and on the Origin qfilie Valleys of Auvergne. 227 



general characterizes these products may be accounted for by 

 the pressure of the superincumbent water ; but, as the same 

 effect might result from the weight of a considerable bed of tuff 

 or scoria?, the occasional occurrence of compact lava beds, even 

 amongst subaerial volcanos, need not so much surprise us. 

 When, therefore, I remarked, that no genuine basalt had, to 

 my knowledge, been found amongst the lavas of those volcanos 

 which are at present in activity, and that their ejections do not 

 appear to exhibit the same col unmar arrangement which belongs 

 to the igneous products of an earlier age, I by no means meant 

 to deny the possibility of such occurring, but only to awaken at- 

 tention to the point, in the hope that other observers might as- 

 certain its truth or falsehood. 



The distinction, however, which I have attempted to maintain* 

 between that irregular prismatic structure, which is the mere 

 result of contraction, and is so well exhibited in the modern lava 

 of Niedermennig, and that of articulated columns which is de- 

 rived apparently from the mutual pressure of spheroidal con- 

 cretions, be it true or false, will not be overturned by the obser- 

 vation, that in some instances a void space exists between the 

 respective columns of the last mentioned kind. For it is evident, 

 that the compression which converted the spheres into polygons 

 would take place whilst the material was yet soft, and that a 

 further degree of shrinking might therefore be expected to occur, 

 before it became perfectly cold. 



This, I conceive, furnishes a sufficient reply to Mr Scrope'*s 

 objection in page 149 of his Memoir on Central France, es- 

 pecially when backed by the ingenious observations of Mr Gre-u 

 gory Watt, published in the Philosophical Transactions for 1808, 

 from which it appears, that, after a melted mass has become in 

 a great degree consolidated, chemical affinities continue to de- 

 velop themselves, and crystalline arrangements to take place. 



But I trust I have already said enough to prevent misconstruc- 

 tion, in case I should in any subsequent treatise, in which it . 

 raiisht be inconvenient to introduce reii>arks of a controversial' 

 nature, think proper to adhere to those views which 1 had taken 



• 3ee P^Qription t)f Volcanos, p. 40. and 422. 



