200 Igneous Bocks and Volcanos, 



so abundantly evolved by certain volcanos. The reaction between 

 silica and carbonates must give rise to carbonic acid, and the 

 decomposition of sea-salt in saliferous strata by silica in the pre- 

 sence of water, will generate hydrochloric acid, while gypsum in 

 the same way will evolve its sulphur in the form of sulphurous 

 acid mixed with oxygen. The presence of fossil plants in the 

 melting strata would generate carburretted hydrogen gases, whose 

 reducing action would convert the sulphurous acid into sulphuretted 

 hydrogen ; or the reducing agency of the carbonaceous matters 

 might give rise to sulphuret of calcium which would be in its turn 

 decomposed by carbonic acid or otherwise. The intervention of 

 carbonaceous matters in volcanic phenomenon is indicated by the 

 recent investigations of Deville, who has found carburetted 

 hydrogen in the gaseous emanations of Etna and the lagoons 

 of Tuscany. The ammonia and the nitrogen of the volcanos are 

 also in many cases probably derived from organic matters in the 

 strata decomposed by subterranean heat. The carburetted hy- 

 drogen and bitumen evolved from mud volcanos, like those of the 

 Crimea and of Bakou, and the carbonized remains of plants in 

 the moya of Quito, and in the volcanic matters of the Island of 

 Ascension, not less than the infusiorial remains fround by Ehren- 

 berg in the ejected matters of most volcanos, all go to show that 

 fossiliferous sediments are very generally implicated in volcanic 

 phenomena; It is to Sir John F. W. Herschel that we owe, so far 

 as I am aware, the first suggestions of the theory of volcanic action 

 which I have here brought forward. In a letter to Sir Charles 

 Lyell, dated February 20, 1836, (Proceedings Geol. Soc. London, 

 vol. 11, p. 448), he maintains that with the accumulation of sedi- 

 ment the isothermal lines in the earth's crust must rise, so that 

 strata buried deep enough will be crystallized and metamorphosed, 

 and eventually be raised, with their included water, to the melting 

 point. This will give rise to evolutions of gases and vapours, 

 earthquakes, volcanic explosions, etc. all of which results must, 

 according to known laws, follow from the fact of a high central 

 temperature ; while from the mechanical subversion of the equili- 

 brium of pressure, following upon the transfer of sediments, while 

 the yielding surface reposes upon a mass of matter partly liquid and 

 partly solid, we may explain the phenomena of elevation and sub- 

 sidence. Such is a summary of the views put forward more than 

 twenty years since by this eminent philosopher, which, although 

 they have passed almost unnoticed by geologists, seem to me to 



