VII.] ON SOME POINTS IN DYNAMICAL GEOLOGY. 77 



earth's surface. Starting from the notion of an igneous centre, 

 Babbage and Herschel adopted the first view, in which I have 

 followed them, maintaining that the heat from a yet uncooled 

 nucleus is the efficient cause of all igneous and volcanic mani- 

 festations. According to Keferstein, on the other hand, the 

 hypothesis of an incandescent nucleus is unnecessary, and the 

 internal heat results from what he called a fermentation among 

 the deeply buried sedimentary layers. A view which unites 

 these two is proposed by LeConte, who suggests that heat 

 from a central source invading the buried sediments may there 

 excite chemical action, which will, in its turn, evolve heat, 

 and thus greatly augment their temperature. It is, however, 

 I think, probable that any chemical processes which may be 

 set up in the buried sediments for their conversion into igne- 

 ous rocks and volcanic products would absorb rather than gen- 

 erate heat. 



In his remarkable study of the Secular Cooling of the 

 Earth (Trans. Eoyal Soc. Edinb., XXIII. pt. 1, p. 157), Sir 

 William Thomson arrives at the conclusion that the observed 

 mean rate of increase in descending in the earth's crust will 

 continue with but little variation for 100,000 feet, but will 

 gradually diminish at greater depths, from an increase of con- 

 ducting power. Estimating with him the rate of increase at 

 one degree of Fahrenheit for fifty-one feet, it would require 

 the depth just named, or about nineteen miles, to give a tem- 

 perature of 2,000 F., which may be supposed sufficient to pro- 

 duce the chemical actions required. But it is probable that 

 the seat of volcanic activity may be much less profound than 

 above supposed, in which case the central heat would be in- 

 adequate. Chemical action, as suggested by Keferstein and 

 LeConte, being rejected as a source of heat, there however 

 remained the hypothesis that thermal effects might result from 

 local physical causes, and that the immense mechanical force 

 which is exerted in the movements of the earth's crust might be 

 converted into heat. This view was, so far as I am aware, first 

 suggested by George L. Vose, whose review of Orographic Geol- 

 ogy, a very valuable contribution to the literature of the sub- 



