582 GEOLOGY. 



ings of the land, and how far to changes in the level of the sea, can- 

 not now be determined; but when we recall that the ocean-level 

 must respond to every deformation which affects its bottom (unless 

 compensated by an equivalent opposite movement), and to every stage 

 of filling, 1 it does not seem strange that its level is in a nearly per- 

 petual state of fluctuation. 



In general, it may be said that the movements of the crust which 

 have been of most importance, from the point of view of continental 

 or biological evolution, are not those which have affected high land 

 or deep sea bottom, but those which have converted sea bottom into 

 land, or land into sea bottom. Such changes are most likely to have 

 taken place where land was low, or water shallow. From the point 

 of view of geology, therefore, the critical level of crustal oscillation is the 

 level of the sea. 



Thickness. 



The Coal Measures differ greatly in thickness in different sections 

 of the country, but, like all the preceding formations of the Paleo- 

 zoic, they are especially thick in the Appalachian mountains, measured 

 by the usual method. 



The thickness of the Millstone Grit (Pottsville conglomerate) 

 alone ranges up to 1700 feet in Pennsylvania, but it thins rapidly 

 to the westward. The Coal Measures are equally variable in thick- 

 ness, ranging from nearly zero to> more than 3000 feet (Penn- 

 sylvania to Alabama). In the interior, the corresponding formations 

 rarely much exceed 1000 feet. In Ohio they are about 1200 feet 

 thick; in Indiana, 1000 feet; in Illinois, 1650 feet; in Michigan 300 

 feet, and in Iowa, 600 feet. In Arkansas, the Coal Measures attain 

 the remarkable thickness of more than 20,000 feet, 2 from which it 

 is inferred that there must have been land close at hand capable of 

 supplying sediments in great quantity, combined, probably, with 

 conditions favorable to deposition in sloping attitudes. This was 

 probably the axis of the Ouachita uplift. In Texas, the Penn- 

 sylvanian has a thickness ranging up to 5000 feet. 3 In the 

 Wasatch mountains, the Carboniferous strata (including the Lower 

 Carboniferous) have been estimated to be about 13,000 feet thick, 



1 Salisbury. Jour, of Geol. Vol. XIII. p. 469. 



2 Cited in Report of the Dept. of Geol. and Nat. Res. of Indiana. 



3 Richardson, Bull. G., Univ. of Texas Mineral Survey. 



