196 Scrope— Origin of Valleys. 



Gergovia and La Serre, and the bed of basalt wbich occupies tbe 

 bottom of the narrow valley between them, and which has flowed 

 out of the recent volcanic vent marked by the Puy Noir. Here are 

 three long sheets of basalt, each of which, by its gradual inclination, 

 all in the same direction, and by the remains of the cone of scoriae 

 marking the vent from which it was erupted, is proved to have 

 flowed in a state of liquidity from the high granite platform into the 

 basin of the fresh-water formation. Each must necessarily have 

 occupied the lowest levels of that basin to which it had access. And 

 hence it is certain that at the period when the lava of G-ergovia 

 flowed into its present position, there could have been no lower de- 

 pression in its immediate vicinity. The hollow therefore into which 

 the lava of the neighbouring plateau of La Serre flowed must have 

 been subsequently excavated ; for the base of that bed is everywhere 

 — that is, on every line drawn transversely across both plateaux — 

 lower by from 300 to 500 feet than that of Gergovia, which is 

 parallel to it, not above three miles distant. Again, by the same 

 reasoning, it is evident that the intervening valley of Chanonat, the 

 bottom of which is now covered by a still more recent bed of basalt, 

 must have been excavated since the production of the lava of La Serre, 

 which overhangs it at the height of more than 500 feet. Here then^ 

 are three distinct steps in the process of excavation. Indeed it should 

 be said four, since the rivulet of the valley of Chanonat has worn 

 away a new channel some twenty to fifty feet in some places below 

 the level of the pebble-bed on which the most recent of the three 

 lava-currents reposes." 



" In its mineralogical aspect the basalt of these different 

 lava-flows affords confirmatory proof that their relative ages corres- 

 pond with their relative heights. That of Gergovia is compact, 

 partly amygdaloidal, and much decomposed externally. That of La 

 Serre has a far fresher aspect, though still possessing the distinctive 

 features of the older basalts. That of the inferior current of Chano- 

 nat is scarcely less recent in appearance than many of the lavas of 

 Etna, of which the date is known. Again the scoria-cone (with its 

 crater) whence the last flowed out is entire and undisturbed. That 

 which gave birth to the lava of La Serre has been much degraded, 

 the heavier and more massive scoria?, bombs, etc., alone remaining. 

 The source of the basaltic cm-rent of Gergovia, the Puy de Berze (for 

 though separated from it now by a deep transverse ravine, at the 

 junction of the granite with the marls, I have no doubt of the two 

 having been once united), retains the character of a vent of eru.ption, 

 but is worn down to a mere stump." 



" Many similar examples might be adduced were it necessaiy, 

 from the same district, leading inevitably to the conclusion that the 

 vast abstraction of matter which has occurred from the fresh-water 

 formation of the Limagne, since the drainage of its lake, was effected 

 gradually and progressively, and went on hand in hand with the occa- 

 sional flooding of parts of this valley and its tributary ravines by 

 lavas emitted in the eruptive paroxysms of the neighbouring volca- 

 noes. In the absence of the sea, and the lake having evidently been 



