Mr. Mallet's Theory of Volcanic Energy. 141 



work of descent of a hammer upon a piece of red-hot iron is 

 transformed into heat within the iron, although the iron be 

 hotter than the hammer becomes through the work upon it ; 

 and in principle the cases do not differ. 



There is in Mr. Mallet's reply one argument which, if based 

 upon true assumptions, would be of real importance. It is as 

 follows : — " There is nothing to warrant the supposition that a 

 crust 400 miles thick, which is the value our author [taking 

 it from Mr. Mallet] assumes for k, would be compressed 

 equally throughout its depth or crush simultaneously through- 

 out its thickness ; nor can it be assumed that volcanic acti- 

 vity is found uniformly diffused throughout the depth of such 

 a crust, but must be supposed (as I have shown in my original 

 paper, § 87) to be confined principally to the upper strata 

 of the crust, where, as may easily be seen, in an elastic and 

 flexible crust local lateral displacements may take place suffi- 

 cient to produce crushing and volcanic action without any 

 dragging of the crust as a whole over the nucleus." 



Taking the latter supposition first ; if the lateral displace- 

 ments be confined to the upper strata, this merely makes the 

 nucleus (for this purpose) larger. And my formula may be 

 applied as for a solid earth, introducing such a value of k as 

 will correspond to the depth to which, at any place, the move- 

 ments are supposed to extend. But it is manifest that no 

 greater temperature can be obtained under these circum- 

 stances. The case in which a high temperature would really 

 be induced would be that wherein the whole pressure was 

 sustained upon a small area only of a vertical section, so that, 

 when the rocks gave way, the crushing should be confined to 

 that area. But in a body like the earth, which has gone on 

 cooling for so long, and whose crust has been so long sub- 

 ject to the compressions required by the theory, it is incon- 

 ceivable that there should be vast, open, vertical chasms, with 

 faces in contact only here and there, or even alternations of 

 hard and soft strata, under such conditions as to have the 

 supposed effect in localizing the work. It is, however, only on 

 the supposition of some such arrangements of the rocks as 

 these that there can be any analogy between the localization 

 of heat in the crust and the instances adduced by Mr. Mallet 

 in a former paper, such as those of the ordinary flint and steel, 

 or the cutting of a file by means of a rotating disk of soft iron. 



As a parting compliment to my paper, Mr. Mallet tells his 

 readers that my concluding sentences contain a "'string of im- 

 probable suppositions not containing any thing new." Mr. 

 Scrope, who, ripe in years and honours, has so lately been 

 removed from us, took a somewhat different view. To set up 



