226 Dr Daubeny on the Diluvial TJieory, 



a strong presumption that no deluge can have swept over the 

 country since the origin of the volcano to which it belongs ; and 

 on this principle, I have already given Mr Scrope credit for 

 pointing out to us the effects produced by rivers upon lava- 

 currents, which, having descended from such craters, we pre- 

 sume to be post-diluvial. But, is the great mass of the volcanic 

 products of Mont Dor or Cantal in this predicament ? Has any 

 geologist pretended to trace the vast sheets of basalt and trachyte 

 which here cover the greater part of two departments, to any 

 thing like a crater, or connect them with cones of scoriae? So 

 far from this having been done, it seems to me quite impossible 

 to determine in what direction they began to flow, or to avoid 

 suspecting that the circumstances under which they were ejected 

 essentially differed from those which exist at present. 



The examples, therefore, brought forward by Mr Scrope can- 

 hot affect the question concerning the origin of such valleys as 

 these, which every one must allow to differ widely from the 

 ravines he has pointed out to us, and which, from their perfect 

 similarity to the valleys of denudation found in other countries, 

 we are bound, in consistency, to attribute to one and the same 

 cause. 



I should, however, lay but little stress upon this classification 

 of volcanic products, were it not true, that the distinction in their 

 ages is found generally to be accompanied with corresponding 

 variations in the characters belonging to them. That such is 

 the case most geologists appear to admit, and even Mr Scrope, 

 although he objects to it in theory, adheres to the distinction 

 in practice. 



That there are exceptions to the universality of this remark, 

 I have in many places of my work admitted, but, generally 

 speaking, it is certain, that those lavas, which I have termed 

 postdiluvial^ and which are posterior to the valleys of the 

 country, put on the appearance of such melted matters as have 

 flowed in the open air ; whilst those to which I have given the 

 name of antediluvial, seem, for the most part, to have been pro- 

 duced under a certain degree of pressure. If, as we may infer 

 from Mr Scrope''s own statements, the ancient lavas of Auvergne 

 were ejected at the bottom of a fresh water-lake, and if that 

 lake chanced to be of a certain depth, the compactness which in 



