250 Rev. 0. Fisher — On the Formation of Mountains. 



crust should take place at any point, the underlying superheated 

 rocks, being thus relieved from the pressure above them, would 

 expand and rise up and fill the hollow." Let us suppose, as the 

 extreme case, that a limited portion of the crust, instead of being 

 upheaved, was entirely removed. Then, not considering differences 

 of density, nor ebullition, the underlying rocks would rise up, and 

 form a sensibly level surface with that around the removed portion. 

 They could not rise higher than this. Suppose now, for the sake of 

 argument, that the removed portion of the crust were to be replaced. 

 It would sink back again into its original position. And the same 

 thing would equally happen if it were broken up before being 

 replaced. It seems then that the upward pressure of the subjacent 

 rock has no power to support any portion of a crust of normal 

 density at a level higher than that of the general surface ; and, conse- 

 quently, that any abnormal elevation of a portion of such crust must 

 be owing to lateral pressure. The question at issue is, what is the 

 cause of that pressure, and what amount and form of elevation would 

 ensue from, it ? Captain Hutton's hypothesis, properly applied, would 

 produce results nearly analogous to that investigated by myself in 

 1866, and by Mr. Mallet in 1872, with this obvious difference — that 

 on Captain Hutton's supposition the crust would expand away from 

 the nucleus ; whereas in ours the nucleus contracts away from the 

 crust. Other points of divergence will be pointed out in the sequel. 



The author of the lecture assumes rocks to expand on an average 

 0-000005 of their (linear) dimensions for 1° F., and on that supposi- 

 tion gives a table of the altitudes, which, according to his theory, 

 would be attained by the arch caused by the expansion of an area of 

 given width, when exposed to a given rise of temperature. 



I have not been able to obtain the same numbers ; but mine do not 

 differ materially from his. The elevation of the highest point of a 

 circular arch, constructed according to Captain Hutton's supposition, 

 comes out with me equal to the following expression — where a is 

 the earth's radius, and e the expansion of rock for the increase of 

 temperature under consideration : 



- (1 + -) + 3- (a„) 2. o 

 whence I obtain the following table : 



Thickness 



600 feet. 



2500 feet. 



10,000 feet. 



25,000 feet. 





Temperature .., 



10° 



50° 



200° 



500° 



Breadth 100 miles 



„ 500 „ 



» 1000 „ 



„ 2000 , 



1567 

 1569 

 1575 

 1600 



7835 

 7845 

 7875 

 8000 



31,340 

 31,380 

 31,500 

 32,000 



78,360 

 78,460 

 78,750 

 80,010 



(N.B. — This table is not to be taken as representing in the opinion of the author 

 of this paper what occurs in nature.) 



* This supposes the arc considerably larger than ex.radius, as will be the case in 

 every instance in the table. 



