^^^^- 55-] GEOLOGY OF THE DAVOS DISTRICT. 391 



limestone and Yerrucano has been already mentioned, occur crystal- 

 line rocks which are less coarsely crystalline than the ancient 

 schists ; and in the slightly weathered surfaces exposed by the 

 railway-cutting, the observer finds again the development of the 

 dark and ferruginous patches which characterize the Casanna 

 Schiefer in other localities. 



It seems, then, that although it is scarcely possible to give an accu- 

 rate definition of the variable Casanna Schiefer, while it is certainly 

 hard to trace a division-line between them and the older crystallines 

 on one side, and perhaps occasionally difficult to mark their junction 

 with the Yerrucano on the other, yet they constitute a definite local 

 horizon with certain characteristics observable in the field, and 

 always occupy a place between the Yerrucano and the ancient 

 crystallines. 



The geological age of these rocks is not easy to determine. While 

 some of them resemble those regarded in other parts of the Alps as 

 Carboniferous,^ some pass by gradations into the great mass of the 

 older crystallines. 



{b) The Rocks of Limited Distribution. 



In the foregoing pages I have dealt with those rocks which 

 more or less certainly represent great geological formations else- 

 where recognizable, and are within this district to be traced in 

 constant sequence from north to south and from east to west. 

 There remains, however, a residuum of rock-types the study of 

 which presents great interest, but at the same time great difficulty. 

 They are found along a line following the general north-easterly 

 and south-westerly strike from Klosters to the neighbourhood of 

 Arosa, always in association with the westernmost development of 

 the dolomite, and may be grouped generally under the following 

 headings : — (i) The Serpentine, (ii) the Eed-and-green Schists, 

 (iii; the Ophicalcareous Grits, (iv) the Radiolarian Hornstones, 

 (v) the Breccias, (vi) the Diabases and Yariolite, and (vii) the Talc- 

 granite. 



(i) The Serpentine. 



The Alpine serpentines possess great interest, not only on account 

 of the problems connected with the nature and origin of the basic 

 magmas in the crust of the earth, but also because of their wide and 

 yet restricted distribution, the difficulty of making sure of their 

 relationship to surrounding strata, and their diwstinct unwillingness 

 to yield to the enquirer the secret of their age. While their zone 

 of distribution is singularly narrow as a rule, the east-and-west 

 extent of their outcrop is very great. 



The serpentine of the Eastern Alps is a more continuous band 

 than has been generally supposed, and that of Davos is probably 

 connected with the serpentine which appears in the Lower Engadin 



' Specimens from the Korbshorn and from the lower Ootschna breccia 

 which I showed to Prof. Heim were at once compared by him with the rocks 

 believed to be Carboniferous in the Glarua area and elsewhere. 



