THEORIES OF ELEVATION AND DEPRESSION. 139 



volcanic action, Babbage explains as follows : Large accumulations 

 of sediments, sncli as occur generally on coasts, would cause a rise 

 toward the surface of all the subjacent isogeotherms. This increase of 

 temperature of the crust would cause a vertical expansion or swelling 

 of the crust at that point, and a consequent rise above the sea-level. 



The great objections to this theory, as applied to these latter cases, 

 are : 1. The elevation of sea-bottom from this cause would not affect 

 the contiguous land; and, 2. That the places where the greatest 

 quantities of sediments are depositing (as, for instance, the deltas of 

 great rivers) are places of subsidence, instead of elevation. 



Herschel's Theory.* — Sir John Herschel assumes, as a general law 

 — what has been proved in a great number of instances — that areas 

 of great accumulation of sediment are areas of subsidence. He agrees 

 with Babbage, that accumulation of sediments must cause an upward 

 movement of the isogeotherms, but he differs from Babbage in believ- 

 ing that this invasion of sediments by the interior heat would produce 

 subsidence instead of elevation. For, according to Herschel, the inva- 

 sion of sediments by the interior heat would produce chemical changes, 

 and sometimes even aqueo-igneous fusion. These chemical changes, 

 whatever other effects they produce, would certainly change loose sedi- 

 ments into denser crystalline rocks (metamorphism), thus producing 

 contraction instead of expansion. The accumulating sediment mean- 

 while would subside, by the pressure of its own weight, on the liquid 

 or semi-liquid thus formed. 



Recent View. — Again : On the view that there exists a sub-crust 

 layer of liquid matter (page 87), not only would loading with sedi- 

 ment cause subsidence of marginal sea-bottoms, but also lightening by 

 erosion would produce elevation of land-surfaces. 



General Theory. — The theory of Babbage accounts with great prob- 

 ability for the rise of ground in the vicinity of volcanoes, and Herschel's 

 theory accounts, perhaps, for the subsidence of deltas and other places 

 where great accumulation of sediments occurs ; and this latter theory 

 has the additional advantage of accounting for metamorphism, and 

 perhaps, also, for volcanic phenomena. But it is evident that some 

 other and more general theory is necessary to account for those great 

 inequalities of the earth's crust which form land and sea-bottom. For 

 example : although loading with sediment may cause sea-bottoms to 

 sink ; and lightening by erosion may cause land- surfaces to rise, yet 

 this does not at all explain how sea-bottoms and land- surfaces came to 

 be such. These great inequalities must be originated by some other 

 cause ; loading and lightening only tend to maintain them. The for- 



* Herschel, Proceedings of the Geological Society, vol. ii. p. 548 ; and Babbage. ibid., 

 p. 72. 



