184 GLACIAL HISTORY. 



In the river-basin of the Aar the blocks coming from the 

 Bernese Oberland extend only into the neighbourhood of Berne ; 

 further to the north the Diluvial debris have had their origin 

 from the basin of the Rhone. The whole of Western Switzerland 

 is covered with materials from the mountains near the Rhone, 

 as has been shown by Prof. Guyot, who has carefully investigated 

 the kinds of rock and the distribution of erratic blocks all over 

 this region, and has discovered that they may be traced to the 

 Alps of the Valais*. He has shown that the mountains covered 

 with mighty glaciers and immense fields of granular snow which 

 separate Switzerland from Italy, between the St. Bernard and 

 the Simplon, were the chief sources of the materials of the innu- 

 merable erratics of the basin of the Rhone. These Pennine 

 Alps, and especially the mountain masses at the head of the 

 valleys of Ering and Bagne (in the Canton of Valais), were the 

 primaeval home of the blocks of talcose granite (arkesinef) which 

 are scattered over a great part of the Rhine basin, and have 

 advanced as far as Seeberg. From Monte Rosa blocks of ser- 

 pentine and of a peculiar variety of gabbro (euphotide) take their 

 origin. A white granite comes from the south side of the 

 Bernese Oberland Alps in the Upper Valais. 



From the Val Ferret a fine-grained Alpine granite has been 

 transported, which forms the vast blocks above Monthey, in the 

 Valais, and is also met with at the foot of the Jura. A similar 

 Alpine granite came from the mountain-mass of Mont Blanc, 

 and reached the lower Valais through the vale of the Trient. 

 The black-spotted grey sandstones of Val Orcine may have 

 originated from this valley, or from the slopes of the Dent de 

 Morcles ; they occupy principally the right side of the basin of 

 the Rhone, and are especially numerous in the neighbourhood 

 of Vevey in the Canton of Vaud. They form a broad zone 

 which bends at the outlet of the valley of the Rhone, and ex- 



*' See Prof. Guyot's important memoir " Sur la distribution des especes de 

 roches dans le bassin erratique du Rhone " (Bull, de la Soc. des Sci. Nat. de 

 Neuchatel, 1847). The question of the place of origin of the erratic blocks 

 has been carefully treated by F. Miihlberg in his work on the Erratic forma- 

 tions of the Canton of Aargau. 



t This is a greenish-yellow variety of granite, consisting of a mixture of 

 quartz, felspar, steatite, amphibole, and chlorite. 



