Great Central Lowland.— With the excep tion of a few minor uplifts, such as the Onichi tas, the Ozarks, the Black others of lesser note, this region is a vast int,.rior region, lying south of the Canadian Shi( I (I, and en closed between the Eastern and W, stern high lands, sending a narrow arm to tin Arctic be tween the Western Highland and the Canadian Shield. The area is one of flat-lying rocks. East of the Missouri River, these are almost wholly Paleozoic, the Carboniferous being the most wide-spread surface formations. From the Missouri River westward, Cretaceous and Tertiary rocks constitute most of the surface. It was during late Carboniferous times that most of this area permanently became land, al though its western portion was again tempo rarily under the sea during the Cretaceous period. Topographically the region varies from almost dead level plains to sharp hilly regions, ranging from a few hundred to 5,000 feet above sea-level. The northern part has been glaciated, and swamps and lakes are abundant. As a whole the region comprises the great agricultural portion of the continent. It is the richest in coal and petroleum of any section of the country. It has very valuable lead and zinc deposits, beds of salt, quarries of building stone and other minerals of lesser value.
The Western Highland—This portion of the continent is more complicated in structure and history than the areas thus far considered. It may be said to consist of a great range of mountains, the Rockies proper, on the east, to the west of which lies a broad plateau region, and beyond this the Pacific border ranges, them selves complex in structure and history. These sections will be taken up each in a separate brief discussion.
The Rocky Mountains, contrary to rather hazy prevalent conceptions, lie nearly a thou sand miles from the Pacific Ocean, at least in the United States. They contain rocks of every age from Pre-Cambrian to Quaternary folded into ranges, rather generally trending north and south. Their final emergence from be neath the sea, and the folding which gave them their present structure, came at the close of the Cretaceous period. During the Tertiary they were the site of much igneous activity, and since their folding they have been greatly modi fied by powerful erosion, several minor uplifts and very recently by glaciation. Unlike the Appalachian region, there are minor areas be tween the numerous ranges, in which the rocks are flat-lying, or nearly so.
The Inter-Mountain plateau, while is con tains some areas of intense folding, s more largely one of nearly flat-lying rocks, greatly modified by faulting. The rocks are of all ages from Pre-Cambrian to Tertiary.
The southern and higher portion of this large region is generally known as the Colorado Plateau. Paleozoic rocks are exposed at a few points, but the surface formations are largely Jurassic and Cretaceous in age. The rocks are nearly flat, as a whole, and the structure is dominated by a few very large faults. Above the plateau surface, which average 5,000-7,000 feet in height, rise several groups of volcanic mountains, among which the San Francisco Peaks and the Henry Mountains are best known. The area is deeply trenched by magnifi cent canons, among which the Grand Callon is easily the foremost.
To the north and west of the Colorado plateau is that portion of the Inter-Mountain plateau most commonly called the Great Basin. This region stands notably lower than the Colo rado plateau and is much more intensely faulted. Paleozoic formations and Tertiary lavas and lake beds are wide-spread surface rocks. The general floor of this region ranges from a few hundred to 5,000 feet above sea level, and above this are many north and south mountain ranges carved out of fault blocks. Between these ranges are extremely flat desert floors, some of them beds of extinct salt lakes.
Still farther north this complicated basin structure has been buried beneath thousands of feet of Tertiary lavas, forming the great Columbia Lava plateau, many thousands of square miles in extent. This has more recently been trenched by the Snake and Columbia rivers, which have cut canons from 2,000 to 4,000 feet in depth.
The Pacific Border ranges are made up of the Sierra Nevadas and Cascades on the east, and the Coast ranges on the west, with the Valley of California, the Willamette Valley and Puget Sound as lowlands separating the two systems.
The Sierra Nevada Range consists largely of Triassic and Jurassic slates, cut by great masses of granite. The range was folded near the close of the Jurassic period, since which time it has been modified by subsequent uplifts, faulting, and very extensive erosion, with recent glaciation. To the north it passes with some what similar structure, under great piles of lava that constitute the numerous volcanic cones of that range, dating from Tertiary time.
The Coast ranges consist of intensely folded and faulted rocks varying in age, but probably dominantly Tertiary. Their structural history has been extremely complex. One in tense period of folding came near the middle of the Miocene. There have been later uplifts, accompanied by some folding and much faulting the latter process going on even to the present time, as witnessed by the great fault responsible for the San Francisco earthquake.