22 STUDIES, SCIENTIFIC AND SOCIAL chap, i 



The granite of the central and highest parts of the 

 Sierra Nevada is flanked near the Yosemite with Silurian 

 slates, lower by some triassic or Jurassic beds followed by 

 enormous deposits of late tertiary gravels, which have 

 been largely preserved from denudation by extensive 

 flows of lava, the remnants of which form the numerous 

 table-mountains so characteristic of the lower slopes of 

 the Sierra. As granite is a metamorphic or igneous rock 

 it can only have been formed deep down in the crust of 

 the earth, where it was exposed to the action of great 

 subterranean heat. It is therefore certain that, when 

 first elevated to fonn the mass of the Sierra Nevada, it 

 was everywhere deeply buried under Silurian and other 

 palaeozoic rocks, and not improbably under a further deposit 

 of mesozoic age. These various beds, of an unknown thick- 

 ness, must all have been denuded away before the granitic 

 core was exposed, and during that process the main lines 

 of the valleys must have been fixed, and the streams might 

 have begun to cut their way into the granite substratum. 



Although granite appears to be, and sometimes is, a 

 very durable rock, it varies greatly in its power of resist- 

 ing denudation, owing perhaps, in part, to the nature and 

 thickness of the overlying rocks, beneath or among which 

 it was forced up, and which in some cases determined the 

 characteristic forms it assumes when exposed to atmo- 

 spheric agencies. These forms are either rude cubical 

 masses, as seen on some of our Dartmoor tors ; peaks and 

 pinnacles, as in some of the Alps of Dauphine and in the 

 Cathedral spires of the Yosemite ; but, more commonly, 

 rounded forms culminating in cones or almost perfect 

 domes or hemispheres, as in the great domes of the 

 Yosemite. (Fig. 7.) It is an interesting fact that all 

 these forms occur also in the granite region of the Upper 

 Kio Negro in Brazil Thj Cocoi Mountain forming the 

 boundary between Brazil and Venezuela is a quadrangular 

 or cubical mass of granite, about a thousand feet high, 

 rising abruptly out of a great undulating plateau of the 

 same rock. Others in the same region are conical or 

 dome-shaped; and on the southern bank of the river 

 Uaupes, about sixty miles from its mouth, is anisolated 



