HISTORICAL GEOLOGY. 145 



The Reptilian, including the Triassic, Jurassic, and Cretaceous, by 

 lines sloping from the right to the left (/), the Cretaceous being 

 distinguished by having the lines broken. 



The Tertiary, by lines sloping from the left to the right ( \ ). 



The surface without markings is occupied by rocks of undeter- 

 mined age ; that on the east is mostly crystalline. 



In Nova Scotia and New Brunswick, the Subcarboniferous is not distinguished from 

 the Carboniferous; and, west of the Mississippi, the limit between the Carboniferous 

 and Permian areas is partly conjectural; and also that in Arkansas between the 

 Carboniferous and Subcarboniferous. In the lettering:, Cr. stands for Cretaceous; C, 

 Charleston, S. C; Ci., lor Cincinnati; V.,Vicksburg, Miss.; B., Black Hills; 0., Ozark 

 Mountains; W\, Witchita Mountains. On rivers, to the west: to., White; n., Niobrara; 

 p., Platte; rp., Republican; s., Smoky Hill; a., Arkansas; c, Canadian; r., Red. 



Thickness of the stratified rocks. — The whole thickness of the 

 rocks in the series has been stated at twenty miles or more. But 

 this includes the sum of the whole, grouped in one pile. As the series 

 is nowhere complete, this cannot be said to be the thickness observed 

 in any one region. The rocks of New York, down to the Archaean, 

 counting all as one series, are about 13,000 feet in thickness. They 

 include only the Silurian and Devonian (excepting the Triassic in the 

 southeast). They thin out to a few feet in the northern part of the 

 State, and have their greatest thickness toward Pennsylvania. In 

 Pennsylvania, the rocks include the Carboniferous ; and the whole 

 thickness is at least 40,000 feet. This is exclusive of the Triassic, 

 which may add a few thousands to the amount. In Virginia, the thick- 

 ness is still greater ; but no exact estimate has been made. In Indi- 

 ana and the other States west, it is only 4,000 feet, although extending, 

 as in Pennsylvania, to the top of the Carboniferous. The greater 

 part of the continent of North America east of the Mississippi is des- 

 titute of rocks above the Carboniferous. 



In Great Britain and Europe, the series of rocks is more complete 

 than in eastern North America. In Great Britain, the thickness to 

 the top of the Silurian is over 60,000 feet ; to the top of the Car- 

 boniferous, or the Paleozoic, 85,000 feet; then to the close of the 

 series, 100,000. This amount is the sum of the thickest deposits of 

 the several formations, and not the thickness observed in any par- 

 ticular place. On the Continent, there are at least 25,000 feet of 

 strata above the Paleozoic. 



Subdivision of the North American continent into regions of 

 partially independent progress. — It is a remarkable fact, illustrated 

 through all American geological history, that the grand features of 

 the continent were early defined ; and that, through all time, from 

 the close of the Archaean, if not also before, the ranges of land which 

 are now the courses of the mountain chains, were the boundaries be- 

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