On Some Points in Chemical Geology. 423 



According, then, to Sir J. Herschel's view, all volcanic phenomena 

 have their source in sedimentary deposits ; and this ingenious hypo- 

 thesis, which is a necessary consequence of a high central tem- 

 perature, explains in a most satisfactory manner the dynamical 

 phenomena of volcanoes, and many other obscure points in their 

 history, as for instance, the independent action of adjacent 

 volcanic vents, and the varying nature of their ejected products. 

 Not only are the lavas of different volcanoes very unlike, but those 

 of the same crater vary at different times ; the same is true of the 

 gaseous matters, hydrochloric, 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 reaction, 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 convert 

 the sulphates into sulphurets, which would yield sulphuretted hy- 

 drogen when decomposed by water and silica or carbonic acid the 

 latter being the result of the action of silica upon earthy carbon- 

 ates. 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 retained by argillaceous sediments), and 

 is in part formed by the action of heat upon azotized organic mat- 

 ter present in these strata, as already maintained by Bischof.* 

 Nor can we hesitate to accept this author's theory of the formation 

 of boracic acid from the decomposition of borates by heat and 

 aqueous vapour, y 



The almost constant presence of remains of infusorial animals 

 in volcanic products, as observed by Ehrenberg, is evidence of the 

 interposition of fossiliferous rocks in voleanic phenomena. 



The metamorphism of sediments in situ, their displacement in a 

 pasty condition from igneo-aqueous fusion as plutonic rocks, and 

 their ejection as lavas with attendant gases and vapours are, then, 

 all results of the same cause, and depend upon the differences in 

 the chemical composition of the sediments, the temperature, and the 

 depth to which they are buried : while the unstratified nucleus of the 

 earth, which is doubtless anhydrous, and according to the calcula- 

 tions of Messrs. Hopkins and Hennessey, probably solid to a great 



* Lekrbuck der Geologie, vol. ii, pp. 115-122. 

 | Ibid, vol i, p. 669. 



