Mr. J. Ball on the Formation of Alpine Lakes, 495 



diluvium, and were subsequently cleared out by glaciers, we 

 ought to find the diluvium still in situ in those portions of the 

 lake-bed which from local conditions must have been protected 

 from the advance of the glacier. As an illustration, I referred 

 somewhat at random to the rocky promontory of Balbianello, 

 which projects from the west shore of the Lake of Como nearly 

 at right angles to the general direction of the lake. If the 

 glacier had descended from the north as a gigantic graving-tool, 

 and had scooped out the diluvium from the channel of the lake, 

 the promontory in question would have protected the diluvium 

 lying on its south side, and it should still be found there at the 

 present time. M. de Mortillet at once accepted my argument 

 as a fair test of his theory, and has given what appears to be a 

 sufficient answer, by stating that the diluvium, or ancient allu- 

 vium, is actually to be found in the locality which I had quoted. 

 During the last summer I repaired to the place in question with 

 the Marchese Giammartino Arconati, who inhabits the well- 

 known villa on the promontory, provided with about 113 metres, 

 or 370 feet, of sounding line. 



It is not necessary that I should give the detail of all the 

 soundings made by me on that occasion ; but I desire to direct 

 attention to four which have the greatest significance. It will 

 be sufficient to say that, on sounding from the point of the pro- 

 montory along its south shore, I found the rocks everywhere 

 descend very steeply, apparently at an angle of about 60° with 

 the horizon. Not only the steepness of the declivity, but the 

 suddenness of the descent from one point to another, showed 

 that we were over ledges of rock rising in a precipice as nearly 

 vertical as is commonly found on the steepest face of a limestone 

 mountain. 



Having passed rather more than halfway along the southern 

 shore of the promontory, we found a slight concavity, or very 

 diminutive cove, where lay on a slope, stretching for some yards 

 from the water, a small mass of shingle, consisting of waterworn 

 stones with a little finer gravel. This is evidently the diluvium 

 referred to by M. de Mortillet, and believed by him to be the 

 remains of the vast mass which once filled the entire basin now 

 occupied by the lakes of Como and Lecco. I immediately pro- 

 ceeded to make soundings at successive points in a line receding 

 from the shore, opposite the shingle bank, measuring the exact 

 distance from the shore at which each sounding was made. The 

 result gives of course a rough outline of the vertical section of 

 the bed of the lake opposite to this point, and is best seen in the 

 annexed diagram, where the distance from the shore and the 

 depth at each poiut are given in English feet, the dotted line 

 showing the outline of the bed. 



