Professor Forbes on the Geology of Auvergne. 7 



configuration of both, the constitution of both, the included 

 minerals of both. 



The next remark I would make is an important one, and 

 refers to the alteration of rocks. The group of volcanic rocks 

 round Clermont forming what may be called the Monts Dome, 

 rise through an elevated table-land of granite, the flanks of 

 which are abundantly covered by sedimentary deposits of the 

 tertiary epoch. Associated with the latter are many of the 

 basalts. Thus in the hill of Gergovia, we have a section of 

 this kind, reckoning from below; 1. Tertiary limestone ; 2. Ba- 

 salt with nodules, and containing crystals of chalcedony ; 3. 

 Tertiary limestone, sometimes oolitic; 4. Basalt; 5. " Calca- 

 reous peperino'*' (Scrope), a sort of volcanic tufa containing veins 

 of compact felspar ; 6. Basalt at the summit. From the middle 

 bed of basalt shoot forth veins into the superincumbent calca- 

 reous tufaceous matter, which, in the places I examined, presents 

 no decided appearance of alteration by heat.- M. Dufrenoy* 

 thinks otherwise, and infers the posteriority of the contained 

 basalt lo the containing strata, which I am disposed to think 

 the phenomena of veins just alluded to would force us to admit, 

 though Mr Scrope ^ draws an opposite conclusion. This differ- 

 ence of opinion is not a little instructive ; and the exact paral- 

 lelism in that, as in other respects, between these phenomena 

 and those of Scotland, as well as the almost total absence of ap- 

 parent torrefaction at Gergovia, serve admirably to connect the 

 most unequivocal volcanic appearances with those of a more du- 

 bious character. 



Mineral Character, — The mineralogical character of the 

 rocks of Auvergne is one of their most interestinjr features. 

 Dr Daubeny % has particularly alluded to the perfectly basaltic 

 character of the recent coulees of Gravenoire, near Clermont. 

 One of these, in particular, on the side next Montaudoux, pre- 

 sents most brilliant crystals of olivine and augite, such as our 

 trap rocks associated with the older formations exhibit. The 

 amygdaloids and compact felspars might every one be matched 

 in Scotland, as well as many of the trachytes of Mont Dor and 



• Memoires pour servir k la Geologie de la France, torn. i. 

 ■\ Central France, p. 92. — Mr Lyell agrees with M. Dufrenoy, Geology^ 

 ilT. p. 259. 

 X I/'ttera on the Geology of Auvergne, Eit Phil. Joanml^ 



