24 STUDIES, SCIENTIFIC AND SOCIAL chap. 



dome-shaped mountain about a thousand feet high, of so 

 regular an outline as to look like a gigantic half-globe. 

 Now, it is evident that these cubical, hemispherical, and 

 conical hills, rising out of a nearly level plateau which 

 extends for several hundred miles around them in every 

 direction, must owe their present position to the slow 

 degradation by atmospheric agency of the vast masses of 

 rock in which they were once buried, but whose destruc- 

 tion they have survived owing to their superior hardness or 

 tenacity. It is true the rocks in Brazil have been subject 

 to tropical rain and heat and to the powerful aid of 

 tropical vegetation ; but, on the other hand, the rocks of 

 the Yosemite have been exposed to the even more power- 

 ful agencies of alternations of intense frost and great sun- 

 heat, as well as of torrents formed by melting snows, and 

 probably of occasional debacles caused by bursting glacier 

 lakes. 



It is well known that granite often weathers very 

 rapidly, sometimes becoming completely decomposed to a 

 depth of twenty or thirty feet, so that it can be dug out 

 with pick and spade. This process of decomposition is 

 greatly facilitated by the action of carbonic acid either in 

 air or water. Now, during the latter part of the tertiary 

 epoch, there was a long period of volcanic action in the 

 Sierra Nevada ; and as both carbonic acid and many other 

 powerful gases are emitted during eruptions, and also 

 permeate the earth and are absorbed by the water, we 

 should have all the conditions for the decomposition and 

 denudation of the granite rocks. The alternations of 

 temperature on the higher parts of the Sierra Nevada are 

 very great. During the long bright Californian summer 

 the action of direct sun-heat on the exposed rocks must 

 be considerable, the air temperature in the Yosemite 

 valley being usually over 80°, while at a height of 8,700 

 feet ice an inch thick is formed at night in June and July. 

 In winter at such elevations — that of the present summit 

 of some of the domes — the temperature must fall below 

 zero of Fahrenheit every night. The alternate expansion 

 and contraction produced by such changes of temperature 

 are among the most powerful agencies in the splitting up 



