Excavation of Valleys. fSL 



mained stationary, because the gneiss has been protected by lava 

 from the ravages of the rivers, which, instead of deepening and 

 widening their orig-inal channels, have, in the first instance, been 

 compelled to disencumber themselves of volcanic matter. 



But the upper parts of the same valleys, to which, as above 

 Antraigues, the lava did not reach, and the flanks of the hills 

 above the contact of each lava-current, have remained during 

 the same time unprotected. The eye nevertheless cannot, in 

 general, recognize any difference between the steep slope of the 

 gneiss, where it has been exposed, and the part which has been 

 encased with lava, so as to discover in the outline any inequality 

 in the degrees of waste. If, then, the loss of gneiss which we 

 can calculate, in certain positions, to have been immense, has 

 not affected, in an appreciable degree, the general shape of the 

 hills or valleys, we may infer that the gradual decay of a conti- 

 nent can give birth to sedimentary formations of vast import- 

 ance in the grand receptacles of transported matter, while, in 

 the mean time, we are unable to perceive any sensible alteration 

 in the land whence the matter is withdrawn. 



All the lava-currents in the Vivarais are cut through in some 

 part of their course, so as to exhibit the subjacent gneiss, with 

 an intervening bed of ancient alluvium. In Auvergne, the most 

 modern lavas have not, in general, suffered so much. This, 

 however, cannot be safely attributed to their inferior antiquity, 

 but to the difference in the form of the valleys, which, in Au- 

 vergne, are wider in proportion to the volume and rapidity 

 of water flowing in them. Where, however, the configuration 

 of the country happens to have been favourable to waste, as in 

 the gorges before mentioned of the Sioule at Chaluzet, the 

 Veyre at St Saturnin, and the Couze de Chambon below Ver- 

 rieres, lavas flowing from craters nearly as entire as those of the 

 Vivarais have been destroyed in as great a degree. 



It may be asked. Whether, if all these lavas be refer- 

 able to the same great epoch, there are not others which must 

 be classed as intermediate between them and the oldest volcanic 

 rocks of Mont Dor .'' That such connecting links in the chain 

 do exist, we observed what amounted, in our opinion, to conclu- 

 sive proofs ; and we cannot doubt that more abundant examples 



