in the Erratic Basin of the Bhone. 255 



of the kinds of rock most distinctive of the basin of the Rhone. 

 They fire composed of a sandstone, often slaty, of a fine grey, 

 very micaceous, sprinkled here and there Avith slaty spots, of 

 greater or less size, and of a deep and dull black, interposed 

 between the laminse. These sandstones contain pebbles and 

 fragments of quartz, gneiss, and other primitive rocks, the size 

 of which varies from that of fine gravel to the bigness of the 

 head. These pebbles are usually so numerous that the slaty 

 structure disappears, and they are so firmly cemented that 

 the hammer cannot detach them without breaking their bed, 

 and at the fracture they appear like spots whose edges are 

 not always clearly marked. The whole forms a rock of great 

 hai'dness. Their primitive site is not confined to the valley 

 of Valorsine ; the rock is likewise in situ on the right bank 

 of the Rhone, above Outre-Rhone, near the Dent de Morcles, 

 and on the mountain of Foully. In two localities it is ac- 

 companied with conglomerates and schists of wine-red, be- 

 longing to the same fofmation. It is from the latter, that is, 

 the right side of the valley, that the greater part of W\q. nu- 

 mei'ous blocks of this species which are in an erratic state in 

 the basin of the Rhone, seem to have been detached. 



9. We must, in the last place, indicate, as a character of 

 the basin of tlie Rhone, which no other neighbouring basin 

 shai'es with it, at least in the same degree, the remarkable 

 abundance of pebbles of all sizes, of a quartz usually yellow- 

 ish, which are distributed over the entii'o surface, and the 

 presence of which, at the outskirts of the basin, invariably 

 announces the proximity of other erratic rocks. 



Distribution of the Species. — The distribution of the spe- 

 cies I have named in the plain is by no means accidental. 

 Here also there is no disorder, no absolute mixture, but an 

 order and a method, which takes place according to certain 

 laws. No doubt v.'e cannot look here for distinct limits of 

 distribution, like those which separate the diffei-ent basins, 

 but we can lay down the following propositions : — 



1. A particular species abounds in one region of the basin, 

 ajid is found rarely, or not at all, in another. 



2. The blocks of diverse species, on leaving the place of 

 their origin, have a tendency to form parallel series, and 

 when thoy reach the plain, tliey spread considerably, but do 



