Vol. 54.] THE STEUCTFItE 01* THE DAVOS "VALLET. 287 



material of great thickness, but for which the lake would drain to 

 the north and would carry with it the waters of the Fluela and the 

 Dischma ; that the contours suggest the former existence of a larger 

 lake stretching south towards Prauenkirch, and that in that part 

 there is evidence of the previous existence of a great detrital fan 

 sufficient to account for the formation of the lake in question. 



It remains to consider whether there is any method of arriving 

 at a knowledge of the fundamental structure of the area, and whether 

 there is any collateral evidence derivable from other sources. 



With the former object in view, it occurred to me that something 

 might be learnt by making a series of transverse sections across 

 the valley. By taking a number of fixed points at a constant level 

 on both sides, and drawing lines down from these to the points 

 where the rock in situ disappears beneath the superficial accumu- 

 lations, one can form an idea of the slope of the sides of the valley. 

 By prolonging the lines under the central alluvium one might 

 arrive at the approximate level of the rock-floor. That such a 

 method would be exact for any particular section is not suggested, 

 but a number of sections would give a relative series of points 

 indicating the real position of the solid rock. 



The contour-line of 2100 metres (about 7000 feet) is one which 

 can be traced easily on the map : it corresponds roughly to the 

 upper limit of the pine-woods ; and at this level there is, all along, 

 a marked alteration in the slope of the mountains, which evidently 

 represents some period of change in the denudation of the district 

 far earlier than the time which we are considering. 



For these reasons I selected it as a fixed point, and drew to scale 

 the nine sections marked on the sketch-map (fig. 1, p. 281). It is 

 often difficult to fix the position of the ' junction ' of the solid rock 

 with the surface-accumulations, and I do not claim any great 

 accuracy for the separate sections. The general result, however, is 

 distinctly confirmatory of the opinions already arrived at. 



Thus the section (C) across the lake from the Seehorn indicates a 

 depth of 150 metres, and this agrees with the results of soundings. 

 Between the Dorfliberg and the Biihlenberg the rock-floor seems to 

 lie at the same depth, while between Davos Platz and Clavadel it is 

 reduced to 120 metres. A section along the bed of the Sertig and 

 across to the Erbberg gives the thickness of the loose material 

 beneath the river as less than 100 metres. These measurements 

 may be far from the truth in themselves, but as a comparative 

 series they indicate that the solid rock-axis is south of Davos. 



Lastly, in trying to determine the former direction of drainage 

 in the district, we have to consider the evidence of ice-movement. 

 Here we find facts entirely in support of the theory of a former 

 northerly discharge. 



The great intrusive serpentine of the Todtalp has well-defined 

 boundaries and the rock is unmistakable.^ It, has been rapidly 



^ See Ball, ' The Serpentine & Assoc. Rocks of Davos,' Zurich, 1897. 

 Q. J. G. S. No. 215. * 



