360 John Ball — Soundings in the Lake of Como. 



To a fourth contributor — Signor Ferdinando Stoppani — are due most 

 of the measurements in the Lecco branch of the Lake. 



Geologists in foreign countries will feel little interest in the 

 personal question here referred to, but they cannot fail to attach 

 importance to the results of such a work, which appears to have been 

 carefully and efficiently conducted. The mode adopted for measuring 

 the horizontal distances between the point where soundings were 

 made, does not seem to have been free from objection ; but the errors, 

 if any, could not be considerable enough to affect the general result. 



It is remarkable that these measurements should have remained 

 unknown to many geologists, even among those who take a 

 special interest in the subject, and that they are not referred to by 

 so careful a writer as Professor Eiitimeyer in the important memoir, 

 " Ueber Thai- und See-bildung," in which he has fully discussed the 

 disputed question of the origin of Alpine Lakes. On this account 

 I believe that the plans and sections accompanying the present paper, 

 being reduced copies of those published by the original observers, 

 will be full of interest to all who have attended to this subject. 



Very little comment is necessary to the full understanding of the 

 accompanying plans. Most travellers are familiar with the form of 

 the Lake of Como. Filling the bottom of a great valley that extends 

 southward from near Chiavenna, at the foot of Splilgen, to Bellagio, 

 it forks at that point. "What is called the Lecco branch extends 

 S.S.E. beyond the town of Lecco, and sends the Adda to carry the 

 drainage of the lake-basin to the Po, across the plain of Lombardy. 

 The Como branch, about equal in length to that of Lecco, is more 

 sinuous, but the general direction is S.S.W. from Bellagio. At the 

 south end it is closed by a barrier of solid rock that rises behind the 

 city of Como to a height of about 200 feet above the Lake. 



The original bed of the Lake, as thus roughly sketched, has been 

 extensively modified by the action of Alpine torrents that continually 

 pour vast masses of detritus into the basin. At its northern ex- 

 tremity the Mera, draining the valleys of San Giacomo and Bregaglia, 

 has filled up the original bed of the Lake for a distance of nearly 

 five miles, and the shore is first reached at the hamlet of Eiva, nearly 

 half-way from Chiavenna to Colico. Still more considerable changes 

 have been effected by the Adda. In a course of over seventy miles 

 this receives numerous affluents from the lateral valleys of the 

 Ehastian and Lombard Alps, until it enters the lake near Colico in 

 the form of a turbid stream, heavily charged with glacial silt and 

 coarser detritus. It is very probable that an arm of the Lake, long 

 since filled up, once extended to Morbegno in Valtellina ; but it is 

 at all events certain that the Adda has completely barred the main 

 channel of the Lake, forming a wide delta that stretches from shore 

 to shore, and cutting off the northern part of the ancient Lake — 

 now called Lago di Mezzola — from the main basin. 



At the opposite end of the Lake, where there is every reason to 

 suppose that the original bed was comparatively shallow, several 

 rapid torrents have carried masses of Dolomitic debris into the Lake 

 at Lecco, and again at Olginate, so that at each place the channel is 



