MOUNTAIN ORIGIN AND STRUCTURE. 



257 



Fig. 229.— Ideal Section of a Mountain-Range. 



core is only metamorphic ; and remnants of unchanged strata, caught 

 up and left among the folds of the crests, show that these strata once 



extended over the top, and that 

 the metamorphic axis is exposed 

 only by erosion. Only carry the 

 metamorphism a step further and 

 the erosion a little deeper, and we 

 have the granitic axis complete 

 (Fig. 230). 

 Mountains are made out of Lines of Thick Sediments.— But the 

 question occurs, What determines the place of a mountain-range? 

 The answer is, A mountain-range while in preparation — before it be- 

 came a range — was a line of very thick sediments. This is a very im- 

 portant point in the theory of mountain origin, and therefore must be 

 proved. The strata of all mountains, where it is possible to measure 

 them, are found to be of enormous thickness. The strata involved in 

 the folded structure of the Appalachian, according to Hall, are 40,000 

 feet thick ; the strata exposed in the structure of the Wahsatch, ac- 

 cording to King,* are more than 50,000 feet thick ; the Cretaceous 

 strata of the Coast Range, according to Whitney, f are 20,000 feet thick ; 

 and if we add to this 10,000 feet for the Eocene and Miocene strata, 

 the whole thickness is probably not less than 30,000 feet. The Alpine 

 geologists estimate the thickness of the strata involved in the intricate 

 structure of the Alps as 50,000 feet. The strata of Uintah, according 

 to Powell, are 32,000 feet thick. 



Now, it must not be imagined that these numbers merely represent 

 the general thickness of the stratified crust ; only, that in these places 

 the strata are turned up and their edges exposed by erosion, and thus 

 their thickness revealed. On the contrary, it may be shown that the 

 same strata are much thinner elsewhere. The same strata which along 

 the Appalachian range are 40,000 feet thick, when traced westward thin 

 out to 4,000 feet at the Mississippi River. The same strata which along 

 the line of the Wahsatch are 30,000 feet thick, when traced eastward thin 

 out to 2,000 feet in the region of the Plains.;); It is evident, therefore, 

 that mountain-ranges are lines of exceptionally thick strata. 



* Fortieth Parallel Survey, vol. iii, p. 451. f Whitney, on Mountain-Building. 



\ King, Fortieth Parallel Survey, vol. i, p. 122. 

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