Geological Hociety. 137 



cavated to the depth of 40 feet. It is inferred that no general inun- 

 dation contributed to this effect, from the total absence of sand, mud, 

 or pebbles, on the surface of the lava of Come j although that current 

 has occupied a low and exposed situation ever since the period when 

 the Sioule began to open for itself its present channel. 



3. Near the volcano of Chaluzet, the Sioule has not only cut 

 through more than 100 feet of compact basalt, but also the gneiss 

 beneath, to the depth of at least 50 feet j the ancient channel of the 

 river being marked bv a bed of pebbles, intervening between the gneiss 

 and the basalt, and' now at a considerable height above the actual 

 stream. And here the authors discovered an ancient mining gallery, 

 driven in horizontally between the basalt and gneiss, so as to exhibit 

 the pebble bed to the distance of .50 or 60 feet ; a proof that this 

 deposit was a true river alluvion, and not merely an external accu- 

 mulation of debris covering superficially a mountain slope. The state 

 of the cone and lava of Chaluzet demonstrates further, that no flood 

 has passed over the country since the commencement of the exca- 

 vation ; and similar inferences are drawn from the condition ot the 

 cone of Montpezat in the Vivarrais. At Thueyts, in the same tract, 

 the gneiss is worn into by the Ardeche, in one instance to 70 feet 

 below an ancient alluvion overlaid by basalt. And in this valley an 

 undulating band of pitchstone, at right angles to the vertical columns, 

 occurs between the prismatic basalt and the subjacent gneiss, afford- 

 ing an exact parallel to the external portions of the dykes which 

 traverse the oolitic strata in the Hebrides. 



4. The lavas of the Vivarrais have suffered more from the action 

 of rivers than the recent currents in Auvergne : but the greater ve- 

 locity and volume of the waters flowing in the narrow and steep val- 

 leys of the former country, may account for this, without supposing 

 the lavas to be much more ancient. In Auvergne there are currents 

 of ages unquestionably intermediate between the oldest and most 

 modern ; the remains of which are in many cases seen to follow the 

 direction of the valleys, reposing upon ancient alluvions, and elevated 

 above the modern lavas and the present rivers. The authors, how- 

 ever, do not admit that relative altitude can be considered as an in- 

 variable criterion of the relative antiquity of basaltic plateaus, as 

 some writers have supposed. 



5. In conclusion, a detailed account is given of the deposits at 

 Mont-Perrier or Boulade, where the fossil remains of various extinct 

 quadrupeds are found alternating with beds of transported materials 

 of different kinds, which rest against the sloping side of a hill to the 

 height of between 200 and 300 feet. This hill itself is essentially 

 composed of tertiary marls, capped with basalt ; but the basalt does 

 not here overlie the alluvions, as has been asserted. 



I'haenomena perfectly analogous to those of Perrier are exhibited 

 on the Allier at St. Maurice, and in the hill of Monton, not far di- 

 stant : and these three sections, as well as that above mentioned at 

 the new jjassage of the Sioule, all concur in ))roving that many val- 

 leys in Auvergne, anciently excavated through gneiss and lacustrine 

 maris capped with old basalt, have at some remote periods been 



NeivSrrirs. Vol. 5. No. 9.6. Feb. 1829. 'A' ''^*^^ 



