Excavation of Valleys. 17 



height now exposed is sufficient to strike an observer inexpe-^ 

 rienced in volcanic phenomena with astonishment. They who 

 have examined modern lavas are aware, that the surface and 

 sides of the stream consist, even while it is moving on, of a 

 thick coat of scoriae and solid lava, and often to a considerable 

 depth, of broken angular masses of heavy lava, so that the ex- 

 ternal form of the current bears no resemblance to that assumed 

 by a liquid mass. 



Below the Etang de Fung, the " cheire" of Come occupies 

 the whole valley down which the Sioule formerly flowed, con- 

 tinuing in full force past the village of Mazayes, which is built 

 upon it. We have here, therefore, an opportunity of observ- 

 ing the original form of a stream of lava, which has taken en- 

 tire possession of a river-gorge, and has subsequently remained 

 undisturbed. Notwithstanding occasional irregularities, the 

 central part of the " cheire" swells to the greatest height, and 

 on approaching the gneiss on either side, it generally slopes at 

 a considerable angle, sometimes not less than 40°. As the gneiss 

 usually meets it at a steep inclination, a lateral valley is formed 

 on each side, destitute of water. If the Sioule, when first 

 dammed up, had been compelled to find an exit in this direc- 

 tion, it would naturally have taken its course through one of 

 these, passing between the lava and the gneiss. But we may 

 also affirm, that it would have flowed down.i4;he western side, 

 for when a " cheire" descends into a valley in a course at right 

 angles to that of the river which it dams up, the barrier of the 

 lake thus caused is necessarily of much greater height on the 

 side where the lava enters. Upon this principle, we may ex- 

 plain why the basalt of the volcano of Chaluzet in Auvergne, 

 and of Jaujac in the Vivarais is confined to one side of the ri- 

 ver, viz. that on which are the respective volcanic vents. In 

 these cases, the rivers opened a new channel between the lava 

 and the primary rock, on the side opposite to that on which the 

 lava first entered. From considering the above facts, it will ap- 

 pear, that when a river has eaten out a new channel, and the lava 

 presents a perpendicular section on one side, we may easily ex- 

 aggerate the quantity of matter removed, unless we bear in 

 mind the probable form of its original termination. It is only 

 when some remnants of tlie lava are observed on both sides of 



APRIL — JUNE 1829. B 



