PRESIDENT’S ADDRESS— SECTION C. oF 
due to the expansion of heated rock, it is difficult to see how, by 
the theory, they could have subsided again, for this subsidence 
could only take place by a retreat of the isogeotherms, for which 
no cause is assigned. 
Another and last objection is that the existence in the Sat- 
pura Basin in India, of sediments 22,500 feet thick, which have 
never been plicated, proves that a rise in the isogeotherms is not 
the direct cause of contortion, for if it were so, there would be 
some proportion between thickness and amount of contortion. 
This objection is, I think, fatal to the theory. 
Eighteen years ago, having convinced myself that the con- 
traction theory was quite incapable of performing the duties 
ascribed to it, I advocated the gradation (or as I named it, the 
Herschel-Babbage) theory, which I thought would afford a com- 
plete explanation of the phenomena. But since then the survey 
of North America has opened out to us a new geology quite 
unlike that of Europe, and the surveys of Australasia and India 
have supplied us with many important facts. Moreover, during 
the last fifteen years the various theories have been discussed in 
all their bearings by many able geologists, and I now see that 
I was wrong in thinking that the gradation theory offered a 
sufficient explanation. It is evident to me now that this theory, 
although containing some truth, explains minor details only, and 
does not touch the fundamental causes. As has been so well 
stated by Mr. W. J. McGee, in the Geological Magazine for 
November, 1888, it accounts for many of the consequent pro- 
cesses, but not for any of the antecedent processes. It accounts 
neither for regional elevation nor for subsidence; it gives no 
sutiicient explanation of contortions, over-thrusts, and granitic 
cores ; and it supplies no adequate machinery for causing alter- 
nating oscillations of the surface. 
Internal Changes in Temperature.—The contraction and grada- 
tion theories, either separately or together, are evidently incapable 
of explaining the facts. As Professor J. D. Dana has lately 
shown, the deep sea troughs are not the result of superficial 
causes, but of work going on in the interior of the globe, and 
we are driven to look to changes in volume in masses of the 
earth’s interior to explain the movements of the surface. Now, 
changes in volume must be due either to changes in density 
caused by changes in temperature, or to changes in the quantity 
of matter at any particular place, caused by internal movements, 
or, possibly, to a combination of both. 
Hydrothermal metamorphism is sometimes cited as a cause 
of increase of volume as well as of decrease of density in rocks ; 
but there seems to be a fallacy here. A combination of water 
with the minerals forming a rock will no doubt decrease the 
density of that rock, but there will be no great increase of 
volume. The metamorphosed rock will not occupy more space 
