28 Messrs Lyell and Murchison on the 



small streamlets crossing the broad plateau of lava, and which 

 was dry when we visited it, has shaped out a channel about 

 twenty feet deep, and which, near its junction with the Fontau- 

 lier, is about 100 paces broad. In its gravel-bed v^e saw enor- 

 mous blocks of gneiss, which it had carried for half a mile over 

 the slightly inclined plane of the lava current. In consequence 

 of accidents, resulting from volcanic causes, the course of the 

 Fontaulier presents us with alluvial accumulations of three 

 distinct dates. The most ancient occurs at various places under 

 the lava current. It is generally not much elevated above the 

 level of the river, and is seen continually at its edge reposing on 

 gneiss, and containing no volcanic matter. The next in age is 

 that seen at the bridge of Montpezat, resting on basalt, and 

 containing volcanic sand as well as primary rocks. The last is 

 the bed of the present river, where, notwithstanding the passage 

 of the Fontaulier for so many miles through basalt, and that 

 pebbles of basalt are introduced by the tributary stream de- 

 scending from Burzet, boulders of primary rock predominate. 



Lava of' Thueyts. 



The lava current proceeding from the volcanic crater of 

 Thueyts, in the Vivarais, distant only a few miles from that 

 last described, is remarkable under several points of view. 



\st. Contrary to the general course of such currents, the main 

 body of lava has ascended the bed of the Ardeche for about a 

 mile and a-half, there occupying the widest part of tiie valley, 

 and filling it up to the gneiss on either side. 



^d. The Ardeche has eaten its present bed between the pre- 

 cipitous rocks of gneiss on its right bank, and the sides of the 

 lava current, exposing in several places a pebble bed between 

 the ends of the vertical prisms of basalt and the subjacent 

 gneiss. The latter is thus shewn to have been cut into since the 

 epoch of the eruption, in one instance twenty-five, in another 

 seventy feet, below the river alluvion. 



3(/, The depth of excavation is immense in the narrowest part 

 of the gorge, where the compact basalt has been eaten into to 

 the depth of ninety feet, and the hard and solid rocks of 

 gneiss below for seventy feet more, down to the present level 



