202 DR. STERRY HUNTS SUGGESTIONS. 



Views of Sir John Herschel. It is to Sir John Herschel that we 

 must attribute the enunciation of the origin of volcanic and igneous 

 phenomena in metamorphism of sedimentary deposits. In 1836 he 

 maintained that the temperature of the earth's crust must be raised 

 in consequence of the accumulation of sediments, and insisted that 

 the heat thus developed in the strata would result in the develop- 

 ment in them of a crystalline condition, and that, under the influence 

 of included water, they might become heated to the melting point, so 

 that gases would be given off, and the phenomena of earthquakes and 

 volcanic eruptions result. He further appealed to the transfer of sedi- 

 ments by denudation, as the primary cause which has initiated 

 internal changes which result in elevation and depression of the 

 earth's crust. As late as 1861, we find this distinguished philosopher 

 urging, in his " Physical Geography," that the accumulation of sedi- 

 ments, and their denudation, is capable of producing any amount of 

 pressure on the one hand, and relief from pressure on the other, that 

 the geologist can possibly require, without calling in the aid of 

 unknown causes. 1 These views, however, in so far as they concern 

 the recognition of pressure and water as agents in volcanic action, 

 were anticipated by Poulett Scrope in 1825 ; 2 and in so far as concerns 

 the nature of the products of metamorphism of sedimentary rocks, 

 identical views were published by Keferstein in 1834. But these 

 hypothetical ideas produced little or no impression, and were pro- 

 pounded anew by Sterry Hunt. 3 



Views of Sterry Hunt. Hunt's views are similar to our own, 

 which were suggested by other evidence. They are well stated in the 

 following paragraph : 



" Two things become apparent from a study of the chemical nature 

 of eruptive rocks ; first, that their composition presents such varia- 

 tions as are irreconcilable with the simple origin generally assigned to 

 them; and, second, that it is similar to that of sedimentary rocks 

 whose history and origin it is, in most cases, not difficult to trace. I 

 have elsewhere pointed out how the natural operation of mechanical 

 and chemical agencies tends to produce among sediments a separation 

 into two classes, corresponding to the two great divisions above 

 noticed. From the mode of their accumulation, however, great varia- 

 tions must exist in the composition of the sediments, corresponding 

 to many of the varieties presented by eruptive rocks. The careful 

 study of stratified rocks of aqueous origin discloses, in addition to these, 

 the existence of deposits of basic silicates of peculiar types. Some 

 of these are in great part magnesian, others consist of compounds like 

 anorthite and labradorite, highly aluminous basic silicates, in which 

 lime and soda enter, to the almost complete exclusion of magnesia and 

 other bases ; while in the masses of pinite or agalmatolite rock we 

 have a similar aluminous silicate, in which lime and magnesia are want- 

 ing, and potash is the predominant alkali. In such sediments as 



1 L. c., p. 117. 



2 See "Considerations on Volcanoes," by G. Poulett Scrope. 1825. 



3 "Chemical and Geological Essays," 1^75, pp. 8, 15, 44, 66. 



