Page:Popular Science Monthly Volume 15.djvu/325

 strata which occupy much of Colorado, Utah, and Nevada, scarcely reach eastward beyond the mountains; and the Cretaceous age was marked by a great submergence which carried the shore-line progressively from the Gulf of Mexico to the Wahsatch, and northward, perhaps, to the Arctic Sea, converting all the area between the Wahsatch Mountains and the Canadian highlands into a sea, in which were deposited in some places 2,000 feet of limestone, the slow accumulation of calcareous matter from the growth and decay of marine organisms.

From Mr. King's careful study of the Mesozoic rocks of Nevada, we learn that the Trias consists of alternations of limestone and quartzite, which, in Star Peak, form a continuous section of over 10,000 feet. The fossils which the limestones contain show that much the larger part of this mass belongs to the Alpen Trias of the Old World, the Halstadt and Saint Cassian beds, and those which form the passage to the Jura.

The Jurassic rocks of Nevada are mostly shales—the deposits from water too shallow for limestones—and contain few fossils. Along the eastern margin of the Jurassic area in the Black Hills, the Jurassic beds are more purely marine, and are far richer in fossils. The upper portion of these beds, which are of an estuarine or littoral character, has lately been discovered to be a vast cemetery of vertebrate animals, some of which are of unequaled size, and in their structure of special interest. Among these are the huge dinosaurs described by Marsh and Cope, some of which far exceed in dimensions any terrestrial animals before known, the largest, according to Marsh, having a length of at least one hundred feet, and a height of twenty-five or thirty.

The uppermost member of Mr. King's Mesozoic section is the somewhat famous Laramie group—the Lignitic formation of Dana, so named because it contains the most important coal-beds of Colorado. The age of this group of rocks has been much discussed by Dr. Hayden and Lesquereux, the distinguished fossil botanist, and it has been represented by them to be Tertiary, on the evidence of its numerous fossil plants; Cope, however, found the remains of Cretaceous vertebrates, and Meek, Cretaceous mollusks, in it; and hence it was said to have a Cretaceous fauna and a Tertiary flora. The writer has, however, for a long time contended that its flora was distinct from that of the Tertiary rocks, and the proof was stronger that it was Cretaceous. Mr. King adduces new and apparently conclusive evidence that it is older than the Tertiary, since, like Cope, Meek, and Stevenson, he has obtained numerous Cretaceous animal remains from it, and finds it to underlie unconformably the Coryphodon beds, the oldest portion of the Eocene.

—The pictures which geology presents to us of the far West during the Tertiary age are totally different from those which preceded them, and, on the whole, more varied and interesting. As