The chief economic product of the Cretaceous is the coal which was laid down in many places in the western part of the United States, and ranging in grade from lignite to anthracite.
California, Oregon and Washington reveal marine Cenozoic sediments: Eocene to a depth of 8,000 to 12,cm ft.; late Miocene in California, 8,000 ft.; and Pliocene and Pleistocene south of San Francisco to a depth of 13,000 feet. Continental Cenozoic deposits are wide-spread in the foot-hills and high plains east of the Rocky Mountains. Great tectonic and volcanic activity characterized the Cenozoic in the United States, and the great Cascadian revolution was initiated, and continued, the great uplift responsible for the Colorado plateaux and the cutting of the Grand Canyon, and the grand volcanoes-Rainier, Shasta and Lassen-which remained active throughout the Pleistocene. The close of the Pliocene was a time of great deformation, a critical period of geologic history in the United States, when the final aspect of the surface as we know it was largely determined.
The Pleistocene or Quaternary, the final division of geologic chronology, was marked by the culmination of the Cascadian revolution, and the extensive glaciation that followed, when more than the northern half of North America and Europe was buried beneath great strata of ice. Three great ice caps formed in America : the Labrador, east of Hudson bay; the Keewatin, west of Labrador; and the Cordilleran, over the Canadian Rockies. Many smaller outlying caps or shields were formed about the margins of these caps. The glaciation was not continuous, but
five stages of ice formation and extension with four interglacial stages have been generally recognized in the United States.
During the Pleistocene glaciation the surface of the areas cov ered by the ice as well as those contiguous to its borders was profoundly modified by the ice itself or by the waters resulting from its melting. About the centres of origin of the several ice caps the terrain was scoured, sculptured, deeply grooved by the moving ice; about the peripheries this erosive action was mini mized and depositional changes were most pronounced. As the ice receded the whole character of the Great Lakes was changed, the drainage systems of the whole area disturbed, and the aspect of the entire area significantly changed.
The extensive lands of Cenozoic time were dominated by mam mals, and even the seas contained representatives of this great group. The life that characterized the Mesozoic was practically extinct before the close of the Eocene; and by the time the Oli gocene had dawned the mammal life had assumed a distinctly modern aspect, though still rather primitive. With increasing des 'Pirsson and Schuchert, of Geology, p. 949.
iccation and cold in the Miocene, and consequent extension of grasslands, ruminants and rodents increased in number and va riety. Prominent among the forms of this time were horses, camels, rhinoceroses, deer, sabre-tooth tigers and many others. The Pliocene is equally interesting for its life, but because the land stood high and little deposition was taking place on land, few fossils formed. The evolution of the horses, the camels and the elephants has been most fascinatingly worked out. The first pri mates, from which man is considered to have descended, appear as lemurs or lemuroids in the American Eocene ; the oldest ape appears in the Egyptian Oligocene ; the oldest ape man in the Javan Pleistocene; and the first dawn-man with the characteristic qualities of the modern Homo sapiens is thought to have origi nated on the steppe plateaux of Central Asia when first appeared conditions necessitating his leaving the trees and walking upright on the grasslands. The evolution of man, Homo sapiens, is a Pleistocene record, of which little that can be authentically sub stantiated has been written in the U.S., though skeletal remains and cultural artifacts found in Florida, Oklahoma, Texas, Colorado, and elsewhere may in time prove valuable pages in the history.