Pacific Border.—The Pacific Border embraces a chain of coast ranges with long and important valleys between these and the Sierra-Cascade mountains. The most northerly range is the Olympic mountains (24b), not unlike the Northern Cascades but lower. The Oregon Coast range (24c) is a gentle anticline of Ter tiary beds, relatively recent in origin. It is succeeded on the south by the Klamath mountains, (24d), a range of long standing, con sisting largely of metamorphosed Palaeozoic rocks, once eroded down to a peneplain but uplifted again, locally as high as 7,000 ft., then deeply and ruggedly eroded. Most of the California coast is occupied by the California coast ranges (24f). These are rela tively low mountains (generally between 2,000 and 4,000 ft.) ranged in parallel ridges due largely to faulting of an earlier, much deformed and much worn down mountain belt. Some broad val leys south of San Francisco are filled with late Tertiary sediments because of depression at that time. Still later the coast seems to have been depressed some 1,5oo ft. below its present level. Shore lines etched against the mountain side are preserved here and there at various levels between the sea and the 1,50o ft. contour. As shown by these old shorelines, the depression that caused San Francisco bay is relatively recent.
Puget sound is the submerged northern end of a trough (24a) 35o m. long between the Cascades and the Coast range. Its southern end is the valley of the Willamette, a southern tributary of the Columbia. Some of the valleys in this long trough are agri culturally rich. Commercially the region of the sound is favoured by its fine harbours and the f our or more transcontinental railroads which these harbours have attracted. The Great Valley of Cali fornia (24e) between the Coast range and the Sierra Nevada is 400 m. long and 5o m. wide. In the upwarp of the adjacent mountain belts, this belt was bent down. As a consequence it was covered with the silt, sand and gravel washed from the mountain slopes. That deposition is still going on in the valley is evident from the habits of the two master streams, the Sacramento from the north and the San Joaquin from the south. These have small gradients and run between high natural levees ; sandbars are abundant ; at the head of San Francisco bay the two streams unite to build an extensive delta. The filling of the valley is mainly from the higher mountains on the east. Deposition is in the form of alluvial fans which in the southern part have crowded the San Joaquin westward to the foot of the Coast range ; the great allu vial fan of Kings river, extending across the valley, has cut off its southern third forming Tulare basin, partly occupied at times by a playa lake. Here the climate is so arid that all inflow is evapo
rated and there is no drainage to the San Joaquin. Irrigation is highly developed, especially in the more arid southern half on the great alluvial fans. Fruits, nuts and raisins are the chief products.
The Sierra Nevada and the Great Valley extend south to lat itude 35° beyond which there are ranges trending nearly east-west from the Mohave desert to the Pacific. The famous fruit-growing lowlands of California lie mainly farther south. The Gulf of California occupies a down-warped trough similar to the Great Valley. In recent geologic time the gulf reached 15om. farther north. This extension was cut off by the delta of the Colorado river and its waters evaporated. It is now the basin of the Salton Sink, still in part below sea level and known, where irri gated, as the rich Imperial Valley.
All the great general systems of rock formations recognized in Europe are represented in the United States, but close correlation across the Atlantic is not possible in all the systems. The gen eralized geologic chronology for the U.S., including part of the Pre-Cambrian from Canada, is shown in the next column.
Archaeozoic Group.—The rocks representing the Archaeozoic era, the oldest group, outcrop in many parts of the United States, as in the cores of some of the western mountains and in some of the deep western canyons, in the worn down bases of the old mountain complexes of Wisconsin, Minnesota, New England and the eastern piedmont plateau of the Appalachians, and other regions of geologically ancient terrain. They have been revealed in deep borings in many other places and are believed to underlie the younger formations everywhere. They have been described as a "complex of basic and acidic surface and deep-seated igneous rocks, of schists and gneisses in part derived from them and in part of unknown origin, and of shreds and small masses of meta morphosed sediments, all unconformably below and older" than the Proterozoic sedimentary rocks.
No incontrovertible direct evidence has been uncovered of Archaeozoic life, though globular masses of metamorphosed lime stone of distinctive structure described from Canada and else where, designated Eozoon canadense , are widely believed to be cal careous depositions made by marine algae. The indirect evidences of life are the presence of sedimentary carbon in the form of graphite, and possibly of bacteria-precipitated limestone and bac teria-oxidized iron oxides, in the rocks of the Archaeozoic era. Locally the Archaean rocks are rich in iron ores, chiefly oxides, as in northern Minnesota, and in graphite, as in the Adiron dacks.