24 Capt. F. W. Hutton — On the Formation of Mountains. 



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. 



(c.) 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 por- 

 tion 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. It shows, however, that the anticlinals could 

 seldom attain the full amount of elevation that is shown in my table, 

 as the abutments must sink; but the table shows that there is an 

 ample margin for such depressions. 



[d.) Mr. Fisher's fourth argument is that the specific gravity of 

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

 be the case, with the rocks that caused the movement, only while 

 they were heated, and even then the difference would be too small to 

 detect. When the rocks cooled by denudation, they would go back 

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

 be the same as before, or probably rather more, owing to compression. 

 The granitic rocks, however, underlying the dome, would have their 

 specific gravity lowered, and this accords with the best observations 

 on mountain chains. 



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 them- 

 selves, where we find that extensive areas which have been deeply 

 buried, and which trnist therefore have been considerably heated, are 

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

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

 when the burying was so long ago that the former more rapid con- 

 duction of the interior heat outwards appreciably affected the result. 



Mr. Fisher then attacks my illustration of the theory taken from 

 the Weald. But the Weald was not "adduced to give verisimilitude 

 to this theory," as he supposes; neither did I "pretend" to any 

 precise measurements, as any unprejudiced reader will see ; but it 

 was given as an example of the way in which the theory might be 

 tested in the field. 



I have not access to any precise data as to the thickness of the 

 beds, or the height or breadth of the anticlinal, and exact measure- 

 ments would have been quite useless unless we also knew exactly the 

 rate of expansion. In geological inquiries mathematical inves- 

 tigation can only be used as a check to our speculations, and as giving 

 ns limits beyond which we cannot go. 



The average thickness of the Cretaceous rocks was taken from 

 Jukes's Manual (1862, p. 602), and the height of the hills in the 

 Weald from Lyell's " Elements of Geology." If the true thickness 



not mean that a horizontal section through it would necessarily he circular. Tt may- 

 be an ellipse, of which the major and minor axes may have any ratio, rrom this 

 point of view all anticlinal curves are parts of dome-shaped elevations. 



