THE CENTURY'S PROGRESS IN GEOLOGY 



spread formations was practically to abandon the Nep- 

 tunian hypothesis. So gradually the Huttonian expla- 

 nation of the origin of granites and other " igneous " rocks, 

 whether massed or in veins, came to be accepted. Most 

 geologists then came to think of the earth as a molten 

 mass, on which the crust rests as a mere film. Some, 

 indeed, with Lyell, preferred to believe that the molten 

 areas exist only as lakes in a solid crust, heated to 

 melting, perhaps, by electrical or chemical action, as 

 Davy suggested. More recently a popular theory at- 

 tempts to reconcile geological facts with the claim of the 

 physicists, that the earth's entire mass is at least as 

 rigid as steel, by supposing that a molten film rests be- 

 tween the observed solid crust and the alleged solid 

 nucleus. But be that as it may, the theory that subter- 

 ranean heat has been instrumental in determining the 

 condition of " primary " rocks, and in producing many 

 other phenomena of the earth's crust, has never been in 

 dispute since the long controversy between the Neptu- 

 nists and the Plutonists led to its establishment. 



ii 



If molten matter exists beneath the crust of the earth, 

 it must contract an cooling, and in so doing it must dis- 

 turb the level of the portion of the crust already solidi- 

 fied. So a plausible explanation of the upheaval of 

 continents and mountains was supplied by the Plutonian 

 theory, as Hutton had from the first alleged. But 

 now an important difference of opinion arose as to the 

 exact rationale of such upheavals. Hutton himself, and 

 practically every one else who accepted his theory, had 

 supposed that there are long periods of relative repose, 



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