8 Professor Forbes on the Geology of Ativergne. 



the Cantalj and the trachytic conglomerates of the latter, which 

 much resemble those of Arthur''s Seat. Where the trachytes 

 are more perfectly crystallized, containing glassy felspar in six- 

 sided tables, as at the Puy de Sancy, and in four-sided prisms, 

 as at the Capucin, both in the Mont Dor, we have a perfect 

 antitype in the rock of the Drachenfels ; whilst the porous but 

 durable lavas of the coulees of Pariou and the Puy de Nugere 

 (the latter forming the well known Pierre de Volvic) very closely 

 resemble the millstone lava of the environs of Andernach. Of 

 the rock called Domite I shall say nothing, because the only 

 proof of the prevailing hypothesis, that it is altered granite 

 ejected like hasty-pudding (to use the phrase of Mr Scrope), is 

 yet untried : we may hope for something Hke a result from Mr 

 Harcourt's experiments at the Low Moor iron-works. The 

 phonolite of the Puy de Griou, in the Cantal, is identical with 

 that of Blackford Hill, near Edinburgh. Again, the lavas and 

 scoriae of the Puy de Gravenoire and the lava of the Puy de 

 Come, forming the vast and sterile Cheire de TAumone, offer 

 the most perfect analogies to the produ(;tions of Vesuvius and 

 other modern volcanos. The felspathose rock of the Mont Dor 

 containing sulphur and alum, may be classed with that of the 

 Solfatara, and the tufas of every part of Auvergne may find 

 parallels at Ischia, at Pausilipo, and in the Campagna di Roma. 



Structure, — Nor does the columnar form occur under less 

 remarkable circumstances. We find basaltic pillars of exquisite 

 symmetry, straight and bent, at Murat, in the Cantal, and in a 

 thousand other sites, which may rank with those of Staffa and 

 the Giant's Causeway. Whilst, what is far more remarkable, 

 we discover the most obvious traces of columnar structure in 

 the relatively modern lavas, as in the very remarkable locality of 

 Pechadoire, near Pont Gibaud, and in the still more singular 

 displays of the Vivarais, a country which I have not visited. 



There is another circumstance respecting the structure of the 

 undoubtedly igneous rocks of Mont Dor well worthy of remark. 

 The slaty structure, so far from being opposed to the crystalline 

 character of the rock, is found frequently combined with it, and 

 that in the three leading characters of rock, trachyte, basalt, 

 and phonolite. The columns of trachyte behind the baths of 

 Mont Dor have so excessively slaty a structure, that it is with 

 great difficulty that we can procure fresh surfaces even in hand 



