390 ME. A. V. JENNINGS ON THE [Aug. 1899, 



important fact to which reference will be made later), as well as its 

 association in parts with ' talc-quartzite.' 



Apart from this most conspicuous mass, it may be mentioned that 



(1) it is well developed as a zone surrounding the Ducan basin ; 



(2) it can be traced on each side of the Strela ridge, being found in 

 the Guggerthali on the south, and east of the Schiabach on the north, 

 of the Davos dolomite ; (3) in connexion with the Casanna-Cotschna 

 limestone-ridge it is found on the north side, though often mixed in 

 breccia with other rocks, while on the south side the position in 

 which it would be expected is occupied by red shales and cherts ; 

 (4) it is to be found (in close association with the ' talc-quartzite,' 

 and also containing red hornstone) along the slopes of the Eastern 

 Ehatikon above Monbiel and Klosters. 



Attention is specially directed to this distribution of the Yerrucano 

 in the Davos district, as it is of much importance in relation to the 

 stratigraphical questions which are discussed on a subsequent page. 



(v) The Casanna Schiefer. 



The name which has come into general use for a series of crystal- 

 line or semi-crystalline rocks found constantly in Graubtinden, 

 where the lower sedimentaries, in the form of Yerrucano, appear in 

 contiguity with the ancient crystallines, might be naturally thought 

 to refer to the Casanna Alp on the south of the Pratigau. This, 

 however, is only one of the many cases in which the repetition of 

 local names in different regions of Switzerland leads to misunder- 

 standing. The formation in question derives its name from the 

 Casanna Pass in the Upper Engadin. It is difficult to give any 

 exact description of the character of the schists that would be applic- 

 able over an extended area. In parts there are foliated rocks so 

 completely crystalline as to be classified as fine-grained gneisses, 

 while elsewhere there are thin black shaly strata of a much less 

 ancient appearance. So far, however, as the Davos district is con- 

 cerned, the predominant rock is a micaceous schist with (in parts) 

 evident graphitic layers and often a great quantity of yellow 

 hydrated iron-oxide. These characteristics were not noted by 

 Theobald, and, so far as I know, have not been elsewhere made the 

 subject of remark ; but in reference to the conclusions in later 

 parts of this paper, it seems to me desirable to emphasize the fact 

 of their occurrence. 



On the Korbshorn, for instance, it is possible to obtain hand-spe- 

 cimens which consist entirely of a white mica-schist with layers and 

 masses of graphite and ochre. The cuttings on the road from Glaris 

 to Monstein show also large rounded masses, apparently concre- 

 tionary, with a similar black and ochreous character. The yellow 

 colouring of the rock now quarried above Erauenkirch, and the vast 

 accumulations of ochreous earth in the Albertitobel, are similar 

 evidences of the distribution of this type of rock. On the eastern 

 side of the Davoser See, where the presence, in thin layers, of the 



