Frofessor Forbes on the Geology ofAuvergne, 11 



It is in the 6th or lowest bed that we have the accompanying- 

 section. Beneath the general overlying trachyte, we have a por- 

 tion of the same unstratified matter passing into a trachytic con- 

 glomerate (a very common rock of this country). To the right 

 of the figure we have this same conglomerate becoming stratified, 

 and passing undistinguishably into common paste-like volcanic 

 tufa ; and in this very tufa we have veins passing from the 

 trachyte above, and even appearing to cut both one and the 

 other. We shall find ourselves reasoning in a circle if we at- 

 tempt to ascribe superior antiquity to any one of these substan- 

 ces. I am disposed to account for rather than explain such 

 phenomena, by changes induced in structure, and particularly 

 a blending of structure subsequent to the deposition of rocks by 

 the action of heat. 



It is worthy of observation, that this bed of tufa contains por- 

 tions of wood in a high state of preservation. 



Relative age of rocks. — The conclusion of the anteriority of 

 basalt to trachyte, from the section just given, and from others 

 in the neighbourhood, as near Quereilh, would not be quite le- 

 gitimate, even supposing that the basalt were .not found to blend 

 with trachyte. For we have seen that, in the case of Gergovia, 

 we must necessarily admit the insinuation of basalt posterior 

 to the date of the strata it traverses, and we have also a very 

 good specimen of this in the Mont Dor, on the great road 

 from the baths to Murat. But there is one remarkable fact 

 which seems necessarily to lead to this conclusion. In the Cautal 

 between Aurillac and Vic-en-Carladez, I have observed masses 

 of basalt inclosed in a trachytic conglomerate, a vast pudding- 

 stone formation with a base of trachyte, passing into and inter- 

 stratified with that rock. 



The common opinion is, that the trachytic eruptions belong to 

 the oldest epoch, that the basaltic were subsequent to them, and 

 the phonolitic still later. Certainly there are many facts seen in 

 the Mont Dor and Cantal which confirm such an idea, yet pro- 

 bably we must not consider this as [universally true, owing to a 

 variety of facts, one of which I have just stated. That the 

 trachytic eruptions were very frequently renewed seems almost 

 undeniable, from the dykes of more compact trachyte which tra- 

 verse the older rocks, and which add so much to the sublimity 



