272 



THE WONDERS OF GEOLOGY. Lect. IIJ. 



valleys, and furrowed by many minor water-courses, all 

 originating near the central eminence, and diverging 

 towards every point of the horizon. The beds of which 

 • this group is composed, consist of scoriae, pumice-stone, 

 trachyte, and basalt ; these rocks dip off from the central 

 axis, and lie parallel to the sloping flanks of the mountain, 

 as is the case in Etna, the Peak of Teneriffe, and all other 

 insulated volcanic mountains. There is no crater; all 

 vestiges having been destroyed since the extinction of its 

 fires ; but streams of lava may be traced, in elevated peaks, 

 over a gorge which occupies the very heart of the mountain, 

 and they extend to a distance of many miles. A remark- 

 able natural section, worn by a cascade, at a short distance 

 from the baths of Mont Dore, exhibits the following beds 

 in a descending series : 



1. Porphyritic trachyte; a volcanic rock, 160 feet in thickness. 

 2. Arenaceous tufa. 3. Columnar basalt. 4. Breccia, made up of 

 volcanic fragments, cemented together by tufa. 5. Thick beds of 

 basalt. 6. White ferruginous tufa, enveloping fragments of granite, 

 basalt, &c, and traversed by veins of the overlying basalt. 



I may add, that the volcanic vents of central France are 

 evidently of very different ages ; some being of immense 

 antiquity, while others are of comparatively recent origin, 

 for they have exploded through the oldest beds of basalt ; 

 but even the most modern belong to a very remote period. 



50. Fresh-water strata of Auvergne. — This district 

 presents a series of alternations of fresh-water limestones, 

 with basalt, scoriae, and other volcanic productions, based 

 on a foundation of granite and gneiss. These beds occur 

 in the following order, beginning with the lowest or most 

 ancient: — 



1st. Clay, sand, and breccia, without organic remains. 



2d. Limestone and calcareous marl, in strata nearly horizontal; 

 about 900 feet thick. These are entirely of fresh- water origin, for 

 they abound in shells of the genera potamides, helix, planorbis, 

 and limncea, which are known to inhabit lakes and rivers. Some 



