Excavation of Valleys. 25 



the serrated outline of the mountains, and to the deep and fis- 

 sured character of the valleys. 



The cone and crater of Montpezat have as perfect and fresh 

 an appearance as the most recent in Auvergne.- The volcanic 

 bombs are extremely numerous, and the ashes are so loose, that 

 there is no vegetation on the greater part of the northern slope. 

 As there are no smooth rounded alluvial pebbles at the bottom 

 of the crater, or on any part of the cone, the angular blocks of 

 gneiss near the summit (some heavier than a man can lift) were 

 not transported thither by water, but must have been ejected 

 from below. These have undergone no visible alteration ; a re- 

 mark that will apply equally to many of the granitic fragments 

 contained both in the scoriae and basaltic lava of this volcano. The 

 black cellular scoriae that were thrown out from the crater towards 

 Thueyts, attest, if possible, still more strongly than the state of 

 the cone, that no general inundation has passed over the coun- 

 try since the eruption. These ashes and scoriae are very light, 

 the fragments varying from the size of walnuts to peas, cover- 

 ing, in beds from three to four feet thick, a rapid acclivity of 

 gneiss. They have in great part disappeared, but, from the 

 abrupt shape of the valley, and their feeble adhesion to the sub- 

 jacent rocky slope, it is clear, that even the force of a very small 

 body of water would have swept away the whole. 



The lava of Montpezat descended from a breach on the NW. 

 side of the crater into the bed of the Fontauher. Now, it is na- 

 tural, and indeed almost necessary, to suppose, that whenever a 

 mountain stream is thus dammed up, a lake must have been 

 the result, which would receive for a time all the sand and rocks 

 brought down continually from the higher mountains. The 

 river issuing from such a lake would exert but little power near 

 its outlet, not being yet charged with foreign matter. These 

 circumstances are still clearly exemplified in lakes caused by 

 similar obstructions in Auvergne, and we shall enumerate some 

 facts there observed, as illustrative of phenomena to be described 

 in the sequel. At the lake of Chambon, for instance, where 

 the lava of Tartaret blocked up the whole valley of the Couze, 

 that river has been unable, near its outlet, to wear to the depth 

 of more than a few feet through the barrier ; wliereas, not far 

 below, at Sailhens, where the body of water i;? not greater, it has 



