386 ME. A. V. JENNINGS ON THE [Aug. 1 899, 



out of recognition. The rocks form only small patches overlying 

 the dolomite of the Ducan district, and certain crushed infolds 

 diiScult to trace in the dolomite-ridges of the west. 



If one approaches the Ducan from Monstein by either of the 

 valleys which converge there, one crosses a broad band of the older 

 crystallines, then Casanna 8chiefer and Verrucano, finally reaching 

 the great walls of dolomite with their (in parts) almost vertical 

 strata. 



In the northern valley, above the little lake at the foot of the 

 Barenthali, are found numerous blocks of a grey limestone con- 

 taining a branching-coral. These belong to the Lithodendron- 

 kalk, and form part of the Rhsetic Dachsteinkalk formation. The 

 corals are usually too much obliterated for palaeontological study, 

 but from comparison with those of other districts there is little 

 doubt that they may be referred to Calamophyllia. Among these 

 blocks are dark or black limestone-shales full of crushed shells, 

 which stand oat distinctly on the weathered surfaces, but are 

 not sufficiently well preserved for identification. There can be 

 little doubt, however, that the beds were rightly referred by 

 Theobald to the Kossener Schichten or Avicula cotitorta-shaiies. 

 These strata, much tilted and contorted, can be traced by the 

 eye, and are seen to form the upper part of the surrounding peaks 

 and to extend upward and northward to the ^Iplihorn. The 

 crumpled cap of the Krachenhorn, on the south, is composed of 

 the same beds. 



West of the Davos Valley the Lithodendron-kdilk is present on the 

 Strela Pass and on the north side of the Cotschna ridge. I have 

 not, however, traced the Kossener Schichten in these localities. It 

 is possible that both occur in the Amselfiuh fold, but I have not yet 

 observed them there. 



(2) The Hauptdolomit. — As elsewhere in the Eastern Alps, 

 this is the most conspicuous member of the Triassic system, forming 

 many of the higher mountains of the district and weathering cha- 

 racteristically into angular peaks like the Tinzenhorn and Schiahorn, 

 or great precipitous rock- walls such as the Ducan, the Kiipfenfluh, 

 and the Amselfiuh. The stratified nature of these, and the folding 

 of the beds of which they are composed, are frequently conspicuous 

 at great distances owing to the lingering snow-lines on shaded 

 ledges. The Ducan and the Piz d'Aela are prominent examples. 



The rock itself is light or dark grey, often streaked in all direc- 

 tions with white calcareous veins and sometimes full of graphitic 

 films, but devoid of fossils. The degree of dolomitization varies 

 considerably, some specimens effervescing readily with cold acid 

 and others very little or not at all. The specific gravity ranges, 

 according to Dr. Ball, from 2'75 to 2*84. In the lower portion the 

 rock may possess a more or less fiaggy structure, and may be diflicult 

 to distinguish from the underlying limestones of the ' Mittelbild- 

 ungen' ; elsewhere the distinction is sharply marked. 



The great masses of the Hauptdolomit appear in some places 

 isolated like those of the Ducan, in others they form a confused 



