Professor Forbes on the Geology of Auvergne, 9 



specimens. As to basalt, its tabular form is too frequent in 

 Auvergne not to be remarked. But I particularly noticed vast 

 slabs of it in the valley above St Bonnet, one of the immense 

 external indentations of the group of Mont Dor. • 



Two extremely picturesque eminences, not very far from the 

 last mentioned valley, called La Thuilliere and La Roche Sana- 

 doire, are composed of what has been called by almost all writers 

 on Auvergne, pJionolite, though it does not coincide precisely 

 with my idea of that rock. It is composed, however, apparently 

 altogether of felspar. In the case of La Thuilliere, the very 

 polygonal arrangement, striking when viewed at a distance, 

 merges into a most completely slaty structure when closely exa- 

 mined, the rock forming vast tabular masses which resemble 

 strata. To these cases we might add that of the modern lava of 

 the Puy Nugere, forming the coulee of Volvic, the stone of which 

 may be cleft into tabular masses parallel to the bed in which the 

 lava flowed, and these again through vertical planes coinciding 

 in direction with the lava stream. These remarks find an im- 

 portant application in considering the tabular structure of rocks 

 of dubious origin, such as granite, which may be even extensive- 

 ly slaty (as it rarely, and though sometimes is, as on the south 

 side of the Canigou in Roussillon, near Prats-de-MolIo), without 

 leading us to the conclusion of an aqueous deposition. 



Passage ofrochs into one another. — I have already observed, 

 that, far from finding geological theories too simple, I imagine 

 that they are rarely complicated in any proportion to the actual 

 complication of nature. Especially the changes which occur 

 after their deposition, and which may be oftentimes repeated, 

 may serve to render inextricable the apparent disorder of super- 

 position. 



It has been a general (and for the most part a well founded) 

 opinion, that trachytic eruptions were prior to basaltic ones ; and 

 that the existence of these rocks may refer to very different pe- 

 riods in the history of our globe. The universality of this view, a 

 study of Auvergne will hardly allow us to acquiesce in. I shall 

 confine myself at present to one curious section, which I was in- 

 duced especially to examine from a reference to it in Mr Scrope's 

 work on Auvergne * where he has called attention to the exist- 



• Note p. 109. 



