaction of the Louis XVI [Louis Seize] (1774-93) period, during which simplicity replaced excess and the classic revival influenced decorative motifs and brought a return to straight lines and symmetry. Slenderness of proportion was emphasized in furniture. Colors were light in tone; ornament was delicate and in low relief, embossed, or painted. Furniture details included slender fluted legs, convex moldings, and rosette, leaf, and flower motifs in the carved frames often painted white and touched with gilt. Upholstery and hangings used varied fabrics. The Revolution abolished the guilds, which had maintained high standards of craftsmanship, and weakened the practice, instituted under Louis XIV, of cooperation between artists and masters of the various crafts in producing fine furniture and decorative accessories.

Jurassic period [from the Jura Mts.], second period of the Mesozoic era of geologic time, lasting from 213 to 144 million years ago. At the start of the Jurassic most of the continents were joined together until the Atlantic began to form and the Americas split off from Africa. Eastern North America was mostly elevated and subject to erosion, which reduced the Appalachian region to a peneplain. Before the end of the period, the Appalachian borderland began to founder as the Atlantic Ocean continued to widen. The Pacific border of North America, from California to Alaska, was submerged for most of the period. In the Early Jurassic, large areas of Arizona, Colorado, and Utah were apparently desert, and the sand was later consolidated into the white and pinkish Glen Canyon and Navajo sandstones, which now enhance the scenic beauty of the district. During the Upper Jurassic, the Logan Sea entered this area from the north. In its various advances and retreats, this body of water covered large areas of Montana, Idaho, Wyoming, Colorado, and Utah, depositing sandstone, shale, limestone, and some gypsum. The retreat of the Logan Sea, toward the end of the period, was followed, probably in the Upper Jurassic but possibly in the Lower Cretaceous Period, by the deposition of the Morrison continental series of clays and sandstones, noted for its richness in fossil dinosaurs. The close of the Jurassic in North America was marked by widespread folding along the western border of the continent, accompanied by the intrusion of lava as the eastern edge of the plate that carries the Pacific Ocean was thrust beneath the westward drifting plate that carries the North American continent. In this disturbance the Sierra Nevada, Klamaths, Cascades, Coast Ranges, and coastal mountains of Canada and Alaska were formed. The history of the European Jurassic is very well known, that system being one of the most complete on the Continent. Studies of oxygen isotopes, the extent of land flora, and marine fossils indicate that climates during Jurassic times were mild—perhaps 15°F; (8°C;) warmer than those of today. No glaciers existed during this period. The plant life of the Jurassic was dominated by the cycads, but conifers, ginkgoes, horsetails, and ferns were also abundant. Of the marine invertebrates, the most important were the ammonites. The dominant animals on land, in the sea, and in the air were the reptiles. Dinosaurs, more numerous and more extraordinary than those of the Triassic period, were the chief land animals; crocodiles, ichthyosaurs, and plesiosaurs ruled the sea, while the air was inhabited by the pterosaurs and relatives. Mammals, making their first appearance, were few and small but undoubtedly became well established during the Jurassic period. The Jurassic saw the appearance of the first bird, Archaeopteryx. See Geologic Timescale (table).

Devonian period, fourth period of the Paleozoic era of geologic time between 408 and 360 million years ago (see Geologic Timescale, table). It was named (1838) by the geologists Sir Roderick Impey Murchison and Adam Sedgwick for Devonshire, England, where they first investigated rocks formed during the period. The Devonian period was a time of great tectonic activity, as Laurasia and Gondwanaland drew closer together. Pangaea began to consolidate the plates containing North America and Europe (see plate tectonics), further raising the northern Appalachian Mountains and forming the Caledonides in Britain and Scandinavia. For much of the Devonian, large areas of North America and Europe, and smaller parts of Africa, South America, and Australia were covered by seas, which withdrew during the Upper Devonian. The Cordilleran area of North America was submerged, depositing from 4,000 to 6,000 ft (1,200-1,800 m) of limestone and shale in Nevada and 2,400 ft (730 m) of quartzites and limestones in Utah. The Devonian period in Europe was marked by considerable volcanic activity and the deposition of two great rock systems: the marine formation of Devonshire, the Rhine valley, and Russia; and the Old Red Sandstone. The climate was relatively warm everywhere on the earth. The most notable Devonian animals were the jawed and bony fishes, which appeared in great numbers toward the close of the period. Conspicuous types were sharks, armored fishes, lungfishes, and ganoid fishes. Common invertebrates of the Devonian were crinoids, starfishes, sponges, and early ammonites; trilobites and graptolites became scarcer. An unusual surge of coral reef growth also occurred and corals were never again as prolific. Of land animals, the chief vestige is the footprint of a primitive salamanderlike amphibian in the Upper Devonian of Pennsylvania. Trees made their first appearance; the Devonian plants were the earliest to be extensively preserved as fossils, but their high degree of development suggests that more primitive forms existed earlier.

Cretaceous period, third and last period of the Mesozoic era of geologic time (see Geologic Timescale, table), lasting from approximately 144 to 65 million years ago. The Cretaceous was marked, in both North America and Europe, by extensive submergences of the continents. Changes both in the earth's surface and in its flora and fauna brought the Mesozoic to a close.

Historical Geology of the Period

The Lower Cretaceous Period

At the beginning of the Lower Cretaceous in North America, the Mexican Sea of the late Jurassic period spread over Texas, Oklahoma, New Mexico, and parts of Arizona, Kansas, and Colorado. Deposits from this inland sea, known as the Comanchean Sea, were chiefly limestone (up to 1,500 ft/457 m thick in Texas) but some continental sediments (i.e., sandstone, shale, and conglomerate) mark the reemergence of land, which brought the Lower Cretaceous to a close. The Comanchean Sea was probably separated by a land barrier from contemporaneous seas in the California areas, where 26,000 ft (7,925 m) of Shastan shales, with sandstone and thin limestone, were laid down. The sediments were derived by rapid erosion from the recently elevated Sierra Nevada and Klamath mts. In Montana, Alberta, and British Columbia the Kootenai deposits of sandstone and sandy shale, which contain workable deposits of good coal, were formed; along the Atlantic coast the unconsolidated sandy clay, gravel, and sand of the Potomac series were deposited.

The Lower Cretaceous opened in NW Europe with the deposition of a continental and freshwater formation, the Wealden sand and clay, best displayed in England. The sea, meanwhile, expanded from the Mediterranean, finally overlaying successive Wealden strata with limestone. There was at the same time an extensive sea in N Europe. At the close of the Lower Cretaceous, there was some recession of the seas; by the Upper Cretaceous, the great transgression of seas submerged lands that had been open since the Paleozoic.

The Upper Cretaceous Period

The Upper Cretaceous opened in W North America with the deposition of continental sands (now the Dakota sandstone), which, however, were covered by the ensuing rise of the Colorado Sea. The Colorado Sea was the greatest of the North American Mesozoic seas and extended all the way from Mexico up into the Arctic, covering most of central North America. The Colorado deposits were composed chiefly of shales, limestone, and some chalk in Kansas and South Dakota. Slight shifting of the sea was followed by the deposition of the Montana shale and sandstone and then by withdrawal of the sea. Near the end of the Upper Cretaceous, conditions in the west were similar to those of the Carboniferous period in other regions; swamps and bogs were formed that later became valuable deposits of coal.

At the close of the Cretaceous the Laramide revolution occurred—at least two different epochs of mountain building and one of relative quiet. In this disturbance the Rockies and the E Andes were first elevated, and there were extensive flows of lava. The Appalachians, which had been reduced almost to base level by erosion, were rejuvenated, and the seas retreated from all parts of the continent. The intermittent character of the Laramide disturbance makes difficult the demarcation of the Mesozoic and the succeeding Cenozoic era.

The striking feature of the European Upper Cretaceous are great chalk deposits from small carbonate-bearing marine algae and calcareous fauna, now exposed in the cliffs of the English Channel. In India the late Upper Cretaceous was marked by an overflow of lava in the Deccan plateau. The area covered by igneous rocks dating from this period now comprises over 200,000 sq mi (518,000 sq km) and was formerly much larger, having been reduced by erosion. Near Mumbai the formation is 10,000 ft (3,000 m) thick.

Movement of the Continents

During the Cretaceous period the massive continents of Gondwanaland and Laurasia continued to separate. South America and Africa had separated, with the consequent widening of the S Atlantic. The N Atlantic continued to expand, although it appears that Europe, Greenland, and North America were still connected. Madagascar had separated from Africa, while India was still drifting northward toward Asia. The Tethys Sea was disappearing as Africa moved north toward Eurasia. Antarctica and Australia had yet to separate.

Evolution of Plant and Animal Life

The Lower Cretaceous is characterized by a revolution in the plant life, with the sudden appearance of flowering plants (angiosperms) such as the ancestors of the beech, fig, magnolia, and sassafras. By the end of the Cretaceous such plants became dominant. Willow, elm, grape, laurel, birch, oak, and maple also made their appearance, along with grass and the sequoias of California. Closely associated with the angiosperms were insects, including a form of the dragonfly, and most were similar to today's insects. This prepared the way for the increase in mammals in the late Cenozoic. The marine invertebrates of the Cretaceous included nautiluses, barnacles, lobsters, crabs, sea urchins, ammonites, and foraminifers. Reptiles reached their zenith, including the dinosaurs Triceratops, Tyrannosaurus, Stegosaurus, Apatosaurus (Brontosaurus), and Iguanodon, and ranged from herbivores to carnivores. Flying reptiles such as the pterosaurs were highly developed, while in the sea there were ichthyosaurs, plesiosaurs, and mosasaurs. Other reptiles living in this period include crocodiles and giant turtles; snakes and lizards made their first appearance at this time. True mammals, which had already appeared in the Triassic period, were rare, as the Cretaceous reptiles dominated.

The climate of the Cretaceous was apparently fairly mild and uniform, but it is possible that toward the end of the period some variant zones of climate had appeared, making the overall climate cooler. Such changes, along with changes in both the earth's surface and its flora and fauna, brought the Mesozoic to a close.

By the end of the Cretaceous, about 75% of all species, including marine, freshwater, and terrestrial organisms, became extinct. The rather abrupt disappearance of Cretaceous life remains a mystery. Theories for the extinctions include one or a mixture of the following: drastic cooling of the globe, retreat of the seas, breakup of the continents (see continental drift), biological disease, reversals of the earth's magnetic field, or a change in atmospheric carbon dioxide and oxygen. Another popular theory was introduced in 1980 by Luis Alvarez and colleagues at the Univ. of California. Alvarez proposed that the earth was struck by an asteroid or comet about 6 mi (10 km) in diameter around 65 million years ago. Such an impact (or collection of impacts) would spread dust into the atmosphere, suppressing photosynthesis and changing the food chain. Evidence for an impact includes an anomalous iridium layer, typical of meteorites, and some probable impact craters dated to the late Cretaceous.

Carboniferous period, fifth period of the Paleozoic era of geologic time (see Geologic Timescale, table), from 350 to 290 million years ago.

Historical Geology of the Period

The Carboniferous period was marked by vast, coal-forming swamps (see also bog) and a succession of changes in the earth's surface that, continuing into the Permian period, ended the Paleozoic era. The Carboniferous is often split into two divisions, the Mississippian and the Pennsylvanian; in the United States the break in the geologic sequence is so sharp that each division is commonly considered an independent period.

The Lower Carboniferous Period

In the Lower Carboniferous, or Mississippian, period, the submersion—on several occasions—of the interior of North America under shallow seas resulted in the formation of limestone, shale, and sandstone. In the Appalachian region, especially in Pennsylvania, great deposits of sandstone and shale were laid down by the erosion products from the eastern coastal highlands. In the far west the Rocky Mt. region was covered by shallow seas that deposited the Madison and Redwall limestones of the Grand Canyon.

The Lower Carboniferous in Europe was a period of submergence and great volcanic activity. E of the Rhine, shales, sandstones, and conglomerates were deposited; and in Russia, the Coal Measures formed. The close of the Lower Carboniferous was marked by mountain building in New Brunswick, Nova Scotia, the S Appalachian region, the SW United States, and Europe.

The Upper Carboniferous Period

In the Upper Carboniferous, or Pennsylvanian, period, there was at least one great submergence. In the E United States great deltas of sediments, now represented by the Pottsville conglomerate, were formed during the early Pennsylvanian. In Kansas, Nebraska, Arkansas, and Texas, the Pennsylvanian beds are chiefly shale, sandstone, and coal; over the Cordilleran (Rocky Mountain) region, marine limestone, with little coal; on the Pacific coast from California to Alaska, limestone and shale. The sea level also oscillated during the period and caused the formation of great marshes with extensive vegetation that was later transformed into coal, with Pennsylvanian strata containing the largest U.S. coal deposits. The Pennsylvanian coal fields of North America include the anthracite field of E Pennsylvania; the Appalachian field, from Pennsylvania to Alabama; the Michigan field; the eastern interior field, in Indiana, Illinois, and Kentucky; the western interior and southwestern field, stretching from Iowa to Texas; the Rhode Island field; and the Acadian field of SE Canada.

In the Upper Carboniferous of Western Europe, the Millstone Grit (the equivalent to the Pottsville conglomerate) is followed by the Coal Measures, which include the Welsh, English, Belgian, Westphalian, and Saar Basin fields. In the Mediterranean region and parts of Asia, the Upper Carboniferous environment resembled that of W North America.

The Upper Carboniferous was a period of marked disturbances caused by collisions of crustal plates. Gondwanaland, the supercontinent containing the continents of Africa and S America, had formed; Euramerica, part of Europe and N America, had fused into a continent to the north; and Angara, today's Asia, was also to the north of Gondwanaland. In Europe the Paleozoic Alps were thrust up; in Asia, the Altai and the Tian Shan; in North America, the Arbuckle and Wichita mts. and the ancestral S Rockies. The Indian peninsula became an active site of deposition; in the Himalayan geosyncline and much of China, mountain building was dominant. Crustal movements in the Andean geosyncline of South America affected the pattern of sedimentation over much of the continent.

Evolution of Plant and Animal Life

The plant life of the Carboniferous period was extensive and luxuriant, especially during the Pennsylvanian. It included ferns and fernlike trees; giant horsetails, called calamites; club mosses, or lycopods, such as Lepidodendron and Sigillaria; seed ferns; and cordaites, or primitive conifers. Land animals included primitive amphibians, reptiles (which first appeared in the Upper Carboniferous), spiders, millipedes, land snails, scorpions, enormous dragonflies, and more than 800 kinds of cockroaches. The inland waters were inhabited by fishes, clams, and various crustaceans; the oceans, by mollusks, crinoids, sea urchins, and one-celled foraminifera.

Cambrian period [Lat. Cambria=Wales], first period of the Paleozoic geologic era (see Geologic Timescale, table) extending from approximately 570 to 505 million years ago. It was named by the 19th-century English geologist Adam Sedgwick, who first studied the great sequence of rocks characteristic of the period near Cambria, Wales. During the Cambrian, the continents and seas differed from present day configurations. Four major continents, Gondwanaland, Angara, and the two sections of Euramerica, were inundated with a rising sea level, accumulating thick sedimentary deposits (see sediment). This sedimentary rock, i.e., conglomerate, sandstone, shale, and limestone, was formed in shallow seas that covered large areas of present-day North America, Europe, and Asia. In the United States, Lower Cambrian formations are found in the Appalachian; the sandstones near Waucoba Springs, S Calif.; and the thick layers of conglomerates and sandstones in Georgia, Tennessee, and North Carolina. Middle Cambrian rocks are found in New Brunswick, near Braintree, Mass. Upper Cambrian formations include the St. Croix sandstone of Wisconsin and the upper Mississippi valley, parts of the Arbuckle limestone of Oklahoma, and the Potsdam sandstone in New York's Adirondacks. In Russia, the Cambrian beds are remarkable in that they comprise mostly undisturbed and unconsolidated sand and clay despite their great age. The Cambrian rocks are the first rock layers to contain many easily recognizable fossils. The known Cambrian fauna—all marine—includes every phylum of invertebrates; the possibility that vertebrate fossils may be found cannot be excluded. The dominant animal was the trilobite, along with sea snails, brachiopods, sponges, and archaeocyathids. The ages of the various rock layers are distinguished according to the different genera of fossils they contain. The sudden appearance of highly developed and diversified fauna in Cambrian rock is best explained by the assumption that more primitive forms flourished during a missing stratigraphic interval between the close of the Precambrian and the beginning of the Cambrian. Remnants of these early organisms were either destroyed by erosion or their soft bodies easily decayed in a short period of time. In addition, at the beginning of the Cambrian, numerous animals eventually developed skeletons, or hard parts, capable of leaving behind fossil remains.

Time required for a body in the solar system to return to the same or about the same position relative to the Sun as seen from Earth. The Moon's synodic period is the time between successive recurrences of the same phase (e.g., the period between one full moon and the next). A planet's synodic period is the time required for Earth to overtake it as both go around the Sun or (in the case of fast-moving Mercury or Venus) for the other planet to overtake Earth. Seealso sidereal period.

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Time required for a celestial body in the solar system to complete one revolution with respect to the fixed stars (as observed from a fixed point outside the system). A planet's sidereal period can be calculated from its synodic period. The sidereal period of the Moon or an artificial satellite of Earth is the time it takes to return to the same position against the background of stars. Seealso day.

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In geology, the basic unit of the geologic time scale. During these spans of time, specific systems of rocks were formed. Originally, the method for defining the sequence of periods was relative; it was based on stratigraphy and paleontology. Carbon-14 dating and similar methods are now used to determine absolute ages for various periods.

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Musical genre incorporating diverse styles from Africa, eastern Europe, Asia, South and Central America, the Caribbean, and nonmainstream Western folk sources. The term was first coined largely in response to the sudden increase of recordings in non-English languages that were released in Great Britain and the United States in the 1980s, but by the early 1990s world music had become a bona fide musical genre and counterpoint to the increasingly synthetic sounds of Western pop music. Initially, African popular music and world music were virtually synonymous, and the genre's biggest stars included the Nigerians King Sunny Ade and Fela Anikulapo Kuti and the Senegalese Youssou N'Dour. Moreover, one of its earliest advocates was the Cameroonian-born Frenchman Francis Bebey. By the 21st century world music encompassed everything from Pakistani singer Nusrat Fateh Ali Khan and the pop-flamenco of the French group the Gipsy Kings to “ambient-global” projects that merged so-called ethnic voice samples with state-of-the-art rhythm programming.

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Style of U.S. popular music sung and performed primarily by African American musicians, having its roots in gospel music and rhythm and blues. The term was first used in the 1960s to describe music that combined rhythm and blues, gospel, jazz, and rock music and that was characterized by intensity of feeling and earthiness. In its earliest stages, soul music was found most commonly in the South, but many of the young singers who were to popularize it migrated to cities in the North. The founding of Motown in Detroit, Mich., and Stax-Volt in Memphis, Tenn., did much to encourage the style. Its most popular performers include James Brown, Ray Charles, Sam Cooke, and Aretha Franklin.

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or rock and roll

Musical style that arose in the U.S. in the mid-1950s and became the dominant form of popular music in the world. Though rock has used a wide variety of instruments, its basic elements are one or several vocalists, heavily amplified electric guitars (including bass, rhythm, and lead), and drums. It began as a simple style, relying on heavy, dance-oriented rhythms, uncomplicated melodies and harmonies, and lyrics sympathetic to its teenage audience's concerns—young love, the stresses of adolescence, and automobiles. Its roots lay principally in rhythm and blues (R&B) and country music. Both R&B and country existed outside the mainstream of popular music in the early 1950s, when the Cleveland disc jockey Alan Freed (1921–65) and others began programming R&B, which until then had been played only to black audiences. Freed's success gave currency to the term rock and roll. The highly rhythmic, sensual music of Chuck Berry, Bill Haley and the Comets, and particularly Elvis Presley in 1955–56 struck a responsive chord in the newly affluent postwar teenagers. In the 1960s several influences combined to lift rock out of what had already declined into a bland and mechanical format. In England, where rock's development had been slow, the Beatles and the Rolling Stones were found to have retained the freshness of its very early years and achieved enormous success in the U.S., where a new generation had grown up unaware of the musical influences of the new stars. At the same time, Bob Dylan, Joni Mitchell, the Byrds, and others were blending the traditional ballads and verse forms of folk music with rock, and musicians began to explore social and political themes. Performers such as the Grateful Dead, Jim Morrison of the Doors, and Frank Zappa of the Mothers of Invention combined imaginative lyrics with instrumental virtuosity, typically featuring lengthy solo improvisation. Janis Joplin and Jimi Hendrix won large followings with their exotic elaborations on R&B. The 1970s saw the rise of singer-songwriters such as Paul Simon, Neil Young, Elton John, David Bowie, and Bruce Springsteen, and rock assimilated other forms to produce jazz-rock, heavy metal, and punk rock. In the 1980s the disco-influenced rock of Madonna, Michael Jackson, and Prince was balanced by the post-punk “new wave” music of performers such as Laurie Anderson, Talking Heads (led by David Byrne), and the Eurythmics—all of whom illustrated their songs with music videos. By the 1990s rock music had incorporated grunge, rap, techno, and other forms.

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Popular entertainment that featured successive acts by singers, comedians, dancers, and actors. The form derived from the taproom concerts given in city taverns in England in the 18th–19th centuries. To meet the demand for entertainment for the working class, tavern owners often annexed nearby buildings as music halls, where drinking and smoking were permitted. The originator of the English music hall as such was Charles Morton, who built Morton's Canterbury Hall (1852) and Oxford Hall (1861) in London. Leading performers included Lillie Langtry, Harry Lauder (1870–1950), and Gracie Fields. Music halls evolved into larger, more respectable variety theatres, such as London's Hippodrome and the Coliseum. Variety acts combined music, comedy acts, and one-act plays and featured celebrities such as Sarah Bernhardt and Herbert Tree. Seealso vaudeville.

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Mechanical musical instrument in which projecting pins on a revolving brass cylinder or disk, encoding a piece of music, pluck tuned steel tongues. It was probably invented circa 1780 in Switzerland. With its modular cylinders or disks, it was a popular domestic instrument until displaced by the player piano and phonograph.

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Art concerned with combining vocal or instrumental sounds for beauty of form or emotional expression, usually according to cultural standards of rhythm, melody, and, in most Western music, harmony. Music most often implies sounds with distinct pitches that are arranged into melodies and organized into patterns of rhythm and metre. The melody will usually be in a certain key or mode, and in Western music it will often suggest harmony that may be made explicit as accompanying chords or counterpoint. Music is an art that, in one guise or another, permeates every human society. It is used for such varied social purposes as ritual, worship, coordination of movement, communication, and entertainment.

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klezmer music(Yiddish; “vessel of song”)

Traditional music played by professional musicians (klezmorim) in the Jewish ghettos of eastern Europe, especially for weddings and other ceremonies. The klezmer tradition has its roots in medieval Europe. By the 19th century its style was well-developed, influenced not only by the liturgical music of the synagogue (which allows only unaccompanied singing), but also that of the local non-Jewish cultures. It is primarily lively dance music. Klezmer ensembles have varied considerably; in the U.S., where a klezmer revival began in the 1980s, a typical band consists of four to six musicians playing some combination of violin, clarinet, trumpet, trombone, tuba, accordion, double bass, and percussion.

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Music composed to accompany a play. The practice dates back to ritualistic Greek drama, and it is thus connected to the use of music in other kinds of ritual. Sometimes limited to the role of introduction or interlude (setting a mood or a historical period, for example), it may also accompany spoken dialogue (see melodrama). Film and television music is sometimes considered incidental music.

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Form of black American music derived from Pentecostal church worship services and from spiritual and blues singing. Recordings of Pentecostal preachers' sermons were immensely popular among African Americans in the 1920s. Taking the scriptural direction “Let everything that breathes praise the Lord” (Psalm 150), Pentecostal churches welcomed timbrels, pianos, banjos, guitars, other stringed instruments, and even brass into their services. Choirs often featured the extremes of female vocal range in antiphonal counterpoint with the preacher's sermon. Other forms of gospel music have included the singing and acoustic guitar playing of itinerant street preachers; individual secular performers; and harmonizing male quartets, whose acts included dance routines and stylized costumes. Gospel music's principal composers and practitioners included Thomas A. Dorsey, who coined the term; the Rev. C.A. Tindley (1851–1933); the blind wandering preacher Rev. Gary Davis (1896–1972); Sister Rosetta Tharpe (1915–73), whose performances took gospel into nightclubs and theatres in the 1930s; and Mahalia Jackson. Gospel music was a significant influence on rhythm and blues and soul music, which have in turn strongly influenced contemporary gospel music.

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Music held to be typical of a nation or ethnic group, known to all segments of its society, and preserved usually by oral tradition. Knowledge of the history and development of folk music is largely conjectural. Musical notation of folk songs and descriptions of folk music culture are occasionally encountered in historical records, but these tend to reflect primarily the literate classes' indifference or even hostility. As Christianity expanded in medieval Europe, attempts were made to suppress folk music because of its association with heathen rites and customs, and uncultivated singing styles were denigrated. During the Renaissance, new humanistic attitudes encouraged acceptance of folk music as a genre of rustic antique song, and composers made extensive use of the music; folk tunes were often used as raw material for motets and masses, and Protestant hymns borrowed from folk music. In the 17th century folk music gradually receded from the consciousness of the literate classes, but in the late 18th century it again became important to art music. In the 19th century, folk songs came to be considered a “national treasure,” on a par with cultivated poetry and song. National and regional collections were published, and the music became a means of promoting nationalistic ideologies. Since the 1890s, folk music has been collected and preserved by mechanical recordings. Publications and recordings have promoted wide interest, making possible the revival of folk music where traditional folk life and folklore are moribund. After World War II, archives of field recordings were developed throughout the world. While research has usually dealt with “authentic” (i.e., older) material not heavily influenced by urban popular music and the mass media, the influence of singer-songwriters such as Woody Guthrie, Pete Seeger, Joan Baez, and Bob Dylan expanded the genre to include original music that largely retains the form and simplicity of traditional compositions.

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Any music involving electronic processing (e.g., recording and editing on tape) and whose reproduction involves the use of loudspeakers. In the late 1940s, magnetic tape began to be used, especially in France, to modify natural sounds (playing them backward, at different speeds, etc.), creating the genre known as musique concrète. By the early 1950s, composers in Germany and the U.S. were employing assembled conglomerations of oscillators, filters, and other equipment to produce entirely new sounds. The development of voltage-controlled oscillators and filters led, in the 1950s, to the first synthesizers, which effectively standardized the assemblages and made them more flexible. No longer relying on tape editing, electronic music could now be created in real time. Since their advent in the late 1970s, personal computers have been used to control the synthesizers. Digital sampling—composing with music and sounds electronically extracted from other recordings—has largely replaced the use of oscillators as a sound source.

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or country and western

Musical style that originated among whites in rural areas of the southern and western U.S. The term country and western music was adopted by the music industry in 1949 to replace the derogatory hillbilly music. Its roots lie in the music of the European settlers of the Appalachians and other areas. In the early 1920s the genre began to be commercially recorded; Fiddlin' John Carson recorded its first hit. Radio programs such as Nashville's Grand Ole Opry and Chicago's National Barn Dance fueled its growth, and growing numbers of musicians, such as the Carter Family and Jimmie Rodgers, began performing on radio and in recording studios. With the migration of Southern whites to industrial cities in the 1930s and '40s, country music was exposed to new influences, such as blues and gospel music. Its nostalgic bias, with its lyrics about poverty, heartbreak, and homesickness, held special appeal during a time of great population shifts. In the 1930s “singing cowboy” film stars, such as Gene Autry, altered country lyrics to produce a synthetic “western” music. Other variants include western swing (see Bob Wills) and honky-tonk (see Ernest Tubb and Hank Williams). In the 1940s there was an effort to return to country's root values (see bluegrass), but commercialization proved a stronger influence, and in the 1950s and '60s country music became a huge commercial enterprise. Popular singers often recorded songs in a Nashville style, while many country music recordings employed lush orchestral backgrounds. Country music has become increasingly acceptable to urban audiences, retaining its vitality with diverse performers such as Willie Nelson, Waylon Jennings, Dolly Parton, Randy Travis, Garth Brooks, Emmylou Harris, and Lyle Lovett. Despite the influence of other styles, it has retained an unmistakable character as one of the few truly indigenous American musical styles.

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Music composed for small instrumental ensembles and performed without a conductor. Traditionally intended for performance in a room or reception hall, often solely for the performers' own pleasure, chamber music is now often heard in concert halls. It began with the 16th-century instrumental consort, and long continued to be associated with aristocratic households. The duo sonata (usually for violin and continuo) and trio sonata appeared in early 17th-century Italy. The string quartet arose in the 1750s and remains the best-known chamber genre and ensemble. The serenade, nocturne, and divertimento were Classical genres for varying instrumental forces, often intended to accompany meals and other activities. Standard ensembles include the string trio (violin, viola, cello), string quintet (two violins, two violas, cello), and piano trio (piano, violin, cello). The chamber orchestra, usually with fewer than 25 musicians, is often used for 18th-century music and usually requires a conductor. Seealso sonata.

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(from Latin, alea: “dice game”) Any 20th-century music, particularly that of the 1950s and '60s, the composition or performance of which incorporates elements of chance. In aleatory music aspects such as the ordering of a piece's sections, its rhythms, and even its pitches are decided at the moment of performance. When not purely improvising, players follow lists of arbitrary rules or interpreted “graphic” notation that merely suggest the sounds. Charles Ives and Henry Cowell had used such techniques, but John Cage became the principal figure in aleatory; other aleatory composers include Earle Brown (1926–2002), Morton Feldman (1926–87), and Pierre Boulez.

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Conservatory of music in Philadelphia, Pa., U.S. It was founded in 1924 by Mary Louise Curtis Bok (1876–1970), wife of the editor Edward Bok, and named for her father, the inventor Charles Gordon Curtis. Her endowment was adequate to assure scholarships for gifted students throughout the world. Many eminent musicians have served on its faculty, including Wanda Landowska, Bohuslav Martinů, and Rudolf Serkin. Graduates include Samuel Barber, Leonard Bernstein, and Gian Carlo Menotti.

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Period in European history traditionally dated from the fall of the Roman Empire to the dawn of the Renaissance. In the 5th century the Western Roman Empire endured declines in population, economic vitality, and the size and prominence of cities. It also was greatly affected by a dramatic migration of peoples that began in the 3rd century. In the 5th century these peoples, often called barbarians, carved new kingdoms out of the decrepit Western Empire. Over the next several centuries these kingdoms oversaw the gradual amalgamation of barbarian, Christian, and Roman cultural and political traditions. The longest-lasting of these kingdoms, that of the Franks, laid the foundation for later European states. It also produced Charlemagne, the greatest ruler of the Middle Ages, whose reign was a model for centuries to come. The collapse of Charlemagne's empire and a fresh wave of invasions led to a restructuring of medieval society. The 11th–13th centuries mark the high point of medieval civilization. The church underwent reform that strengthened the place of the pope in church and society but led to clashes between the pope and emperor. Population growth, the flourishing of towns and farms, the emergence of merchant classes, and the development of governmental bureaucracies were part of cultural and economic revival during this period. Meanwhile, thousands of knights followed the call of the church to join the Crusades. Medieval civilization reached its apex in the 13th century with the emergence of Gothic architecture, the appearance of new religious orders, and the expansion of learning and the university. The church dominated intellectual life, producing the Scholasticism of St. Thomas Aquinas. The decline of the Middle Ages resulted from the breakdown of medieval national governments, the great papal schism, the critique of medieval theology and philosophy, and economic and population collapse brought on by famine and disease.

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or Old Stone Age

Ancient technological or cultural stage characterized by the use of rudimentary chipped stone tools. During the Lower Paleolithic (circa 2,500,000–200,000 years ago), simple pebble tools and crude stone choppers were made by the earliest humans. About 700,000 years ago, the first rough hand ax appeared; it was later refined and used with other tools in the Acheulean industry. A flake-tool tradition emerged in the Middle Paleolithic, as exemplified by implements of the Mousterian industry. The Upper Paleolithic (40,000–10,000 BC) saw the emergence of more complex, specialized, and diverse regional stone-tool industries, such as the Aurignacian, Solutrean, and Magdalenian. The two principal forms of Paleolithic art are small sculptures—such as the so-called Venus figurines and various carved or shaped animal and other figures—and monumental paintings, incised designs, and reliefs on the walls of caves such as Altamira (in Spain) and Lascaux Grotto (in France). The end of the Paleolithic is marked by the emergence of the settled agricultural villages of the Neolithic Period.

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or New Stone Age

Final stage of technological development or cultural evolution among prehistoric humans. It is characterized by the use of stone tools shaped by polishing or grinding, the domestication of plants or animals, the establishment of permanent villages, and the practice of such crafts as pottery and weaving. The Neolithic followed the Paleolithic Period (and in northwestern Europe the Mesolithic) and preceded the Bronze Age. Its beginning is associated with the villages that emerged in South Asia circa 9000 BC and flourished in the Tigris and Euphrates river valleys from circa 7000 BC. Farming spread northward throughout Eurasia, reaching Britain and Scandinavia only after 3000 BC. Neolithic technologies also spread to the Indus River valley of India by 5000 BC and to the Huang Ho valley of China by circa 3500 BC. The term is not applied to the New World, though Neolithic modes of life were achieved independently there by circa 2500 BC.

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