310 THE POPULAR SCIENCE MONTHLY. 



Mesozoic. At the close of the Palaeozoic ages great changes took 

 place in the topography of the central and western portions of the con- 

 tinent, 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 Palaeozoic rocks in the central basin, the Archae- 

 an 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 dur- 

 ing 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 inter- 

 val between the Wahsatch and the Sierra Nevada, no Cretaceous rocks 

 are found. On the Pacific side of the great Sierra, however, Creta- 

 ceous strata lie nearly horizontal, abutting against the upturned Juras- 

 sic 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 

 Palaeozoic rocks below. This conformability is, however, more appar- 

 ent than real, for the region between the Wahsatch and the Mississippi 

 gives abundant evidence of elevation and subsidence during the Meso- 

 zoic 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 com- 

 plete 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 



