8 THEORY OF IGNEOUS ROCKS AND VOLCANOES. [L 



in the theory of Scrope and Scheerer. If now we admit that 

 all igneous rocks, ancient plutonic masses as well as modern 

 lavas, have their origin in the liquefaction of sedimentary 

 strata, we at once explain the diversities in their composition. 

 We can also understand why the products of volcanoes in dif- 

 ferent regions are so unlike, and why the lavas of the same 

 volcano vary at different periods. We find an explanation of 

 the water and carbonic acid which are such constant accompani- 

 ments of volcanic action, as well as the hydrochloric acid, sul- 

 phuretted hydrogen, and sulphuric acid, which are so abundantly 

 evolved by certain volcanoes. The reaction between silica and 

 carbonates must give rise to carbonic acid, and the decompo- 

 sition of sea-salt in saliferous strata by silica, in the presence of 

 water, will generate hydrochloric acid; while gypsum in tin- 

 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 carburetted hydrogen gases, 

 whose reducing action would convert the sulphurous acid into 

 sulphuretted hydrogen ; or the reducing agency of the carbona- 

 ceous matters might give rise to sulphuret of calcium, which 

 would be, in its turn, decomposed by carbonic acid or other- 

 wise. The intervention of such matters in volcanic phenom- 

 enon is indicated by the recent investigations of Deville, who 

 has found carburetted hydrogen in the gaseous emanations 

 of the region of Etna and the lagoons of Tuscany. The 

 ammonia and the nitrogen of volcanoes are also in many cases 

 probably derived from organic matters in the strata decom- 

 posed by subterranean heat. The carburetted hydrogen and 

 bitumen evolved from mud-volcanoes, 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 infusorial remains found by Ehren- 

 berg in the ejected matters of most volcanoes, 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 



