414 COSMOS. 



below the most ancient rock formations, which were then 

 generally supposed to be granite and gneiss. Resting on 

 some feeble analogies of inflammability, it was long believed 

 that the source of volcanic eruptions, and the emanations of 

 gas to which they for many centuries gave rise, was to be 

 sought for in the later upper Silurian floetz strata, containing 

 combustible matter. A more general acquaintance with the 

 earth's surface, profounder and more strictly conducted geo- 

 logical investigations, together with the beneficial influence 

 which the great advances made by modern chemistry have 

 exercised in the study of geology, have taught us that the 

 three great groups of volcanic or eruptive rock (trachyte, 

 phonolite, and basalt), when viewed as large masses, appear, 

 when compared together, to be of different ages, and for the 

 most part widely separated from each other. All three, how- 

 ever, have come later to the surface than the Plutonic gran- 

 ite, the diorite, and the quartz porphyry later than all the 

 silurian, secondary, tertiary, and quartary (pleistocene) form- 

 ations ; and that they frequently traverse the loose strata of 

 the diluvial formations and bone-breccias. A striking vari- 

 ety* of these intersections, compressed into a small space, is 

 exhibited, as we learn from liozet's observations, in Auvergne. 

 While the great trachytic mountain masses of the Cantal, 

 Mont-Dore, and Puy de Dome penetrate the granite itself, 

 and at the same time inclose in some parts (for example, be- 

 tween Vic and Aurillac, and at the Giou de Mamon) large 

 fragments of gneissf and limestone, we find also the trachyte 

 and basalt intersecting as dikes the gneiss, and the coal-beds 

 of the tertiary and diluvial strata. Basalt and phonolite, 

 closely allied to each other, as the Auvergne and the central 

 mountains of Bohemia prove, are both of more recent forma- 

 tion than the trachytes, which are frequently traversed in 

 layers by basalts4 The phonolites are, on the other hand, 



* Eozet, Memoire sur les Volcans cT Auvergne, in the Memoires de la 

 Soc. GtoL de France, 2me Scrie, t. i., 1844, p. 64 and 120-130: "The 

 basalts (like the trachytes) have penetrated through the gneiss, the 

 granite, the coal formations, the tertiary formations, and the oldest 

 diluvian bed. The basalts are even frequently seen overlying masses 

 of basaltic bowlders ; they have issued from an infinite number of open- 

 ings, several of which are still perfectly recognizable. Many of them 

 exhibit cones of scoria) more or less considerable, but nowhere do we 

 find craters similar to those which have given out streams of lava." 



t Resembling the granitic fragments imbedded in the trachyte of 

 Jorullo. See above, p. 303. 



J Also in the Eifel, according to the important testimony of the mine 

 director, Von Dechen. See above, p. 226. 



