1885.] 



Regional Metamorphism . 



431 



Of the enormous tangential pressure exercised in the elevation of 

 these chains, some idea may be formed when we consider the amount 

 of compression which those portions of the crust have undergone. 

 Thus, for example, as I mentioned in the paper before referred to, 

 Heim estimates that in the Alps the compression has been to the 

 extent of 72 miles ; and in a recent paper by Professor Claypole, 

 he arrives at the conclusion, after a careful investigation of the 

 magnitude and width of each fold, that in the Appalachian moun- 

 tains " a tract of the earth's surface measuring originally 153 miles 

 from south-east to north-west has been so crusted and compressed 

 that its present breadth along the line of section is only 65 miles, ' r 

 and of this, in one part, — the Cumberland Valley, — " 95 miles of 

 country have been compressed into 16 miles." 



These vast compressions could not have taken place without the 

 transformation into heat of a large portion of the mechanical work, 

 though the degree and centralisation of the heat would depend on the 

 rapidity and completeness with which the compression and deforma- 

 tion had been effected. Need we therefore be surprised to find 

 that, in some of the newer mountain-ranges, a small residual portion 

 of the heat thus mechanically evolved still existing and causing slight 

 aberrations in the position of the underground isothermal lines : the 

 same cause may possibly account for other exceptional cases. 



The only sufficiently complete set of observations on a mountain- 

 chain of this character that have yet been made are those I have 

 before alluded to, by Dr. StapfP in the St. Gothard Tunnel. Parti- 

 culars of these observations will be found in my paper on "Under- 

 ground Temperatures," and I need therefore here only mention that 

 they show at the north end of the tunnel in the part where an axis of 

 elevation of late geological age (Pliocene) traverses the range, that 

 the thermic gradient, which normally equals about 57 feet for 1° F., is 

 there not more than 38 feet ; and for this Dr. Staph states that there 

 was no obvious explanation. 



In further support of this view, I would refer to the exceptional 

 frequency of thermal springs in mountain chains. Some of these are 

 no doubt due to the presence of eruptive rocks, but in many cases there 

 are none of these rocks in the neighbourhood, and yet hot springs are 

 common. Others may, of course, be due to the normal temperature 

 of the depth from which the water rises, but their numbers and their 

 position often militate against this view. In the Alps they are not 

 infrequent, and sometimes occur at very high levels. In the Pyrenees 

 the number of thermal springs exceeds 150, while the Professors 

 Rogers ascertained that there were 56 such springs in the Appalachian 

 chain of mountains. Seven of these are on lines of fault or inversion ; 

 the others issue on lines of anticlinal axes, or at points near to them. 



If I have in these few remarks shown cause for believing that we 



