Page:Popular Science Monthly Volume 15.djvu/324

 —At the close of the Palæozoic ages great changes took place in the topography of the central and western portions of the continent, all of which are for the first time made clearly known by Mr. King's graphic and lucid descriptions in his chapters on the "Mesozoic Areas of the Fortieth Parallel." After the deposition of the great and conformable series of Palæozoic rocks in the central basin, the Archæan continent, which formed the western limit of these older deposits, and which had continued dry land to the close of the Carboniferous age, was sunk under the waves of the Pacific, and thus remained during the Triassic and Jurassic ages—long enough for the deposition of about 20,000 feet of sediments, of which considerably more than half belong to the Trias. Then the great fold of the Sierra Nevada was raised high above the ocean-level, carrying with it all the table-land of the Great Basin, which has not since been covered with salt-water. The rocks composing the Sierra Nevada are chiefly the Jurassic and Triassic beds, here often completely metamorphosed and converted into crystalline slates and massive granites, in which lie the auriferous veins that have supplied the $1,000,000,000 of gold already taken from the California placers and quartz mines. This paroxysm, or rather period of elevation, occurred before the Cretaceous age; for, in all the interval between the Wahsatch and the Sierra Nevada, no Cretaceous rocks are found. On the Pacific side of the great Sierra, however, Cretaceous strata lie nearly horizontal, abutting against the upturned Jurassic and Triassic slates, and reaching to a height of some 1,200 feet above the present ocean-level.

East of the Wahsatch a very different history is recorded, for here the Triassic, Jurassic, and Cretaceous strata were deposited not only in a series conformable among themselves, but apparently with the Palæozoic rocks below. This conformability is, however, more apparent than real, for the region between the Wahsatch and the Mississippi gives abundant evidence of elevation and subsidence during the Mesozoic ages; but these changes of level were continental rather than local, and over an area of hundreds of thousands of square miles the surface on which the strata were deposited was so nearly level that no want of parallelism in their planes of deposition is visible to the eye. That great changes of level did take place in this region is evident from the facts, first, that on the area over which the Triassic beds were deposited, extending from the Colorado to the Mississippi, mechanical and shallow water deposits alone prevail. No limestones occur here in the Trias, but it is made up of great sheets of cross-stratified and tide-swept sand of brilliant colors, chiefly bright red—due to the complete oxidation of their iron from aeration, and the absence of organic matter—with here and there heavy beds of gypsum, and formerly of salt; all the records of the intermittent action of a shallow sea. This sea-bottom over the interval between the Rocky Mountains and the Mississippi became dry land at the close of the Trias; for the Jurassic