B Professor Forbes on the Geology of Auvergne, 



Charpentier and D'Aubuisson bear a candid testimony in their 

 own persons.) 



The effects of heat are of two kinds, producing rocks (by 

 which we mean being the direct agent of their protrusion,) and 

 altering them. Each of these, but especially the latter, affords 

 a field of inquiry of the most interesting character, and often 

 fills up a chain of evidence as solid and as convincing as any even 

 in mathematical physics. Of both of these Auvergne contains 

 examples, though perhaps the production is more frequent than 

 the change of character. To go over what has been already so 

 well enforced, the general deductions from the phenomena of 

 Auvergne, is by no means my purpose ; I would rather allude 

 to one or two individual points which may pretend to something 

 of originality ; and first, I would remark, that the striking con- 

 viction which, as we have said, a sight of this country has 

 seldom failed to impart as to the origin of trap (undoubtedly 

 one of the most fundamental positions in geology), we attribute 

 quite as much to the topographical condensation of the evidence 

 as to its superior force. We discover in Auvergne vast plateaux 

 of basalt, of the origin of which we know nearly or quite as little 

 as of that of Salisbury Crags, near Edinburgh, or the Whin Sill 

 of the north of England. In close apposition to these, we have 

 f!ows of lava, as distinct, as rugged, and as obviously igneous 

 as those of Vesuvius ; — following the course of the actual valleys 

 issuing from scorified craters, rolling their tide of desolation into 

 the midst of fertility, — in short, presenting as complete a picture 

 of volcanic energy, even in its most frightful form, as can any 

 where be seen, with the single exception that all is cold and 

 hard. Though, as later writers have pointed out,* there may 

 be an insensible passage from the basalts of these platforms to 

 the more modern streams, we seldom fail to discover a general 

 reference to an older or a newer class, one anterior, the other 

 posterior, to the existing condition of the valleys. The basaltic 

 jylateaux we cannot trace to their source, the coulees of lava we 

 can ; the evidence of a common origin is perhaps not greater 

 than that which a trap rock country, and a modern volcanic 

 country compared would afford, only that you have no sepa- 

 ration of the two in point of space ; — you may stand upon 

 basalt and break lava ; you may compare at one instant the 



• Scrdpe on the Volcanos of Central France. Lyell's Geology, vol. iii. 



