10 Professor Forbes on the Geology ofAuvergne, 



ence of basalt underlying trachyte, a fact which he says has been 

 denied by M. Beudant and other French geologists. That there 

 is a basalt there can be no doubt, and I was hence led to trace the 

 very curious relations of these and other strata at the same point. 

 This section is a natural one formed by a rivulet descending in 

 a cascade near the baths of Mont Dor. The general section is 

 a very simple one, in a descending order : — 



1. Trachyte. 



2. Volcanic Tufa. 



3. Trachyte. 



4. Volcanic Tufa. 



6. Basalt passing into Trachyte. 

 6. Volcanic Tufa. 



Of bed No. 5. it is to be remarked, that though one portion of 

 it be truly basaltic, black, compact, and heavy, it gradually 

 passes into a distinct trachyte by imperceptible gradations, a cir- 

 cumstance which may possibly have escaped Mr Scrope. First 

 the augite gradually disappears ; the rock becomes less dark and 

 much less heavy ; then crystals of glassy felspar commence, and 

 the transformation is complete. It may be observed, that the 

 basalt in question does not appear to contain olivine, but it is a 

 perfectly well characterized basalt in other respects. 



+ + + + +' T Unstratified Trachyte passing into Conglomerate, r 





Debris. 



l^-v^Vl Trachyte. 

 lo"o°o°l Conplomeratc. 



