ciated with these rocks are the lava sheets which form the Orange Mountains of New Jersey, the Palisades of the Hudson, and the Mount Holyoke range of Massachusetts. These rocks are known as the Newark formation and were not accumulated in the open sea, but in brackish or fresh waters, in basins whose character is little known. The Cretaceous and Tertiary beds of the Atlantic border form southern New Jersey, and the outside lowlands of all the States southward to Florida, and make the coastal plain of this region. They slant gently down and become continuous with the beds that lie below the marginal waters of the Atlantic. Owing to their comparative recency, they are often partially or wholly unconsolidated and occur as sands, gravels, clays, and marls. But they may consist also of well-indurated sandstones and limestones.
Similar statements may be made concerning the formations and the lands that border the Gulf. Here belongs the entire State of Florida, which is already given. It is low not because of denudation, but because of gentle and limited uplift of the undisturbed and youthful strata which lie beneath its surface. As has been intimated, the ancient Mississippi discharged, not far from the present mouth of the Ohio, into a gulf that thus lay between south-reaching lands on the east and west. Its successive burdens of land waste served gradually to fill the embayment, and its delta reached more and more to the south, encroaching, as it is still, upon the Gulf. Mesozoic deposits of Triassic, Jurassic, and Cretaceous age are found along the eastern base of the Rocky Mountains, where they are upturned at various angles, as in the vertical or highly inclined strata of the Garden of the Gods. Eastward, at a little remove from the mountains, these beds, which have yielded perhaps the most remarkable series of fossil vertebrate remains that have been obtained in any country, become horizontal and are often covered with still younger stratified formations, which all together make the underlying masses of the Great Plains. The breaking and upturning of the strata seen in the Rocky Mountain foothills points to the main uplift of these mountains which took place at the close of the Mesozoic era.
After the Rocky Mountain revolution, the Great Plains area ceased to be a region of salt-water deposition and was characterized by swamps, and great lakes of brackish or fresh water. The Laramie formation belongs to this era of low-lying lands in that region when the sea was excluded, and some of the largest coal deposits of the West were accunmlated in the marshes of the time.
Over many thousands of square miles in Colorado, Kansas, Wyoming, and Nebraska are sheets of incoherent or partly consolidated gravels, sands, and clays, which have usually been attributed to sedimentation in such lakes. It is probable, however, that in part, at least, these beds are due to torrents carrying down enormous volumes of wash from the moimtains and distributing it in their wanderings over the plains. Present conditions were not approached in this region until the late Tertiary. By that time the entire belt, including the plains and Rocky Mountains, had received a massive uplift, by which the lakes were drained, and the plains given an eastward slant, from altitudes of 5000 to 6000 feet at the base of the mountains, to the low prairies west of the Mississippi River.
The rocks of the Colorado plateaus consist of some thousands of feet of sedimentary beds of Mesozoic and younger rocks, overlying a Paleozoic and Archæan foundation. The Colorado River furnishes a great natural section, since for some distance it has sunk its channel through the Paleozoic strata and cut far down into the basal granites. Like the Great Plains, it was long a region of marine deposition, followed by lakes and streams as the lands emerged. Here, too, great and widespread uplifts took place, in which the strains were so great as to produce profound fractures and dislocations or faults, and attended at times by large outflows of lava. These upflows sometimes stopped below the surface, and domed up the overlying strata, making a kind of mountain known as laccolithic, of which the Henry Mountains, in Utah, are the type. Largely by such faults, running in a north and south direction, the ancient strata of the Great Basin have been cut into large blocks, and so tilted that the higher edges of the blocks make the parallel ranges of Utah and Nevada.
The initial uplifts of the Sierran mountain belt were made in Mesozoic time, and the strata involved were formed from the waste of the older lands in the present Great Basin region to the eastward. But it was not until late Tertiary time that the entire block or mass of the Sierras was lifted to a great height and tilted to the west. In connection with this uplift a lofty fault scarp developed, which now forms the steep eastern front of the mountains. This crest, therefore, is toward the east, and the principal drainage is down the gentler western slope into the valley of California. The Sierras continue northward as the Cascade Mountains of Oregon and Washington. The Willamette Valley in Oregon and the Puget Sound Valley in Washington are the analogues of the valley of California, and they are separated from the ocean by a young range of mountains, known in its various parts as the Coast Range, the Klamath Mountains, and the Olympic Mountains.
Thus it appears that the Western United States have had a composite history. It began with island nuclei which grew by sedimentation and uplift. The several great ranges of mountains mark several periods of folding, faulting, and uplift, while both mountains and plains rose by massive and wide-ranging or continental movements, thus adding to the height of the mountains and making the plains into plateaus of from 3000 to 8000 feet in altitude. With these disturbances, especially in Tertiary times, were the most extensive outflows of lava of which this continent shows any record. These are found either as remnant sheets and volcanic necks, or as vast sheets scarcely changed since their outflow. They occur in nearly every Cordilleran State, as on both sides of the Rocky Mountains in Colorado, in New Mexico, in Utah, in Mount Shasta, and the great cones of the Cascades, and especially in the lava plateaus of the Snake and Columbia rivers in Idaho, Washington, and Oregon. As a lingering episode of their interior volcanic energy we may, perhaps, recognize the geyser phenomena of the Yellowstone Park.
Through all the periods which have been passed
- Vol. XIX.—42.
