242 
PROFESSOR G. H. DARWIN ON THE DISTRIBUTION OF STRAIN 
taking into consideration the whole surface of the globe, the process of mountain- 
making diminishes with the increase of the time, and so also does the rate of continental 
evolution. 
(25) 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 (1) The small calculated depth of the unstrained 
surface, especially in early geological periods; and (2) The small proportion 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 throughout, 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 
mountain chains. 
VIII. Note on Mr. Davison’s Paper on the Straining of the Earth’s Crust in Cooling. 
By G. H. Darwin, LL.D., F.P.S., Fellow of Trinity College, and 
Plumian Professor in the University of Cambridge. 
Received June 15,—Read June 16, 1887. 
Mr. Davison’s interesting paper was, he says, suggested by a letter of mine published 
in ‘ Nature ’ on February 6, 1879. In that letter it is pointed out that the stratum of 
the Earth where the rate of cooling is most rapid lies some miles below the Earth’s 
surface. Commenting on this, I wrote :—■ 
“ The Rev. O. Fisher very justly remarks that the more rapid contraction of the 
internal than the external strata would cause a wrinkling of the surface, although he 
does not admit that this can be the sole cause of geological distortion. The fact that 
the region of maximum rate of cooling is so near to the surface recalls the interesting 
series of experiments recently made by M. Favre (‘Nature,’ vol. 19, p. 108), where 
all the phenomena of geological contortion were reproduced in a layer of clay placed 
on a stretched india-rubber membrane, which was afterwards allowed to contract. 
Does it not seem possible that Mr. Fisher may have under-estimated the contractibility 
of rock in cooling, and that this is the sole cause of geological contortion ? ” 
Mr. Davison works out the suggestion, and gives precision to the general idea con¬ 
tained in the letter. He shows, however, that there is a layer of zero strain in the 
