374 NATURAL SCIENCE. Nov., 



out of thick sediments. To go no further than our own island, Sir 

 Archibald Geikie has clearly shown that the range of mountains in the 

 Highlands of Scotland, the remains of which the Geological Survey has 

 been investigating for many years, were built up of sediments many 

 thousands of feet thick. It is the same, so far as is known, with all 

 the existing great mountain ranges of the world, such as the Alps and 

 Appalachians, the sediments of which they are composed being in 

 many cases estimated at from 8 to 10 miles thick ; (3) the accumula- 

 tion of sediment, together with a consequent sinking of the sea-bottom, 

 leads to a rise of the isogeotherms which first affect the strata of the 

 crust on which the sediments repose, and next the sediments 

 themselves. 



These are the three leading principles in which we agree, but 

 there are also minor points in which our ideas run parallel or partially 

 so, but it will hardly be necessary to state them here. 



But while our main principles so far run abreast, I attribute the 

 elevation and folding of the chains in their initial stages, not to the 

 contraction of the earth, but to the internal expansion caused by this 

 heating of the sediments and the crust of the earth upon which they 

 repose. These effects are intensified and continued until the building 

 of the range is complete by other related and cumulative causes. It 

 thus becomes a case in my view of direct cause and effect. Professor 

 Le Conte apparently thinks that this expansion will have no effect at 

 all in the way of elevation or folding, relying upon a criticism of 

 Davison, which I shall presently deal with in the form stated in 

 Le Conte's own words. The whole cause of both the folding and 

 elevation is assumed to be in Le Conte's Formal Theory lateral pres- 

 sure, and in his Physical Theory this lateral pressure is assumed to be 

 due to the contraction of the earth. The hydrothermal softening of 

 the earth's crust under the line of thickest sediments, in his view, 

 determines the concentration of this lateral pressure in the localities 

 in which it takes place. 



Granting all the postulates involved, we here undoubtedly have 

 a machinery which looks effective for mountain-building ; but it is 

 one thing to make a formal statement which is sufficient, and another 

 to make -its assumptions square with physical facts. 



The theory was undoubtedly in better case when geologists were 

 content to look upon the earth as in two simple conditions, namely, 

 possessing an unshrinkable crust of unknown thickness — that is, of any 

 thickness required for theoretical purposes — entirely in compression, 

 and a nucleus whose function is to shrink. Such theories, even if 

 afterwards proved to be wrong, are of value in focussing thought ; 

 they are often stepping-stones to the truth. When, however, we 

 come to apply quantitative analysis, with the help of the physical 

 knowledge of the day, to the Contraction Theory the problem assumes 

 a different complexion. We find that the shrinking nucleus is but a 

 shrinking shell, in thickness some one-twentieth part of the earth's 





