II.] ON SOME POINTS IN CHEMICAL GEOLOGY. 15 



schel* subsequently showed that, as a result of the internal 

 heat thus retained by accumulated strata, sediments deeply 

 enough buried will become crystallized, and ultimately be raised, 

 with their included water, to the melting-point. From the 

 chemical reactions at this elevated temperature gases and vapors 

 will be evolved, and earthquakes and volcanic eruptions will 

 result. At the same time the disturbance of the equilibrium 

 of pressure consequent upon the transfer of sediments, while 

 the yielding surface reposes upon a mass of matter partly liquid 

 and partly solid, will enable us to explain the phenomena of 

 elevation and subsidence. 



According, then, to Sir John Herschel's view, all volcanic 

 phenomena have their source in sedimentary deposits ; and this 

 ingenious hypothesis, which is a necessary consequence of a 

 high central temperature, explains in a most satisfactory man- 

 ner the dynamical phenomena of volcanoes, and many other 

 obscure points in their history, as, for instance, the indepen- 

 dent action of adjacent volcanic vents, and the varying nature 

 of their ejected products. t Not only are the lavas of different 

 volcanoes very unlike, but those of the same crater vary at dif- 

 ferent times ; the same is true of the gaseous matters, hydro- 

 chloric, hydrosulphuric, and carbonic acids. As the ascending 

 heat penetrates saliferous strata, we shall have hydrochloric 

 acid, from the decomposition of sea-salt by silica in the presence 

 of water ; while gypsum and other sulphates, by a similar re- 

 action, would lose their sulphur in the form of sulphurous acid 

 and oxygen. The intervention of organic matters, either by 

 direct contact or by giving rise to reducing gases, would con- 

 vert the sulphates into sulphurets, which would yield sulphu- 

 retted hydrogen when decomposed by water and silica or by car- 

 bonic acid ; the latter being the result of the action of silica 

 upon earthy carbonates. We conceive the ammonia so often 

 found among the products of volcanoes to be evolved from the 

 heated strata, where it exists in part as ready-formed ammonia' 

 (which is absorbed from air and water, and pertinaciously re- 



* On the Temple of Serapis, Proc. Geol. Soc., Vol. II. pp. 548, 596. 

 *h For a further development of this theory, see Essays VI. and VII. 



