and on the Ongin of the Valleys of Aavergiie. SI 7 



If, then, a tradition had come down to us of a comet, which, 

 h'j its approach to our globe, had occasioned fearful ravages on 

 its surface, and if observation furnished nothing to contradict, 

 but much to confirm, the general truth of the report, it would 

 imj)ly an excess of scepticism to deny the fact, merely because 

 centuries had passed away without any similar event having 

 taken place. 



Nevertheless, I presume not to decide whether the pheno- 

 mena in question are such as require us to have recourse to 

 such an hypothesis ; or whether, as others have contended, they 

 admit of being more simply explained by reference to operations 

 of which we are eye-witnesses. 



It has long been my persuasion, that the formation of valleys, 

 and the origin of sedimentary deposits, are questions of so com- 

 plex a description, and involve such a variety of different con- 

 siderations, that they can only be treated properly by those who 

 have long directed their chief attention to geological pursuits, 

 and have taken pains to make themselves fully masters of every 

 point which the study embraces. 



Now, with this feeling, it would ill become me to attempt to 

 break a lance with Mr Lyell, on the general merits of the ques- 

 tion Ite has so elaborately discussed, having for some time past 

 been obliged, by the intcrfererJce of other pursuits, to confine 

 myself to those bramihes of geology which relate more or less 

 inmiediately to volcanic operations, and being, therefore, in- 

 debted for most of the information I possess, respecting the de- 

 tails of diluvial action, to the investigations of others. 



I am, therefore, quite ready to defer to the decision which na- 

 turahsts may be disposed to adopt with respect to the origin of 

 valleys in general, and am only prepared to contend, that the 

 volcanic products met with in Auvergne are referable to two 

 different periods, which, so long as the diluvial theory is ad^ 

 niitted, must be viewed as being separated one from the other 

 by the epoch of the deluge which overflowed that country. 



One thing, ^t least, appears certain, namely, that the valley«» 

 of that district have lesulted from the same cause or causes that 

 produced them in the parts of France contiguous ; and, there- 

 fore, so long as we refer them in other places to the operations 

 of a deluge, we are entitled to call those volcanic products m 



JAUUAViY — MAttClI 18-31. 1' 



