Elevation and Subsidence of the Surface of the Earth, 411 



on, and the contraction of the rocks from this cause would more 

 than counterbalance the relief of pressure, and subsidence in- 

 stead of elevation would be the result. Where deposition was 

 going on, the isogeothermals would be rising ; but as deposition 

 is more rapid than the outward conduction of heat, the under- 

 lying rocks would not be able to uplift the incumbent mass, 

 which being loose sand and mud would render no assistance, and 

 the area would sink. The underlying rocks, therefore, as well 

 as the lower parts of the new formation, would be undergoing 

 compression from forced subsidence, and at the same time ex- 

 pansion from an increase of heat, and foldings and contortions 

 would be the consequence. 



If, now, we assume, what the facts tend to show, that subsi- 

 dence equals deposition, we can calculate the amount of com- 

 pression the underlying rocks will undergo from subsidence, and 

 the amount of their expansion from the increase of heat due to 

 the thickness of the new deposits, the two together being the 

 amount of squeezing available for contortions. The following 

 Table gives approximately the sum of these two in decimal 

 parts of a mile for areas of 100, 200, and 300 miles breadth. 

 The upper line gives the thickness of the formation in feet, 

 which is supposed to be equal to the depth of the subsidence : — 



Miles. 



1000 

 feet. 



5000 

 feet. 



10,000 



feet. 



15,000 

 feet. 



20,000 

 feet. 



25,000 



feet. 



100 

 200 

 300 



•014 



•028 

 •04 



•095 



•15 



•21 



•23 

 •33 

 •42 



•415 

 •5 



•675 



•7 



•9 



1 

 1-5 



Prom this Table we can deduce the following, which will be 

 sufficiently accurate for practical purposes in testing the theory: — 



Thickness of beds, in feet... 1000 

 Amount of compresaion yoVo 



5000 



10.000 



15,000 



20,000 



3 



25,000 



These results maybe thought to be much too small to account 

 for the great contortions and foldings that we see in mountainous 

 districts; but it must be remembered that these districts have 

 been elevated and depressed many times, as proved by the uncon- 

 formities among the strata, and that the contortions now observed 

 will be the sum total of all these movements. In the section 

 given by Professor A. Ramsay of North Wales, from Snowdon 

 to Aran Mowddwy (Mem. Geol. Surv. of Great Britain, vol. iii.), 

 I find that, after making due allowance for faults and dykes, the 

 compression of the rocks has been ^(. . But even in this section, 

 admirable as it is, the data are not sufficient for comparing the 



