18 Messrs Lyell and Murchison on the 



theinew exfc^ktion, that we can form a very exact estimate ol 

 tba extertt of the Vaste. 



The waters' of the Sioule being, as before mentioned, dami 

 mfed up at the Etang de Fung, opened for themselves a new 

 passage nearly at right angles to their former course, in order to 

 join the Monges. It becomes, therefore, an interesting matter 

 of inquiry, to learn how the new valley was formed, what was 

 the nature of the argillaceous hill mentioned by M. de Montlo- 

 sier, and whether the entire mass was softened, as he supposed, 

 previous tb excavation. As no one has yet described this 

 locality, we may be permitted to offer a few details concerning 

 it. The barrier which originally separated the courses of the 

 two rivers, consisted of alluvial clay resting on gneiss, so that 

 the deflected stream hollowed out its bed sometimes entirely 

 through clay, sometimes through the subjacent primary rock. 

 The Sioule is now seen immediately after its entrance into the 

 new valley, washing on its right bank the foot of a precipice of 

 alluvium 140 feet perpendicular, undermined, and in a state of 

 gradual decay. This bank consists of blue and red clay not la- 

 minated, but shewing by slight shades of colour that it was de- 

 posited horizontally, and exhibiting here and there some irregu- 

 lar sandy beds, about one foot thick, containing small pebbles 

 of gneiss and quartz, but no volcanic fragments, so that we may 

 conclude the whole to have resulted from the waste of primary 

 rockfe*. Not far from this section, the subjacent gneiss is seen 

 on the same bank, worn through to the depth of twelve feet. 

 The river next flows for some distance through an alluvial plain, 

 boundetl by hill S3 of clayey and then passes through a narrow 

 ^3rge:of gneiss, which rock on the right bank overhangs the 

 riverin a diff 40 feet perpendicular, still slowly wasting. A 

 platesEU cf ancient basalt is seen at the haght of about 250 feet 

 or mare above, the Sioule on either bank, and from the general 

 Structure of the country, we may infer that here, as at Mount 

 Perrier (afterwards to be mentioned) alluvial deposits between 



• Primary clay-slale exists at the distance of a few miles from the Etang 

 de Fung, on the western slope of the granite of the Puys of the Monts 

 l)ome. It may be seen between Montari and Brameau, where it is ver- 

 tical. The destruc tion at some former period, of parts of this formation, 

 may have occasioned the immense preponderance of argillaceous matter in 

 the above-mentioned alluvium. 



