Excavation of Valleys. 29 



of the river *. A single arch, only fifteen paces in width, spans 

 the precipitous chasm ; and the eye can at a glance restons the 

 former outline of these rocks, and estimate the amount of ba- 

 salt and gneiss removed by the action of the stream below. 



4</^, A phenomenon hitherto we believe unnoticed in lava cur- 

 rents of this age, occurs at the spot just mentioned. Between 

 the lower terminations of the vertical columns of basalt and the 

 subjacent gneiss, is an undulating zone or band of black pitch- 

 stone, at right angles to the axes of the prisms. The position 

 of this pitchstone is of high interest, in affording an exact paral- 

 lel to the layers of that mineral described by Professor Sedg- 

 wick, and one of the authors of this paper, as forming the ex- 

 ternal portions of basaltic dikes which traverse the lias, and 

 higher members of the oolitic series in the Hebrides ; and also 

 because similar layers of a black vitreous rock approaching 

 pitchstone, have been recently observed by the other author of 

 this memoir, forming the exterior of dikes in the crater of 

 Somma, or ancient part of Vesuvius. This band at the Geule 

 d'Enfer seldom exceeds one foot in thickness, and it pecu- 

 liarly resembles in mineral structure that of the dikes of Car- 

 saig, on the south coast of Mull, in its black colour, fracture, 

 and state of compactness. The ends of the columns terminating 

 upon it are somewhat scoriaceous and cellular. A bed of sand 

 and pebbles is, in one part of the gorge, seen to be interposed 

 between the pitchstone and the gneiss f. 



The greater portion of the river beds in the Vivarais are 

 without water in summer, and the white-bleached exterior of the 

 primary boulders contrasts so strongly with the black basalt, 

 that we may easily form some estimate of the relative degrees of 

 waste undergone by these rocks. In the bed of the Ardeche, 

 before it receives from the Alignon the basaltic pebbles of Jau- 

 jac, the basalt scarcely bears to the gneiss a greater proportion 

 than one to fifty. Its channel is blackened for a while by the 

 influx of the basalt of the Alignon ; but again, in descending the 

 stream, the primarv rocks everywhere predominate, even near 



• See Plate II. 



•f- A bed of pitchstone is described by Mr Scrope in the Ponza Isles to be 

 interposed between the trachyte and conglomei-ate. Geol. Trans, vol. ii. 

 2d Series, p. 228. 



