HvrroN.—On the Formation of Mountains. 285 
“rigid” spherical shell that he is examining is kept in equilibrium by its 
attraction towards the centre (ùe, its weight), and by the pressures tangential 
to great circles round the cireumference of the shell (2.е., the lateral thrust of 
the arch or dome), but he calculates the amount of the latter, and shows that 
itis independent 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 rigid crust is partly supported by the lateral 
thrust of the arch. 
(b) 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 abnormal elevation of a portion of such crust must be owing 
to lateral pressure," because it might be owing to an increased upward 
pressure 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 Jower beds that are undergoing compression, 
while the upper are поё; 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 
theory, and they virtually 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 occurred so long ago that the former more rapid conduction of heat 
outwards appreciably affected the result. 
