yo PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



miles, on the one side over the Ober-Alp Pass down the valley of the 

 Upper E-hiue, on the other over the depression of the Eurka Pass 

 down the Upper Rhonethal to within a very few miles of Brieg*. 

 Infolded with this, upon the western side of the Ober-Alp Pass, is a 

 mass of Jurassic rock, which may be traced from the lower slopes 

 of this pass along the upper valley of the Heuss over the Furka Pass 

 as far as Obergestellen. This series, where I have examined it, con- 

 sists of a black limestone, often slightly cleaved, which, lithologically, 

 is not unlike some of the darker varieties of British Carboniferous 

 Limestone, and of a satiny black slate. According to the Swiss Geolo- 

 gical Map, it is, in one place, associated with the cream-coloured 

 calcareous rock named raucliwaclce or rauliivacke. The schists of the 

 supposed Casanna group, notwithstanding the crushing which they 

 have undergone, are readily distinguishable from the most slaty mem- 

 bers of the Jurassic series, so that they are evidently separated from 

 the latter by a vast interval of time. 



Bounding, on the southern side, the troughed series of schists men- 

 tioned above, rises another mass of crystalline rock, orographically of 

 even greater importauce than the last, for it is the watershed of this 

 portion of Europe. The St. Gothard Pass affords one of the shortest 

 and simplest sections of the central part, and its tunnel has given- 

 a section yet more complete, which is nowhere quite so much as 

 two miles away from the course of the high road. The Lukmanier 

 on the east, the Gries and Simplon on the west, besides other less 

 familiar passes, afford excellent opportunities for parallel and compa- 

 rative sections. The northern slopes of the St. Gothard, from a 

 short distance above Hospenlhal, where the supposed Casanna group 

 ends, give us a series of mica-schists and micaceous gneisses 

 which, as we ascend, become rather less distinctively micaceous. 

 These rocks, which may be taken as beginning at about 900 feet 

 above Andermatt, and extending to a little below the highest part 

 of the pass (about 2200 feet above Hospenthal), afford us numerous 

 most interesting associations of very micaceous schists with gneiss 

 (sometimes very porphyritic, the crystals of felspar being occasional^ 

 well defined and quite two inches in length). At present I will say 

 no more than that we are compelled to adopt one of the following 

 explanations : — either we have here the result of the metamorphism 

 of successive beds of different, materials — in which case foliation 

 corresponds with stratification — or a granite has been intruded, often 

 rather uniformly, into a mica-schist (which, whatever may have 

 been its origin, was at that time a mica-schist), and both have been 

 subsequently modified by pressure. The strike of the apparent 

 bedding and foliation is roughly S.W.-N.E., the dip very high, 

 usually inclining somewhat on the S.E. side. The plateau forming 

 the uppermost part of the pass is occupied by the peculiar porphyritic 

 granitoid rock to which the Swiss geologists have given the name 



* Of rocks so crushed I speak with much hesitation and deference, but I have 

 sometimes doubted if these may not be newer than the Casanna series, and, 

 at any rate in part, represent the still higher schists of which 1 shall presently 



