HuTTON. — On the Formation of Mountains. 285 



" rigid" spherical shell that he is examining is kept in equilibrium by its 

 attraction towards the centre {i.e., its weiglit), and by the pressures tangential 

 to great circles round the circumference of the shell {i.e., the lateral thrust of 

 the arch or dome), but he calculates the amount of the latter, and shows that 

 it is indejDendent of the size of the shell, except so far as the size alters the 

 weight / and I really fail to see the difference between this and stating, as 

 I did, that each portion of the I'igid crust is partly supported by the lateral 

 thrust of the arch, 



(&.) Mr. Fisher explains very clearly that the interior could not rise higher 

 than the surface by its own pressure, but it does not necessarily follow from 

 this that " any abnoi-mal elevation of a portion of such crust must be owing 

 to lateral pressure," because it might be owing to an increased upward 

 pi'essure caused by the sinking of some adjoining area. This shows that the 

 anticlinals could seldom attain the full amount of elevation shown in my 

 table, for the abutments must sink ; but the table shows an ample margin for 

 such depressions. 



(c.) Mr. Fisher says that the rocks would crush, and not rise up in anti- 

 clinals. But in order to crush there must be some space to crush into, and, 

 by the deposition theory, it is the lower beds that are undergoing compression, 

 while the upper are not ; and, in order to relieve the compression, the upper 

 beds must be forced up, either by fractures being formed and certain parts 

 only raised, or else altogether, into one or more dome-shaped elevations. The 

 last requires much the least work, and is therefore the way in which the 

 pressure would be relieved. On the other hand, by the contraction theory, 

 the upper beds are subject to the greatest compression, and, having no weight 

 upon them, they would undoubtedly crush. 



{d.) Mr. Fisher says that the specific gravity of the disturbed rocks 

 ought to be less than it was before. This would be the case with the rocks 

 causing the movement only while they were heated, and even then the 

 difference would be too small to detect. When the rocks cooled they would 

 go back to their original length by faulting, and the specific gravity would be 

 the same as before. 



These are all the arguments that Mr. Fisher can find against the deposition 

 theoiy, and they viitually resolve themselves into this question : "When rocks 

 are expanded by heat do they, or do they not, crush up ? The best answer is 

 found by examining the rocks themselves, where we find that rocks which have 

 been deeply buried, and which therefore must have been considerably heated, 

 are not crushed but thrown into anticlinal and synclinal curves ; and that the 

 deeper they have been buried the more they have been folded, except when the 

 burying occuri'ed so long ago that the former more rapid conduction of heat 

 outwards appreciably affected the result. 



