CAMBRIAN from aCambria,) an ancient name for Wales) is a term applied to the earliest time period of the Paleozoic Era, and to the system of rocks laid down during that period. It was first applied by Murchison and Sedgwick to rocks of England and Wales, about 1835. In Cambrian time animal life on the earth was already highly differentiated. All the great of the animal kingdom except the vertebrates were present then and definitely characterized. The principal types, so far as the fossil evidence goes, were Brachiopods and Trilobites, but many others existed, such as mollusks, marine worms, siliceous sponges, graptolites and jellyfish, and by the end of the period starfish and crinoids. It is probable that plants, such as seaweeds, existed, but the evi dence is very obscure. The climate during the Cambrian period was probably warrrl even up to the Arctic Circle, but not torrid. This even ness of temperature may have been due to a much larger part of the earth's surface being covered by water than in later time, or to a dif ference in the composition of the atmosphere, more carbon dioxide being present. At the beginning of Cambrian time the North Ameri can continent was probably larger than now. On the east a sound stretched from Labrador to Alabama, separating an island known as Old Appalachia, which extended east beyond the present coast. Central United States was largely land. On the west, a second sound ex tended from the Arctic south into Nevada and California, separating another land mass on the west, which also probably reached beyond the western coast.
During Cambrian time the continental land mass slowly sank, and by the end of the Cam brian a great interior sea covered the whole Mississippi Valley and large areas both east and west. Changes in land and sea were also going on in other continents, large parts of which were under water during later Cambrian time. The period was without notable folding, but was marked by great volcanic activity in Wales and Scotland. In China, Norway and possibly at other points, rocks of very early Cambrian, or else of very late Algonkian Age, bear evidence of glacial origin.
The Cambrian system is fairly well defined at its base, since the rocks are deposited upon the upturned, eroded edges of Algonkian and older strata, indicating a great time break. The top of the Cambrian grades into the Ordovician, so that geologists are not agreed as to the line of demarcation. In North America the rocks of the Cambrian system are divided into three series, as follows: (1) The Lower Cambrian, or Georgian, (2) the Middle Cambrian or Acadian, and (3) the Upper Cambrian, or Potsdam (Saratogan). The Lower Cambrian rocks are
largely restricted to the long narrow sounds already described. As the continent gradually sank, the areas of Cambrian deposition became larger, and Middle and Upper Cambrian sedi ments were laid down over much of central United States from the Appalachians to the Sierra Nevada, south as far as northern Ark ansas. They have since been eroded from parts of this area, and in still other parts lie hidden below more recent deposits. Cambrian rocks outcrop in large areas in the Appalachian Mountains, the Adirondacics, central Wisconsin, the Oiarks and in many isolated patches in the Rocky Mountains, as well as at numerous other points. The rocks indicate generally a period of tranquil change, the ocean slowly ad vancing over the sinking continent and islands, just as one may see it to-day along great stretches of coast. The rocks are chiefly shal low water formations, including conglomerates, sandstones and shales, though limestones are by no means unknown. In a few places as at South Mountain, Pa.,, there are rocks represent ing lava and volcanic ash interstratified with detrital sediments. In the Appalachian region, the Cambrian sediments vary from 3,000 to 12,000 feet in thickness. Over central United States they are much thinner, but in British Columbia they are reported to reach the enormous thickness of 40,000 feet.
In Europe the Cambrian rocks are generally developed more fully than in North America; thus the conglomerates, sandstones, shales, slates and quartzites of the Welsh Cambrian are fully 20,000 feet thick and contain much volcanic material. They are rocks indicating shallow-water conditions, and show three divi sions. They extend from Wales, along Sweden, Norway and Lapland into Russia, having in Sweden a thickness of 2,000 feet. To the east the Cambrian formations thin out, and in central Russia die out altogether, the Ordovician rest ing directly on the Archwan. There are con siderable areas of Cambrian in Germany, Bohemia, France, Portugal and Spain; also in northeast China, in the Salt Range in India, in Australia and in Argentina.
Bibliography.— Report of the British As sociation, Sedgwick 1835; Chamberlin and Salis bury, 'Geology' (Vol. II, New York 1907) ; Cleland, H. F., 'Geology Physical and Histori cal' (New York 1916) • Dana's 'Manual of Geology' (New York
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