324 D. Forbes — On Volcanos. 



volcanos may be from one another whence they are taken, proves 

 them to be altogether identical in general mineral and chemical con- 

 stitution. 



Mallet, as the result of experimental researches, supposes that 

 Vesuvius in the later eruptions drew its supply of molten matter 

 from a depth of somewhat more than eight miles below the surface ; 

 and geologists have variously estimated the thickness of the total 

 sedimentary rocks which are known to them at the surface of the 

 earth at from seven to seventeen miles : we also know from obser- 

 vations in deep mines and artesian wells, that the temperature of the 

 earth's crust increases as we descend, about two degrees for every 

 hundred feet in depth ; so that at this rate a temperature sufficiently 

 high to keep lava in fusion would be met with at less than twenty 

 miles below our feet. Taking all these and other data into due 

 consideration, I cannot arrive at any other conclusion than that all 

 volcanos are connected with one another in depth, and have one 

 common source, not necessarily situated at any enormous depth 

 below the surface, but in which the molten matter, whilst always 

 retaining certain general characters, has undergone considerable 

 changes in composition, mineralogical and chemical, from time to 

 time in the world's history ; for under the term " volcanic rocks," I 

 would here include all eruptive rocks without exception, whether 

 called granites, syenites, porphyrites, basalts, or lavas ; all of which 

 I regard as but so many members of one chronological series, or 

 simply as the products of the volcanic action of different geological 

 epochs. 



So much for the molten products of volcanos. Now a few words 

 on their gasiform emanations, which consist in greater part of the 

 vapour of water, i.e., steam along with volatile chlorides, hydro- 

 chloric and suliDhuric acids, nitrogen and sulphuretted hydrogen 

 gases. The sulphur seen to be sublimed in so large quantities is 

 probably derived from the mutual reaction of the sulphurous 

 acid and sulphuretted hydrogen gases as they come into contact 

 with one another. 



Now, if it be true that we have a vast accumulation of molten 

 matter at a certain distance below the surface, which observation 

 further informs us must in major part consist of the silicates and 

 sulphides of the metallic elements, then, in my opinion at least, it 

 only requires the assumption that watei* from the sea should, by 

 some means or other, find its way down into such a reservoir, to ac- 

 count for all the phenomena of volcanos, both mechanical as well as 

 chemical. The greater part of the water so introduced would be at 

 once converted into steam, which in its turn would become still fur- 

 ther expanded by a heat so great as that of molten lava, and would 

 develope an enormous power. Calculations have been made which 

 show that water even when heated to a much less temperature would 

 exert an " ejection force," as it has been termed, even exceeding that 

 developed in eruptions of the highest volcanos known. Another 

 portion of the water, with the air carried down along with it, acting 

 upon the highly -heated sulphides, would become decomposed, and 



