412 THE GEOLOGY OF THE DAVOS DISTRICT. [A.^g« ^^9^i 



EXPLANATION OF PLATES XXVI & XXVII. 



Plate XXVI. 



Geological Sketch-map of the Davos District, on the scale of tttttqqq, 

 or 1'6 miles to the inch. 



Plate XXVII. 



Section of the Mountain-folds in the Davos District, on the scale 



(vertical and horizontal) of , , or 1'6 miles to the inch. 



Discussioif. 



Prof, Cole commented on the tendency in Alpine sections to 

 represent the Casanna Schiefer as conformable below the Verrucano, 

 and enquired as to whether this was not merely a generalization, 

 based upon local occurrences of the one rock below the other. He 

 presumed that the Casanna Schiefer belonged in this case to the 

 underlying crystalline series. 



Dr. HiNDE also spoke. 



Prof. BoNNEY said that the term * Casanna Schiefer' appeared to 

 him rather vague, but it generally designated rocks of the crystalline 

 group. He expressed his sense of the value of the communication 

 which he had now been the means of making known to the Society, 

 saying that on one point only he felt some doubt — namely, as to 

 whether the serpentine was intrusive in the Triassic rocks. Of the 

 peculiar ' ophicalcareous grits ' on the western side of the serpentine- 

 mass, three explanations might be offered: (l)that they were ' crush- 

 grits,' in which case, as it seemed to him, no conclusion could be 

 drawn ; (2) that some afforded evidence of contact-metamorphism, 

 which would prove the Author's point, but on which he thought there 

 was room for much doubt ; (3) that the serpentine was present as 

 fragments of a pre-existing rock, to which view, for reasons given, 

 he personally inclined, though he admitted the question to be 

 difficult, inasmuch as he had not visited the sections. Having 

 expressed dissent on this point, it was only just that he should 

 conclude by reading an extract from an answer which he had 

 received from the Author : — 



' With respect to the most difficult question dealt with in the 

 paper, he regretted much that his conclusions did not entirely 

 agree with those arrived at by Prof. Bonney from microscopic 

 examination of th« red-and-green Schiefer. He fully recognized 

 the value and interest of Prof. Bonney's theory as to the origin 

 of these rooks, and was quite prepared to admit that it was a 

 possible explanation and perhaps the true one. He felt, however, 

 that the field-evidence for the intrusion of the serpentine through 

 at least the Lower Trias was very strong indeed. If the serpentine 

 were Palaeozoic, and had contributed material to the formation 

 either of the Verrucano or the Lower Triassic strata, he thought that 

 the fact would be easily recognizable in other localities. The very 

 curious rocks in the neighbourhood of the Todtalp, which Prof. 

 Bonney considered as favourable to his view, were so local in 

 distribution, so restricted to a region of evident dynamic disturbance, 

 intrusions, overthrusts, and brecciation, that the Author was inclined 

 to regard them as due to these agencies.' 



