3i6 



Sierra Club Bulletin 



tion of the manner in which a broad deep glacier clasps and de- 

 nudes a dome. When we narrowly inspect it, and trace the 

 striae, we perceive that it has been eroded at once in front, back 

 and sides, and none of the fragments thus removed are to be 

 found around its base. Here I would direct special attention 

 to the fact that it is on the upper side of this rock at A, just 

 where the pressure was greatest, that the erosion has been least, 

 because there the layers were pressed against one another, in- 

 stead of away from one another, as on the sides and back, and 

 could not, therefore, be so easily broken up. 



QUANTITY OF GLACIAL DENUDATION 



These simple observations wx have been making plainly in- 

 dicate that the Sierra, from summit to base, was covered by a 

 sheet of crawling ice, as it is now covered by the atmosphere. 

 Its crushing currents slid over the highest domes, as well as 

 along the deepest canons, wearing, breaking, and degrading 

 every portion of the surface, however resisting. The question, 

 therefore, arises. What is the quantity of this degradation? 

 As far as its limit is concerned it is clear that, inasmuch as gla- 

 ciers can not move without in some way and at some rate low- 

 ering the surfaces they are in contact with, a mountain range 

 may be denuded until the declivity becomes so slight that the 

 glaciers come to rest, or are melted, as was the case with those 

 concerned in the degradation of the Sierra. However slow 

 the rate of wear, given a sufficient length of time, and any 

 thickness of rock, whether a foot or hundreds of thousands 

 of feet, will be removed. No student pretends to give an arith- 

 metical expression to the glacial epoch, though it is universally 

 admitted that it extended through thousands or millions of 

 years. Nevertheless, geologists are found who can neither 

 give Nature time enough for her larger operations, or for the 

 erosion of a mere cafion furrow, without resorting to sensa- 

 tional cataclysms for an explanation of the phenomena. 



If the Sierra were built of one kind of rock, homogene- 

 ous in structure throughout its sections, then perhaps we 

 would be unable to produce any plain evidence relative to 

 the amount of denudation effected ; but, fortunately for 

 the geologist, this is not the case. The summits of the 



