M. A. Guyot on the Erratic Basin of the lihine. 25 



crystalline rocks, among which the bottom of this great val- 

 ley lies. This rock, of a coarse slaty structure, is remarkable 

 for the great abundance of brown mica, which gives its gene- 

 ral colour to the mass ; it is distributed in large shining 

 plates, and in pretty extensive layers ; it is less rich in fel- 

 spar than in quartz, which often forms large irregular crys- 

 tals, the size of which interrupts the regularity of the laminae 

 of the rock. 



We may add to the three preceding species, as a rock which 

 usually accompanies the two first, the rose-coloured and 

 greenish talc-slates and conglomerates detached from the 

 heights which boi'der the left side of the valley of the anterior 

 Rhine, and which seem to belong to the formation which 

 predominates in the mass of Sernfthal. The progress of these 

 iverse species is as follows : — 



The gi'anites of Ponteljas descend from the valley of the 

 anterior Rhine, which they represent in the plain, always 

 occupying the left bank in conjunction with the rose-coloured 

 and green talc-slates. They pass the Col de Tamins and the 

 valley of Tamina, although in small number. The principal 

 mass follows the flanks of the Galanda, enters the valley of 

 the lake of Wallenstadt, covering the declivities above Flums, 

 on the left bank, as above "Wallenstadt and Amraon, on the 

 right bank. Near Wesen, they are driven back by the red 

 conglomerates of the Sernfthal, which issue from the valley of 

 the Linth, and follow the limit of the basin of the Rhine along 

 the heights indicated above, becoming all the time less nume- 

 rous and more insulated. I met with some blocks as far as 

 the heights of Chateau de Kybourg, and in the neighbourhood 

 of Winterthour. But they do not fill this branch of the basin 

 of the Rhine only ; we still meet with them, although much 

 rarer, and mingled with the granites of Julier, on the right 

 bank of the Rheinthal, along the sides of Sentis below Wild- 

 haus, and the heights of Stoss. They are still frequent on 

 the heights which surround St Gall, and along the right bank 

 of the basin, as far as the vicinity of Winterthour and the 

 hill of Irchel, when they meet with those which have followed 

 the first route by the valley of Wallenstadt and the Gaster. 



The Julier granites descend the broad valley of Oberhalb- 



