16 Messrs Lyell and Murcluson an the 



llic shape of a long dike or mound, rising in the middle be- 

 tween 100 and 200 feet high above the plain ; and its sides, 

 which are about 50 feet high, instead of slanting down gently 

 to the subjacent granite, descend at the rapid angle of 40° or 

 45°. The same lava, in a lower part of its course, presents a 

 long steep face towards the lake of Aidat, of which it occasioned 

 the formation, by damming up the course of the stream. 



Several of the " cheires," descending from the western gra- 

 nitic plateau from the chain of Puys of the Monts Dome, exhi- 

 bit the same phenomenon in a striking manner ; but it will be 

 sufficient to allude to that branch of the lava-current of Come, 

 which chansed the course of the Sioule, and of which M. de 

 Montlosier has given so faithful and animated a description *. 

 " But what," says he, " has this branch of the Iava-cun*ent of 

 Come effected ? After encrusting all the higher portions of 

 Mont Ceysatt, it descended to the bed of the Sioule, and barred 

 the course of that river, by presenting to it an impenetrable 

 barrier. The waters thus checked, must, in rising gradually, 

 at length have overrun this parapet, had they not found, some- 

 what higher up upon the left, an argillaceous hill, through 

 which they insinuated themselves, and in which, after softening 

 the entire mass, they made enormous excavations, and finally 

 poured themselves into the bed of the Monges one and a-half 

 league higher than the original point of junction of that stream 

 with the Sioule." 



The barrier of lava before mentioned, caused above the point 

 where it stopped the course of the Sioule, a stagnant water 

 called the Etang de Fung. The side facing towards the Etang 

 is a steep rocky slope 80 feet in height, the general inclination 

 of Avhich is at an angle of 45°, and entirely devoid of vegeta- 

 tion. The lava is compact, and does not appear to have suf- 

 fered at all from decomposition, and as there has been no 

 stream at the bottom to undermine and remove a talus, any ex- 

 ternal waste can only have had the effect of diminishing the ori- 

 ginal declivity. On the other hand, this barrier of lava would 

 present a still loftier precipice, were not its base concealed by 

 a deposit of white alluvial clay, formed in the Etang de Fung 

 before its drainage, the depth of which is unknown. But the 



* Essai SUV la Theorie des Volcanos d'Auvergne, p. 33. 



