Rev. 0. Fisher — On the Formation of Mountains. 249 



below it, will equal the weight of a piece of rock of the same section 

 as the stratum, and 2000 miles long, enough to crumple up and 

 distort any rocks." 



Captain Hutton supposes, after Babbage, that the deposition of a 

 stratum at the bottom of the ocean would raise the temperature of 

 the rocks underlying the original surface by 2° Fahr. for every 100 

 feet laid down upon it, and " that the irresistible pressure caused by 

 the expansion of the rock could only be relieved by the whole 

 stratum bulging upwards, and forming an arch, or, more properly, a 

 dome." And, by calculating the altitude of this arch, he deduces 

 the height of the resulting mountains. 



My present object is to explain why I think this reasoning not 

 conclusive. For the theory is one that would be likely to obtain 

 favour, partly because it appeals to the known denudation of the 

 surface, which has been lately so well estimated by Mr. CroU and 

 others, and partly because it seems to support those uniformitarian 

 doctrines, which are supposed to be of paramount value in geological 

 speculation. I also wish to bring forward some general conclusions 

 which have occurred to me in the course of testing the proposed 

 theory. 



In the first place, then, I think that Captain Hutton is mistaken in 

 his notion of the conditions of equilibrium of the crust. He says, 

 " Each portion of this rigid crust must be maintained in its place by 

 three forces, viz. its weight, the lateral thrust of the arch, and the 

 outward pressure of the superheated interior," where, by the " thrust 

 of the arch," he seems to mean the horizontal pressures at the ends 

 of a section of the crust considered as a spherical rigid shell. Now 

 we have no right to consider the crust rigid ^ when regarded in por- 

 "tions of sufficient dimensions to admit of these lateral pressures being 

 otherwise than sensibly in the same straight line but in opposite di- 

 rections. It is also evident by the merest inspection of any natural 

 section that the rocks cannot be considered by any means rigid. 

 The true statement of the conditions of equilibrium of a portion of 

 the crust considered rigid (for which purpose it must be taken 

 infinitesimally small) is that it is kept in equilibrium by two sets of 

 forces, (1) the two horizontal pressures upon its ends acting in 

 opposite directions and equal in amount, and (2) the vertical forces 

 consisting of the weight of the portion of crust under consideration, 

 and the upward pressure of the rocks beneath, also equal to one another. 

 These two sets of forces must be separately in equilibrium, because 

 they are each set at right angles to the other. Hence it is not true 

 that " if one or more of these forces change in amount, the equili- 

 brium will be subverted ;" for it is evident that the horizontal 

 pressures may be both equally increased, or both equally diminished, 

 without affecting the equilibrium : and this is the case we have 

 to consider: 



Captain Hutton argues, I think truly, that, " if an upheaval of the 



^ It might be thought, on a cursory view, that I have taken this view myself which 

 I now condemn in my paper on the Elevation of Mountains, p. 4, but a little con- 

 sideration wiU show that it is not so. 



