Excavation of' Valleys. 23 



account for some ranges of basaltic prisms lying on the gneiss, 

 nearly horizontal, and presenting their ends towards the goyge. 

 For, in the valley of Antraigues in the Vivarais, where a section 

 is aflPorded of basaltic current abutting against a precipice of 

 gneiss, the prisms are nearly horizontal ; whereas they are ob- 

 lique on the slope beneath, and vertical where they rest on the 

 level bed of the ancient river, in accordance with a fact now 

 generally established, that the axes of the prisms are always at 

 right angles to the cooling surfaces. i, ' 



The comparative destructibility of the basalt and the gneiss 

 is by no means unfavourable to this hypothesis ; for although 

 there are here, as usual, in the gneiss formation, some strata of 

 a hard granitic texture, yet in general it is a slaty rock, contain- 

 ing much mica and decomposing felspar, and its high inclination, 

 usually at an angle of about 65°, and numerous fissures parallel 

 to the main course of the river, facilitate disintegration. This 

 tendency to waste is evinced by the huge slaty masses encum- 

 bering the foot of the cliffs or the middle of the river-bed. The 

 basalt, on the other hand, is extremely compact, resists decom- 

 position, and is only perishable in some parts where the prisms 

 happen to be vertical, and exposed to the mechanical action of 

 an undei'mining current. 



Mr Scrope has observed, that the undisturbed and perfect 

 state of the cone of loose scoriae whence the lava of Chaluzet 

 proceeded, demonstrates that no denuding wave, or extraordi- 

 nary body of water, has passed over this spot since the eruption ; 

 and we may add, that here as at Come, the upper scoriaceous 

 surface of the " cheire" preserves its original asperities, and is 

 in no part strewed over with sand or alluvial pebbles. Now, 

 if we compare the extent of waste at Chaluzet with that under- 

 gone by the " cheire" of Come below Fontgibaud, it is difficult 

 not to conclude with Mr Scrope, that the eruption of Chaluzet was 

 considerably anterior. But we can have no hesitation in assuming 

 that Chaluzet is at least as ancient a volcano as Come. Whence 

 it follows, that in times comparatively recent, there has passed 

 through the narrow gorge of the Sioule near Chaluzet, Ist, The 

 whole contents now removed from the higher valley before alluded 

 to, connecting the Sioule and the Monges ; 2dly, All that has 



