22 Messrs Lyell and Murchison on ttic 



tions had been aftbrded, there might have existed some difficulty 

 in proving that this alluvium did at any time pass horizontally 

 under the basalt, constituting a river bed, and not merely de- 

 bris covering a mountain slope ; but we obtained the most unan- 

 swerable evidence in support of this fact at Les Combres. There, 

 within a very short distance of its termination, and when the 

 lava-current has diminished to the thickness of thirty feet, it 

 rests upon a pebble-bed, having between it and the stream about 

 eighteen feet of gneiss *. Fortunately at this point there is an 

 ancient excavation (said to have been a mine of the Romans), 

 and a gallery has been driven in horizontally through the upper 

 part of the gneiss and the superposed alluvium, so that the 

 lower ends of the columns of basalt form the roof, resting on 

 the pebble-bed, which can be traced for the space of fifty or 

 sixty feet inwards on either side of the gallery, occupying, regu- 

 larly throughout, the space between the basalt and the gneiss. 

 In this alluvial bed are fragments of old basalt, with others of 

 gneiss, and much wliite quartz and schist. Many of these boul- 

 ders are quite rounded, whilst others are flat and slightly angu- 

 lar, as in the bed of the present river, and the surface of the 

 gneiss on which they are deposited is slightly undulating. 



The higher portion of the alluvial bed consists of fine brown 

 sand, identical with bands occupying similar positions at Mont- 

 pezat and other localities in the Vivarais, and the ends of the 

 prisms of basalt terminating upon this sandy bed are here, as 

 in that district, scoriaceous and cellular. 



No remnant of the lava of Chaluzet is now found on the right 

 bank of the Sioule, but it must once have reached it at many 

 points, as the gorge is every where narrow, and at Pranal can- 

 not far exceed fifty or sixty feet in width, although here the 

 basalt, resting on gneiss, forms an impending and vertical cliff, 

 about 100 feet in height. For two-thirds of this thickness the 

 basalt is prismatic, and this structure implies its former liqui- 

 dity and slow cooling on the spot, and that it did not form the 

 exposed surface of a " cheire." It may indeed be contended, 

 that the Sioule excavated the present chasm between the basalt 

 and the gneiss, and chiefly through the latter, because this would 



» See riatc 1. 



