24 M. A. Guyot on the Erratic Basin of the Rhine. 



high Rhetian Alps by the valley of the Rhine, three may be 

 named as peculiarly characteristic of this basin. These are 

 the porphyroidal granites of Pontelja, or of Trons, the green 

 granites of Juliers, and the brown gneiss of Montafun, three 

 species, each of which corresponds to one of the principal 

 affluents of the valley of the Rhine, as they have been named 

 above. 



The porphyroidal granites are a species of protogine, dis- 

 tinguished at first glance, by narrow and elongated rectangu- 

 lar crystals of white felspar, usually mdcles, from a few lines 

 to an inch or upwards in length, and which are distinctly de- 

 lineated in the granitic mass. The quartz is ingrains, pretty 

 numerous, but of small size ; the deep-green mica is dissemi- 

 nated in flakes or masses ; a talcose substance, as in the pro- 

 togines of Mont Blanc, tinges a part of the masses with a 

 delicate green, but without ever altering the whiteness of the 

 large made crystals ; small linear crystals of black amphibole 

 appear numerous in some specimens, very rare in others ; final- 

 ly, we notice here and there, in nearly all, a few minute crys- 

 tals of yellow sphene. 



According to the observations of M. Arnold Escher, these 

 porphyi'oidal granites come from the ravine of Ponteljas, 

 scooped out of the southern mass of the Doedi, above Trons, 

 in the valley of the anterior Rhine. This locality seems to 

 be the only one that produces them, and indeed I did not 

 find a fragment of them in this valley behind Trons, nor in 

 any other of the Orisons. 



The granites of Julier are distinguished from the preced- 

 ing by the absence of the large twin crystals of felspar, by 

 the abundance and size of the quartz crystals, but above all, 

 by the predominance and bright hue of the green talcose sub- 

 stance which colours almost the entire mass of felspar, and 

 communicates a green colour to the rock, which is not ob- 

 served in the Ponteljas granites. They are known also, on 

 the first stroke of the hammer, by their very great tenacity, 

 which is a property not possessed by the latter. These granites 

 belong not only to Julier, but in a considerable degree to the 

 northern chain of the Engadine. 



The gneiss of Montafun has its origin in the masses of 



