414 T. 31. Reach — Compressive Stress in Rocks, etc. 



Taking then for granted the prevalence of these vertical 

 movements in recent geologic times it is obvious that the sub- 

 sidence of a low arch of elevation must tend to put the sur- 

 face rocks into lateral compression. This will be largely 

 governed no doubt by the existence or non-existence of faults 

 and joints, and it is readily seen that when the strata are com- 

 paratively unbroken and continuous as in Ohio the most favor- 

 able conditions prevail. Doubtless the surface rocks adjust 

 themselves to such movements by these ant^clinals where they 

 exist and minor subsidences may locally occur. In the Iro- 

 quois Beach the rising grade appears to be in a northeasterly 

 direction and to vary from 1*60 feet to 6 feet per mile.* 



Were this arch to subside to the horizontal it would be quite 

 sufficient to develop considerable lateral pressure in the surface 

 rocks. So long as the limit of elasticity of the rocks is not 

 exceeded it is quite conceivable that the energy may have been 

 stored up for a great length of time only wanting favorable 

 conditions for its release. 



It would take too long to discuss in one paper the cause of 

 these differential vertical movements, but I would refer those 

 interested to Chap. XXII of the Origin of Mountain Ranges 

 where the larger bendings of the earth's crust are treated of. 

 To call in the usual explanation of all lateral pressure phe- 

 nomena, viz : tangential thrust arising from the shrinking of 

 the earth's nucleus, is to call in an agency which were it the 

 true one would have a more universal effect. 



The fact that these conditions of active lateral pressure in 

 surface rocks are recorded as unusual raises a strong presump- 

 tion that tangential thrust is not the potent agent in geologic 

 change that some maintain, for were it so most of the surface 

 crust would on artificial penetration exhibit powerful signs of 

 pent up energy, for it was clearly proved by me in 1886 and 

 shortly afterwards independently by Mr. Davison that in a 

 cooling solid globe the greatest compression takes place on the 

 surface and the same reasoning applies as I have shown in the 

 same work to a globe with a hard crust of the requisite thick- 

 ness even if the nucleus be molten. 



Park Corners, Liverpool. England, 

 Jan. 2. 1891.' 



* The Deformation of the Iroquois Beach, by Dr. J. W. Spencer. This Journal, 

 Dec. 1890, p. 447. 



