John Ball — Soundings in the Lake of Como. 361 



narrowed to tlie dimensions of a river, and the soundings cease to 

 give any secure indication of the original form of the lake-basin. 



From these statements it will be inferred that the portions of the 

 lake-basin, where alone it is possible to obtain an approximation to 

 its original form, are those lying between Gera and Lecco, along 

 with the entire line of the Como branch, the distances being nearly 24 

 miles, if we measure from Gera to Lecco, and more than 31 miles 

 from the former village to Como. Future observers, may perhaps, 

 ascertain what amount of transporting power is exercised by currents 

 setting in the direction of the stream through the deeper parts of a 

 lake-basin ; but for the present it seems safe to assume that this is 

 but inconsiderable, and that nothing but fine glacial silt can be 

 transported to any considerable distance from the point where a 

 stream enters the Lake. Detailed measurements of the " cones of 

 ejection " formed by the deposit round the mouths of streams are 

 vp-anting to establish this securely, but some evidence is given in the 

 annexed plans. The promontory on which stands the village of 

 Mandello is the upper portion of the " cone of ejection " formed by 

 the Neria torrent, an impetuous stream laden with the Dolomitic 

 debris of the Grigna mountain, the remainder of the cone being 

 prolonged under the surface of the Lake. The section from Santa 

 Anna to Mandello (Fig. 14) does not cut the front of the cone at an 

 exact normal ; but it shows that at the utmost this does not extend 

 more than about 1150 yards from the shore, or about half-way across 

 the present (contracted) bed of the Lake, with a slope of more than 

 1 in 10. The section made from Onno (see Fig. 1), more nearly 

 opposite the point of the same promontory, showing a maximum 

 depth of only 90 yards, has not been published, but as it indicates a 

 slope of at least 30 in a horizontal distance of 875 yards, measured 

 along what is probably a tangential section crossing the base of the 

 cone, it confirms the inference as to the comparative steepness of the 

 subaqueous portion of such a "cone of ejection." 



With such materials as I j)Ossess for forming a judgment, I am 

 disposed to think that the cone of ejection of the Adda does not at 

 the utmost extend beyond the line connecting Domaso with Mon- 

 tecchio, the fact being that in modern times the river falls into the 

 Lake of Mezzola, and its detritus serves to fill up that portion of 

 the original Lake of Como. It is probable that a thin stratum of 

 glacial silt covers the bed of the Lake throughout its northern half; 

 but even this can scarcely be present in the southern branches, and 

 especially in the Como branch of the Lake. This is in every way 

 remarkable. It is a closed basin except where it joins the main 

 body of the Lake opposite Bellagio, and throughout a coast-line 

 exceeding 40 miles in length it receives but two unimportant 

 torrents — at Como and Argegno — and it may safely be said that 

 there are no existing causes that tend materially to alter the form of 

 its bed. How singular this is may best be learned by studying the 

 annexed sections (Figs. 8, 9, and 10), which, be it remembered, are 

 drawn to true scale. The object of this paper is to supply facts, and 

 not to discuss their interpretation ; and it is sufficient here to call 



