256 M. A. Guyot on the Distribution of Bocks 



not fail to preserve a respective disposition, analogous to that 

 which they occupied in their primitive sites. The blocks of 

 the right flank of the valley occupy, in the plain, the right 

 side of the basin ; those of the left flank, the left side ; those 

 of the most central valleys cover the central regions of it. 



3. Groups composed each of a single species of rock to the 

 exclusion of every other, are found here and there in the 

 midst of rocks of various species, but always in conformity to 

 the conditions of the preceding rule. 



A word on the distribution of each of the species I have de- 

 scribed will afi^ord proof of this. 



The Pennine rocks, the arkesine, chloriteous gneiss, and 

 granular chlorites, are by far the most widely diff"used ; they 

 cover three-fourths of the surface of the basin. We have 

 said that they always go together, and form a group which 

 conducts itself almost like a single species. If we take them 

 at their point of departure from the mouth of the valley of 

 Viege and Val d'Erin, we see them follow the left flank of 

 the valley of the Rhone, without ever passing to the opposite 

 side. At the outlet of the valleys of Entremont and Trient 

 they are joined by the granites of Mont Blanc, which accom- 

 pany them, and form the outer boi'der. On issuing from the 

 valley of the Rhone, they spread themselves in the plain in a 

 vast fan-shaped expansion, and fill the basin of Leman, and 

 that of the lakes Neuchatel and Bienne. We find them at 

 the same time along the exterior slopes of the Chablais 

 chains, at the foot of the Saleve, in the whole of the plain of 

 Geneva ; they crown Mont De Sion with prodigious blocks. 

 They constitute the great majority of the large blocks sus- 

 pended on the back of the Jura from Fort Ecluse to the foot 

 of the Dole, as well as the less numerous blocks scattered in 

 the plains in the country of Gex and the heights of De la 

 Cote, as far as the vicinity of Lausanne. Further to the east, 

 these same rocks, but in blocks of smaller size, and compara- 

 tively less frequent, strew the slopes of the Jura, and form, 

 along with the granites of Mont Blanc, the superior limit of 

 the erratic formation. In the plains, where granites scarcely 

 appear, they again predominate, and cover with their debris 

 the whole plain of the Aar, the molassic hills between Soleure 



