362 John Ball — Soundings in the Lake of Coma. 



attention to the very remarkable section given in Fig. 9. Every 

 one who has attended to the subject will desire additional informa- 

 tion as to the nature of the lake-bottom, which fills the almost 

 perfectly level space between the perpendicular cliffs at either side. 

 The suj)erficial deposit brought up by the sound from a depth of 447 

 yards, is a greenish grey mud of impalpable fineness, which was 

 examined under the microscope by MM. Casella and Bernasconi. 

 It appears to be exclusively composed of organic matter, consisting 

 mainly of Diatomacese and Confervacese either in an entire or frag- 

 mentary condition. 



Signer Gentilli appears to adhere to the theory of lake excavation 

 by glaciers, but declines to discuss the application of that theory to 

 the facts disclosed in respect to the Lake of Como. He makes one re- 

 mark, however, which appears to me to be absolutely at variance 

 with them. Supposing the rocks on either side to be of equal hard- 

 ness, and similarly stratified, it is safe to affirm that if they had been 

 hollowed out by glacial action, or by aqueous erosion, the slope 

 would be steepest on the concave side of the bend in those parts of 

 the Lake where the glacier stream was turned aside from its previous 

 direction. Three' such bends are conspicuous in the map of the Lake 

 (Fig. 1), and correspond to the positions of Castello di Musso, 

 Axgegno, and Eiva Palanzo. The erosion theory leads one to expect 

 at each of those places a much more rapid slope than on the opposite 

 side, and Signer Gentilli refers to the section between Argegno and 

 Cavagnola as confirmatory of the theory. I do not find this assertion 

 to be justified by the section, copied accurately from Signer Gentilli's 

 memoir, given in Fig. 8. It is true that if we divide the section 

 into two equal parts the average depth is rather greater on the 

 Argegno side, but the deepest sounding of all lies on the opposite 

 side, and, what is more important, the declivity of the bed, near the 

 lake-shore, is actually steeper on the Cavagnola side. A depth of 

 190 yards is there found at a distance of 100 yards from the shore, 

 while it is necessary to go 120 yards from the Argegno shore to obtain 

 an equally deep sounding. If there be any doubt in regard to this 

 section, there can be none as to that between Castello di Musso and 

 Olgiasca (Fig. 5). There are some important differences between 

 the section here referred to, extracted from the paper of MM. 

 Casella and Bernasconi, and that given by Signer Gentilli, doubtless 

 because not made exactly between the same points, but they agree 

 completely in showing that for a distance of several hundred yards 

 from the shore on either side the slope is twice as rapid on the east 

 side as on the opposite shore, being the exact contrary of the result 

 required by the erosion theory. 



The details of the section between Carate and Eiva Palanzo have 

 not been given to the public. 



When the beds of other Alpine lakes shall have been surveyed 

 with the same care that has been bestowed on that of Como, geolo- 

 gists will be able to reason much more securely than they have 

 hitherto done on the causes that have given them their present form. 



