806 



THE WONDERS OF GEOLOGY. Lect. VIII. 



meaning is often obscure and frequently unintelligible, and 

 many of which admit of a double interpretation. 



The mechanical effects of water as a fluid, everywhere 

 so manifest in the sedimentary formations, are no longer 

 apparent ; but the powerful agency of the same substance, 

 in the several conditions produced by high temperature — 

 as vapour, steam, and gas,* — is universally exhibited in the 

 upheaved mountain-chains, the protruded igneous matter, 

 the rent and dislocated rocks, and the rugged peaks and 

 precipitous glens, which are the characteristic features of 

 the physical geography of primary regions. 



In the phenomena attendant on the earthquakes and 

 volcanoes of modern times, we have proofs of the continued 

 energy of those physical forces which produced the results 

 that will form the principal subjects of this discourse. The 

 nature and effect of volcanic action, to which we have 

 previously, but incidentally, alluded, now, therefore, de- 

 mand our especial consideration, in order that we may 

 comprehend the origin and formation of the metamorphic 

 and plutonic rocks ; or, in other words, of those mineral 

 masses which have acquired a crystalline structure from 

 exposure to the influence of intense heat under great 

 pressure, and have been elevated into their present position 

 by subterraneous movements. I propose, in the first place, 

 to consider the nature and effects of igneous action as 

 exhibited in existing volcanoes ; secondly, to describe the 

 hypogene and plutonic rocks, and the changes produced in 

 contiguous mineral masses by their influence ; and, lastly, 

 to offer a few general remarks on some of the most impor- 

 tant subjects that have engaged our attention. 



2. Nature of Volcanic Action. — Volcanic action is 

 defined by Humboldt to be the influence exerted by the 



* The beautiful experiments of Mr. Grove prove that water may be 

 reduced to its elementary compounds — oxygen and hydrogen — by 

 intense heat alone. See Philos. Trans. 184b'. 



