Cn. XXXIL] TERTIARY VOLCANIC ROCKS. 545 



CHAPTER XXXIL 



ON THE DIFFERENT AGES OF THE VOLCANIC ROCKS Continued. 



Volcanic rocks of the Pliocene, Miocene, and Eocene periods continued — Au- 

 vergne — Mont Dor — Breccias and alluviums of Mont Perrier, with bones of 

 quadrupeds — River dammed up by lava-current — Range of minor cones from 

 Auvergne to the Vivarais — Monts Dome — Puj de C6me — Puj de Pariou — 

 Cones not denuded by general flood — Velay — Bones of quadrupeds buried in 

 Bcorise — Cantal — Eocene volcanic rocks — Tuffs near Clermont — Hill of Ger- 

 govia — Trap of Cretaceous period — Oolitic period — I^ew Red Sandstone pe- 

 I'iod — Carboniferous period — Old Red Sandstone period — "Rock and Spindle" 

 near St. Andrew's — ^Silurian period — Cambrian volcanic rocks. 



Volcanic Rocks of Auvergne. — The extinct volcanos of Auvergne and 

 Cantal in Central France seem to have commenced their eruptions in the 

 Upper Eocene period, but to have been most active during the Miocene 

 and Pliocene eras. I have already alluded to the grand succession of 

 events, of which there is evidence in Auvergne since the last retreat of 

 the sea (see p. 196). 



The earliest monuments of the tertiary period in that region are 

 lacustrine deposits of great thickness (2, fig. 676, p. 547), in the lowest 

 conglomerates of which are rounded pebbles of quartz, mica-schist, 

 granite, and other non-volcanic rocks, without the slightest intermixture 

 of igneous products. To these conglomerates succeed argillaceous and 

 calcareous marls and hmestones (S, fig. 607) containing upper Eocene 

 shells and bones of mammalia, the higher beds of which sometimes al- 

 ternate with volcanic tuff of contemporaneous origin. After the filling 

 up or drainage of the ancient lakes, huge piles of trachytic and basaltic 

 rocks, with volcanic breccias, accumulated to a thickness of several thou- 

 sand feet, and were superimposed upon granite, or the contiguous lacus- 

 trine strata. The greater portion of these igneous rocks appear to h^-ve 

 originated during the Miocene and Pliocene periods; and extinct quad- 

 rupeds of those eras, belonging to the genera Mastodon, Rhinoceros, 

 and others, were buried in ashes and beds of alluvial sand and gravel, 

 which owe their preservation to overspreading sheets of lava. 



In Auvergne the most ancient and conspicuous of the volcanic masses 

 is Mont Dor, which rests immediately on the granitic rocks standing 

 apart from the fresh-water strata.* This great mountain rises suddenly 

 to the height of several thousand feet above the surrounding platform, 

 and retains the shape of a flattened and somewhat irregular cone, all the 

 sides sloping more or less rapidly, until their inclination is gradually 

 lost in the high plain around. This cone is composed of layers of scoriae, 

 pumice stones, and their fine detritus, with interposed beds of trachyte 



* See the map, p. 195. 

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