1826.] Di\ Dauheny an Volcanos. ^5 



sulphurous acid would be continually emitted, which, haivever, 

 would be soon decomposed by the hepatic gas present. The 

 water resulting from this process, would percolate into the 

 recesses of the rock, act upon any portions of the alkaline and, 

 earthy metalloids, that might have escaped the original action, j 

 and give birth to a fresh volume of hydrogen gas, ready in its 

 turn to dissolve a new portion of sulphur, and thereby tocontri-; 

 bute to a repetition of the same pheenomena." P. 392. : 



We have no room for further extracts, but must proceed to 

 give some account of the second department of the inquiry ; 

 namely, as to the degree in which volcanos have contributed 

 towards the production of the older constituents of our globe. 



He decides in favour of the general, if not the universal 

 volcanic origin of trap, by considering the analogy in chemical 

 and geological characters that exists between it and lava, and 

 by accounting for the distinctions between the two, from the 

 differences of circumstances under which they were respectively 

 produced. He even shows, that as these circumstances became 

 more similar to those that exist at present, the characters of the ?^ 

 erupted masses approach more nearly to those of the actual 

 products of subterranean fire ; and he therefore establishes three 

 classes of volcanic formations, the first produced since the 

 commencement of the present order of things ; the second, 

 during the deposition of the tertiary rocks ; the third, cotempo- 

 raneous with the more ancient strata. 



Of these, thefirstclass, beingformed in the open air, possesses 

 the characters of bodies exposed at present to artificial heat ; 

 the third, being of submarine origin, has those characters modi- 

 fied by the influence of great pressure ; whilst the second, being 

 formed under water, but under a body of fluid less considerable 

 than existed in the former case, possesses characters intermediate 

 between the other two. 



The author details, at some length, the subordinate differences 

 arising out of these fundamental distinctions, and then proceeds 

 to notice the arguments that have been advanced, both for and 

 against the igneous origin of granite and serpentine. 



These questions, however, he leaves undecided, or at least 

 considers to req[uire some further elucidation. 



The author likewise regards the opinion which prevails, as to 

 the increasing heat of the earth from the circumference to the 

 centre, as open to some objection ; and is led from his own 

 observations in mines, to consider their temperature as influenced 

 by local causes in a greater degree than is generally suspected. 



He concludes by speculating on the final causes of volcanos, 

 which he regards as the safety-valves, through which elastic 

 fluids, generated by processes going on in the interior of the 

 earth, find a vent, and consequently as the best safeguards 

 against destructive earthquakes. 

 New Series, vol. xn. q 



