The life of the Jurassic includes a cosmopolitan land flora re vealing a rich, varied growth of Cycads and the beginning of mod ern types; a great number of insects; diversified and advanced reptiles, both terrestrial and marine; the earliest birds; and a wealth of marine invertebrate and vertebrate forms including sharks of modern types and the first flat fishes.
The Jurassic Mariposa and Auriferous slates (Gold Belt senes) of the Sierra Nevadas contain the gold-bearing veins of quartz in which lie the "mother" lode from which the rich placer de posits of California have been derived.
The Comanchean, by which American geologists distinguish the Lower Cretaceous, which corresponds approximately to the lower series of six or perhaps seven stages generally included within the Lower Cretaceous of Europe, appears in the United States as a continental phase in limited areas along the Atlantic coast and in extensive regions in the Rocky Mountains; and as two distinct marine faunal provinces : the Shastan, a Pacific coastal overlap, and the extensive Comanchean of the Gulf of Mexico over Arizona, New Mexico, Texas, Oklahoma, Kansas, Arkansas and Colorado from which the group derives its name. The Co manchean extension proper was limited in extent in the United States during Middle Comanchean time, but became more wide spread in Late Comanchean. The waters of this sea apparently were shallow, muddy and brackish from the fresh water poured in by the rivers debouching upon it. The Shastan overlap at first filled chiefly the Great valley of California and the Willamette Puget sound basins but later spread widely to become a shelf sea into which rivers from the highlands to the eastward poured a great thickness of generally coarse-grained sediments. The con tinental deposits of the Great Plains country from Montana southward into New Mexico were apparently laid down over a relatively low, flat plain occupied by small shallow lakes and la goons, rimmed by broad marshes in which many great dinosaurs lived. Some of these same dinosaurs lived also in the Triassic swamps of the Atlantic coast, where the deposits indicate fresh water deposits laid down in river valleys in the earlier part of the period, and estuarine beds along the seaboard in the later part.
Toward the close of the Comanchean, crustal movement is evi dent along the Pacific coast throughout the coast ranges and the Cordilleran Intermontane disturbance initiated the process by which the high plateaux of the western part of the United States and northward and southward were elevated. Along the Atlantic
coast the Piedmont and Appalachian regions were elevated some what, thus rejuvenating the streams flowing from them.
The record of the life of the Comanchean indicates the wane of the cycads and the rapid rise, probably in the eastern United States, of the angiosperms, the ancestors of the dominant forms of modern floras, and the growth of the first "hardwood" forests -oaks, elms, poplars, maples and magnolias, for instance ; the maximum development, both in size and variety, of the dinosaurs; and the decline of the ammonites in the seas.
The last system of the Mesozoic, the Cretaceous, records the last wide-spread transgression of the sea over the United States. The extensive peneplanation of the preceding periods had re duced much of the land to river or coastal plains or low rolling hill lands, and when the Cretaceous submergence began it pro gressed fast over these. The whole Pacific coast was submerged, practically the whole interior west of the Mississippi, the Gulf States as far north as Illinois, and the Atlantic coast as far north as New Jersey. The broad Coloradoan interior sea extended from the Gulf of Mexico to the Arctic ocean. Always a shallow sea, the sediments rapidly accumulating after the Laramide revolution in late Cretaceous time soon filled it, but while it persisted deltas, bars and low islands were formed in its shallow, muddy, marshy waters. The Laramide revolution was a profound orogenic change by which the mountains of western North America and South America were raised to a grandeur that they have never lost.
The life of the Cretaceous embraces the end of the dinosaurs and their contemporaries. The close of the period marked the culmination of the "mediaeval" period in the evolution of animal life, a period long passed by the evolution of the flora; for the flora of the Cretaceous was distinctly modern, including birch, beech, maple, oak, walnut, sycamore, tulip, laurel, holly, ivy, sweet gum, breadfruit and hazelnut. The first sedges and grasses ap peared. Large reptilian carnivorous birds appeared, and many mammals. The Cretaceous was a time of extinction for many marine forms, of toothed birds and dinosaurs, and the destruc tion of reptilian supremacy.