Scrope — Origin of Valleys, 197 



drained towards the commencement of tlie volcanic era, there are no 

 other causes to which we can refer this gradual excavation than 

 those which are still seen in operation wherever rains, floods, frosts, 

 and atmospheric decomposition act upon the siu'face of the earth. 

 To these agents, then, we must refer the effects in question, of which 

 with an unlimited allowance of Time, no one will pronounce them 

 incapable." (Scrope, on the Geology of Central France, 1827.) 



The facts, on which this argument is based, may be obsei-ved in 

 the models in relief of this district, which I have deposited in the 

 Jermyn Street and South Kensington Museums, as well as in the 

 accompanying Plate, in which the profiles of the principal heights, 

 within the distance of about seven miles north and south of Cler- 

 mont, are. brought together. All these form distinct steps in the 

 evidently gradual process by which the -freshwater formation has 

 been excavated, and the n;Lain valley of the AUier with its tributaries 

 formed. Each hill is capped by a plateau of basaltic lava, which 

 acting as a protection to the mass beneath it against the erosive 

 power of the vertical fall of rain, has preserved the portion of the 

 surface which it covers in the state at which it existed at the period 

 when it was flooded with lava from the neighbouring heights. 



I showed in the same work that the same inference was deduci- 

 ble from an examination of the lava-currents of the Bas Vivarais, 

 and of those in the vicinity of Le Puy, namely, that " the erosive 

 force of the streams which now border or intersect them, together 

 with the action of rain, frost, and other atmospheric forces, have 

 alone hollowed out there extensive systems of deep, and in some 

 instances (as that of the Loire itself near Le Puy) wide valleys." 



Mr. Mackiiitosh, as well as some other writers who depreciate the 

 erosive influence of " rain," though admitting to some extent that of 

 " rivers," seems to forget that intermediately between the two there 

 are constantly at work the millions of rills, rivulets, brooks, and 

 torrents, which convey the pluvial water through rivers to the sea, 

 and in seasons of flood are constantly carrying solid particles, silt, 

 sand, pebbles or stones along their beds, which grinding against the 

 rocks that form the sides or bottom of these, exercise a very con- 

 siderable force upon them. Those who have watched the descent of 

 floods in any mountain region, such as the Alps or Apennines, espe- 

 cially after violent local rains, or the fall of a' waterspout, will have 

 observed that the torrent appears to be composed as much of stones 

 as of water. Every pebble and boulder is in motion, all descending 

 more or less rapidly to a lower level ; all exerting an abrading force 

 upon other stones, or on the banks of the stream. In the Vivarais 

 I have seen blocks of gTanite, many tons in weight, moved to con- 

 siderable distances by torrents which are dry in summer. The large 

 l^risms of basalt, separated by the action of frost, or an imdermining 

 stream from a cliff of this rock, lie for some time perhaps at its 

 base, but are soon carried lower by a flood, and at the distance of a 

 mile or two are seen to have been worn down to rounded blocks. 

 Further down they appear as boulders, and still further as pebbles, 

 which sooner or later find their way into the Khone, and ultimately, 

 in the shape of sand or silt, to the sea. 



