546 MONT DOR, AUVERGNE. [Ch. XXXII. 



and basalt, which descend often in uninterrupted sheets, until they reach 

 and spread themselves round the base of the mountain.* Conglome- 

 rates also, composed of angular and rounded fragments of igneous rocks, 

 are observed to alternate with the above ; and the various masses are 

 seen to dip off from the central axis, and to lie parallel to the sloping 

 flanks of the mountain. 



The summit of Mont Dor terminates in seven or eight rocky peaks, 

 where no regular crater can now be traced, but where we may easily 

 imagine one to have existed, which may have been shattered by earth- 

 quakes, and have suffered degradation by aqueous agents. Originally, 

 perhaps, like the highest crater of Etna, it may have formed an insig- 

 nificant feature in the great pile, and may frequently have been destroyed 

 and renovated. 



According to some geologists, this mountain, as well as Vesuvius, 

 Etna, and all large volcanos, has derived its dome-like form not from 

 the preponderance of eruptions from one or more central points, but 

 from the upheaval of horizontal beds of lava and scoriae. I have 

 explained my reasons for objecting to this view in Chapter XXIX., 

 when speaking of Palma, and in the Principles of Geology.f The 

 average inclination of the dome-shaped mass of Mont Dor is 8° 6', 

 whereas in Mounts Loa and Kea, before mentioned, in the Sandwich 

 Islands (see fig. 640, p. 490), the flanks of which have been raised by 

 recent lavas, we find from Mr. Dana's description that the one has a 

 slope of 6° 30', the other of 7° 46'. We may, therefore, reasonably 

 question whether there is any absolute necessity for supposing that the 

 basaltic currents of the ancient French volcano were at first more hori- 

 zontal than they are now. Nevertheless it is highly probable that 

 during the long series of eruptions required to give rise to so vast a pile 

 of volcanic matter, which is thickest at the summit or centre of the 

 dome, some dislocation and upheaval took place ; and during the disten- 

 sion of the mass, beds of lava and scoriae may, in some places, have 

 acquired a greater, in others a less inclination, than that which at first 

 belonged to them. 



Respecting the age of the great mass of Mont Dor, we cannot come 

 at present to any positive decision, because no organic remains have yet 

 been found in the tuffs, except impressions of the leaves of trees of 

 species not yet determined. We may certainly conclude, that the ear- 

 liest eruptions were posterior in origin to those grits, and conglomerates 

 of the fresh-water formation of the Limagne, which contain no pebbles 

 of volcanic rocks ; while, on the other hand, some eruptions took place 

 before the great lakes were drained ; and others occurred after the 

 desiccation of those lakes, and when deep valleys had aleady been exca- 

 vated through fresh-water strata. 



In the annexed section, I have endeavored to explain the geological 

 structure of a portion of Auvergne, which I re-examined in 1843. J It 



* Scrope's Central France, p. 98. 



\ See chaps, xxiv. xxv. and xxvL 7th, 8th, and 9th editions. 



X See Quarterly Geol. Journ. toL ii. p. 77. 



