Site of the Ancient City of the Aurunci. 241 



much more prolific in minerals than it appears to be at pre- 

 sent, very few, at least out of the large number of species, 

 found within the range of Vesuvius existing in its modern 

 lavas, whilst they abound amongst the ejected masses im- 

 bedded in the tuff of Monte Somma. 



Having now endeavoured to trace the particulars in which 

 the processes of an igneous character going on at the present 

 day differ from those which gave rise to the rocks commonly 

 called plutonic, I will next briefly consider which of the com- 

 monly received theories of volcanoes is most reconcilable with 

 the phenomena which have just been pointed out. 



I may remark, in the first place, that th^re are only two 

 modes of explaining volcanic action, which deserve a moment's 

 attention, when viewed by the lights of modern science. 



One set of philosophers, inferring from the oblate sphe- 

 roidal figure of the globe, that it was once in a state of fluidity 

 from igneous fusion, and, again, presuming, from the increas- 

 ing temperature observed as we descend deeper and deeper 

 into its recesses, that it may retain enough of its heat at the 

 present time to be preserved in a state of fusion below cer- 

 tain depths, propose a very simple mode of explaining the 

 evolution of melted matter from volcanoes, by attributing it 

 to the contraction of the crust of the globe upon its fluid 

 contents, by which a portion of the latter is from time to 

 time expressed at the points of least resistance. 



Others, considering that all the matters ejected from a 

 volcano contain an inflammable base united with oxygen — 

 that the latter need not be supposed to have been present in 

 the interior of the earth in quantity sufficient to combine with 

 all the principles for which it could exert an affinity — and, 

 therefore, that these bases may, without violence, be supposed 

 to exist in an unoxidized state at a certain distance from the 

 surface — have proceeded to shew, that, assuming such to be 

 the fact, all the phenomena of volcanic action may be ex- 

 plained according to the received principles of chemistry, by 

 the access, first, of sea water, and afterwards, of atmospheric 

 air, to the interior of the globe. 



