340 Scientific Intelligence. 



tory, and diminished as the time increased. It does not neces- 

 sarily follow that the early mountain ranges were the loftiest and 

 most massive, but probably they were ; and very possibly also, 

 the displacement, by crushing and folding, of two neighboring- 

 portions of rock was greatest in early times. But, taking into 

 consideration the whole sui'face of the globe, the process of 

 mountain-making diminishes with the increase of the time, and so 

 also does the rate of continental evolution. 



"It cannot be said that the contraction theory on the hypothesis 

 of solidity is entirely free from objections. Two very obvious 

 ones have already been alluded to in the course of this paper, 

 namely (l) The small calculated depth of the unstrained surface, 

 especially in early geological periods; and (2) The small propor- 

 tion of folded rock to stretched rock directly produced by secular 

 cooling. But I do not think that these objections are by any 

 means fatal to the theory. Assuming the earth to be practically 

 solid, and to have been originally at a high temperature through- 

 out, I believe it may be concluded that the peculiar distribution 

 of strain in the earth's crust resulting from its secular cooling has 

 contributed to the permanence of ocean-basins, and has been the 

 main cause of the growth of continents and the formation of moun- 

 tain chains." 



In the course of his discussion the author takes up the argu- 

 ment of the Rev. O. Fisher on the insufficiency of the contraction 

 theory, and gives several reasons why it should be regarded as 

 inconclusive. The subject discussed by Mr. Davison is further 

 considered by Prof. Darwin in a note appended to the paper of 

 the former; he shows that some of the conclusions may be reached 

 somewhat more simply, and furthermore makes some deductions as 

 to the results of distortion and the magnitude of the effects accom- 

 plished. Prof. Darwin calls attention to the fact that the stretch- 

 ing of the earth's crust which is of importance from a geological 

 point of view is the excess of the actual stretching above that due 

 to rise of temperature — this if negative is a contraction and is 

 shown by a crumpling of strata. 



Assuming the time elapsed since consolidation to be 100 million 

 years, the present depth of the stratum of no straiu is two miles, 

 and the depth is proportional to the time since consolidation. 

 For the upper layers of the earth it is found that the integral 

 effect is always a stretching, and this is negative ; that is, it is a 

 crumpling, as was to be expected. As to the amount of the 

 crumpling, it is found that in ten million years 228,000 square 

 miles of rock will be crumpled up and piled on the top of subja- 

 cent rocks. Prof. Dai-win concludes : 



1. " The numerical data with which we have to deal are all of 

 them subject to wide limits of uncertainty, but the result just 

 found, although rather small in amount, is such as to appear of 

 the same order of magnitude as the crumpling observed geologi- 

 cally. 



" The stretching and probable fracture of the strata at some 



