ORIGIN OF MOUNTAIN RANGES. 



567 



and then decreases again 

 more rapidly to zero at 

 the surface. This is shown 

 in diagram, Fig. I. In this 

 figure the curve represents 

 the relative rate of con- 

 traction whether radial or 

 circumferential of the sev- 

 eral layers. We use it, 

 however, only to represent 

 the latter. For in con- 

 sidering the radial con- 

 traction, it is not the 

 relative rate of the sev- 

 eral layers that immedi- 

 ately concerns us, but 

 their rate of radial descent. 

 Now this is a summatioti 

 series and therefore in- 

 creases to the very surface, 

 but at different rates of 

 increase. The law of in- 

 crease of radial descent as 

 we come toward the sur- 

 face is shown in diagram, 

 Fig. 2 1 in which the rate 

 of increase is greatest at 

 seventy miles, just where 

 the curve changes from 

 concavity to convexity. 

 If now we superpose these 

 two diagrams the depth a 

 at which the two curves, 



1 1 have taken these figures 

 from Claypole, but modified this 

 one so as to make it a truer repre- 

 sentation of the law. 



400 -Miles 

 b 



Fig. 1. .? .?=Surface; a £ = depth along radius; a x b= 

 curve of contraction. 



c , a c 



-.s 



400:- MUes 



Fig. 2. c <5=curve of radial descent. 



a c 



400+6 Miles 



Fig. 3. d d= level of no strain. 



