Excavation of' Valleys. 21 



terially impeded the flow of the liquid matter, and may thus 

 probably have occasioned those great protuberances and irregu- 

 larities of surface, which, although characteristic of all lava- 

 currents, are remarkably displayed in this. These peculiar 

 features are several times repeated, for whenever an expansion 

 of the valley offers itself, the lava is spread out into a broader 

 and thinner sheet, and at those points where salient promonto- 

 ries of gneiss-rock seem to have barred its passage, the volcanic 

 matter swells up to great heights. Since the mineralogical ac- 

 count of this current has been described by Mr Scrope *, 

 we shall here merely state, that in general characters it is iden- 

 tical with most others of nearly a contemporaneous origin, in 

 having an upper part light, cellular, and scoriaceous, which 

 passes downwards into a basaltic mass, occasionally divided into 

 irregular and converging columns, and frequently terminating 

 at its base in most pei'fect vertical prisms. The deepest sec- 

 tion of the lava occurs directly under the village of Chaluzet, 

 where the left bank of the Sioule consists almost entirely of 

 vertical cliffs 400 feet in depth, exposing in their higher parts 

 black and red scoriae, in their lower columnar basalt. These 

 rocks abound in caverns, some occurring in scoriae black and 

 cellular, as those of Vesuvius, whilst others, situated at lower 

 levels, are grotesquely hollowed out of the compact basalt. 

 Amongst the latter, the arcades of Pranal are remarkably pic- 

 turesque. 



The lower ends of the prisms of basalt generally terminate at 

 some height above the present bed of the Sioule, and whenever 

 not obscured by debris, they are seen to rest upon a bed of peb- 

 bles, of varying thickness, below which the subjacent gneiss is 

 cut through, down to the actual level of the river. Near Pra- 

 nal this pebble-bed is observable in the cliff, about fifty feet 

 above the stream, and the space thus denuded exposes a face of 

 gneiss, nearly vertical, in which lead-mines are now extensively 

 worked -f. If, however, no other than these longitudinal sec- 



• Geolog^j of Central France, p. 85. 



t The greatest depth of gneiss worn into by the river under tl)e basalt, 

 appeared to us, after a careful examination, never to exceed fifty or sixty 

 feet. Mr Scrope had conjectured the amount of excavation through gneiss 

 (called inica-schist l>y liim), at 200 feet. 



