TRUE VOLCANOES. 443 



aeriform envelope is strengthened by a comparison of the 

 thickness of the present seams of coals with that of the 

 thin coal-strata (seven lines in thickness) which, according 

 to Chevandier's calculations, our thickest woods in the 

 temperate zone would yield to the soil in the course of 

 100 years. 57 



In the infancy of geognosy, previous to Do'lomieu's ingenious 

 conjectures, the source of volcanic action was not placed 

 below the most ancient rock-formations, which were then 

 generally supposed to be granite and gneiss. Besting 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 give 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 

 geological investigations, together with the beneficial influence 

 which the great advances made by modern chemistry have 

 exercised on 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 gra- 

 nite, the diorite, and the quartz-porphyry, later than all the 

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

 mations, and that they frequently traverse the loose strata 

 of the diluvial formations and bone-breccias. A striking 

 variety 68 of these intersections, compressed into a small space, 

 is exhibited, as we learn from Rozet's observations, in Au- 

 vergne. While the great trachytic mountain -masses of the 

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



57 Cosmos, vol. i, pp. 283 5. 



53 Kozet, Memoire sur les Volcans d'Auvergne, in the Memoires de la, 

 Soc. Geol. de France, 2me Serie, t. i, 1844, pp. 64 and 120130 : "The 

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

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

 diiuvian beds. The basalts are even frequently seen overlying masses 

 of basaltic boulders j they have issued from an infinite number of 

 openings, several of which are still perfectly recognisable. Many of 

 them exhibit cones of scorise more or less considerable, but nowhere do 

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



