and on the Origin oftlie Valleys of Auvergnc. 211 



exist between the two; the specific gravity of trachyte, in gene- 

 ral, whatever may be the case with regard to that particular mo- 

 dification called Domite, differs not very materially from that 

 of some augitic lavas which chance to be deficient in magnetic 

 iron ; and its chemical composition holds out no greater prospect 

 of any such distinction existing ; neither is it very intelligible in 

 what manner, supposing a sort of nucleus to be formed by the 

 first portion of the lava> in the manner Mr Scrope represents, 

 the succeeding ejections, of which the latest must have been 

 sufficiently fluid to rise to the height of near 3000 feet above 

 the level of the table-land, should so immediately have obtained 

 a sufficient degree of adhesiveness, as to attach themselves mere- 

 ly to the external sides of the cone, without spreading further 

 into the adjacent plain. 



I need hardly say how much more difficult it would be to ex- 

 tend the hypothesis to such a case as Chimborazo. But after all, 

 where is the advantage of resorting to so forced an explanation, 

 in order to avoid supposing that with regard to volcanic moun- 

 tains, which most geologists are now ready to admit with respect 

 to others ? 



A Wernerian, who rejected volcanic agency altogether, and 

 attributed all the different inclinations of rocks to subsidence, 

 &c., might consistently enough object to the theory of craters 

 of elevation, which Von Buch has proposed ; but a vulcanist, 

 who sees the proofs of the heaving up of rocks by the agency 

 of elastic vapours in all the chains of mountains he visits, ad- 

 mits the very principle as a general truth, which he rejects in 

 the particular case to which it is most applicable ; for how can 

 mountains be elevated without a void being occasioned under- 

 neath them ? or why should that be considered absurd with re- 

 spect to domite, which is probable in the case of granite ? 



What other effect, indeed, should we be disposed to attri- 

 bute to volcanos, so long as we are all agreed in assigning to 

 elastic vapours, coupled with the influence of an exalted tempe- 

 rature, the principal share in their production ? 



What more natural than to ascribe to such forces, either 

 singly or combined, not only the fusion and ejection of the sub- 

 stances most contiguous to the focus of their action, but also 

 the softening and heaving up of the rocks somewhat farther re- 



