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Sierra Club Bulletin 



unmistakably marked as these long-abandoned highways of the 

 dead glaciers. 



If, from some outlook still more comprehensive, the attentive 

 observer contemplates the wide flank of the Sierra, furrowed 

 with canons, dimpled with lake basins, and waved with ridges 

 and domes, he will quickly perceive that its present architectural 

 surface is not the one upon which the first snows of the glacial 

 winter fell, because, with the simple exceptions of the jagged 

 summit-peaks from whose neve fountains the glaciers de- 

 scended, there exists over all the broad flank of the range not 

 one weak rock form. All that remain to roughen and undulate 

 the surface are strong domes, or ridge-waves, or crests, with 

 pavement-like levels or solid-walled canons between. All the 

 rest have been broken up and swept away by the glaciers. Some 

 apparent exceptions to this general truth will present them- 

 selves, but these will gradually disappear in the light of patient 

 investigation. The observer will learn that near the summit 

 ice-fountains there are absolutely no exceptions, even in ap- 

 pearance, and that it is only when he follows down in the paths 

 of the glaciers, and thus comes among rocks which were longer 

 left bare by them in their gradual recession, that he begins to 

 find instances of rocks at once weak in structure and strong in 

 form. 



The regular transition from strong to weak rocks will in- 

 dicate that the greater weakness of those farther removed from 

 the summits, is due to some force or forces which acted upon 

 them subsequently to the time they were sustaining the wear 

 and tear of the glaciers. The causes of this after-weakness are 

 various. First we may note the most apparent — ^the slow de- 

 composition of the mass of the rock by the atmosphere, under 

 favorable conditions of heat and moisture. Some varieties of 

 granite crumbled rapidly by the decomposition of their feldspar 

 throughout the mass. Rocks traversed by feldspathic veins, 

 that are otherwise strong, fall apart on the decomposition of the 

 veins, into a heap of loose blocks. Frost also, combined with 

 moisture, produces a wasted, shattered appearance. But by far 

 the most general and influential cause of the feeble condition 

 of old rocks, which formerly withstood the terrible ordeal of 

 glacial action, is the subsequent development of one or several 

 of their cleavage planes. For example, here is (Fig. 13) a 



