192 PEOF. T. G. BONNET ON CRYSTALLINE SCHISTS AND THEIR 



strata in the Alps, — correlations which were of the highest theoretic 

 importance, and had affected the more recent geological maps pub- 

 lished in Switzerland, — we began our work by carefully examining 

 these slopes. Before giving the results it is necessary to jjreface 

 them, for the sake of those unacquainted with the district, with a 

 brief description of its physiography. 



The upper waters of the Rhone, Eeuss, and Rhine occupy an oro- 

 graphical trough between the range of the Lepontine Alps and that 

 which prolongs the line of the Central Oberland. Through a nar- 

 row gap in the latter the Reuss escapes northwards ; and the passes 

 of the Purka on the west, and the Oberalp on the east, limit its 

 basin, and form watersheds in the trough. This is bounded north and 

 south by schists, gneisses, and gneissoid granites ; but there is a 

 rather marked lithologieal difference, as has often been pointed out, 

 between the rocks of the two massifs. Enfolded between them 

 and not reaching to so great an elevation above the sea, are various 

 schists, sometimes more or less calcareous, which have a more 

 modern aspect than the above-named rocks, though they are often so 

 much crushed that any assertion as to their original condition 

 demands great caution. Furthermore, a long strip of dark Jurassic 

 rock, often calcareous, — sometimes a blackish limestone (not unlike 

 one of those in the English Lower Carboniferous series), — extends 

 from the valley of the Rhone over the top of the Eurka Pass, is 

 restricted after a time to the left bank of the Reuss, and finally dies 

 out on the hillside about due north, so far as I can see, of the vil- 

 lage of Andermatt, its disappearance being of course due to denu- 

 dation. If then, as a glance at Yon Fritsch's map will show, this 

 Jurassic limestone is identical with the white marble of Altkirch, 

 there must have been a displacement of the latter southwards of 

 two or three hundred yards. This, however, would not be a diffi- 

 culty of any real importance ; I only mention it to show that the 

 masses at present are not really in the same line of strike. Judging 

 from the outcrops, the slopes up which the road winds to the Ober- 

 alp Pass are mainly composed of gneissoid rock, which obviously 

 has been greatly modified bj pressure *; between this (the " sericite 

 gneiss " of the Swiss map) and the Urnerloch is the marble in 

 question, with certain apparently associated rocks ; while on the 

 other side of the " sericite gneiss," between it and the Lepon- 

 tine massif, is phyllite and black slate, referred by the Swiss geolo- 

 gists to the Carboniferous series t. According to Yon Eritsch, the 

 former group of rocks consists of a lenticular strip of marble, on the 

 south of which is a band of Jurassic rock, which in one place is 

 fringed by rauchwacke. Where the marble narrows to a point at 

 its eastern extremity, a mark indicates the occurrence of fossils. 



* More than one variety of rock is present, but I have thought it needless 

 to enter upon details. The most conspicuous feature in the part nearest to the 

 marble is a cleavage-foliation ; but on examining a fractured surface at right 

 angles to this, we see that the rock has been a granite or granitoid gneiss. 



t Perhaps also some of the newer group of schists occur here, but I did 

 not carefully re-examine this section last summer, no controversy existing in 

 regard to it. 



