3 i2 PREHISTORIC EUROPE. 



formed from a study of this and other sections in the neighbour- 

 hood were shortly as follow : — 



1. That the lignite and its associated deposits were accumu- 

 lated in a lake which they succeeded gradually in silting up. 



2. That, afterwards, flooded streams and torrents flowed over 

 the surface of the dried-up lake, ploughing at first into the 

 lacustrine sediments, but eventually burying them deep under a 

 great accumulation of coarse shingle and gravel. 



3. That a time came when this extreme torrential action 

 ceased, and the streams thereafter commenced to erode their 

 beds, and to cut deep trenches down through the conglomerates 

 into the lacustrine sediments. 



The conglomerates, I have no doubt, are, as Stopanni main- 

 tains, of precisely the same age as the morainic detritus which 

 overlies the white marls of Pianico in the Val Borlezza. They 

 belong, in short, to a glacial epoch, when torrential water, 

 derived from melting snow and n&vi, descended in great volumes 

 from all the hills surrounding the basin of Leffe. Unfortunately 

 the section at Leffe does not disclose the nature of the bottom 

 upon which the lacustrine sediments repose. In the Val Borlezza, 

 as we have seen, the lower part of the lacustrine deposits is 

 abundantly charged with glaciated stones. No glaciated stones, 

 however, occur in the neighbourhood of Leffe. I believe, how- 

 ever, with Stopanni, that the great embankment thrown across 

 the opening of the valley is unquestionably of fluvio-glacial 

 origin. To this view Professor Eiitimeyer has objected that the 

 Eomna at its exit from the Val Gandino flows over limestone 

 and not conglomerate. 1 This is true, and it is no less certain 

 that the basement-beds of the lacustrine series must be under 

 the level of the Eomna, where it first begins to flow across the 

 limestone-strata. But we have no reason to suppose that the 

 present exit of the river was that which obtained in preglacial 

 times. When the Val Gandino was dammed up by the cone de 

 dejection of the Serio and converted into a lake, the old course 

 by which the Eomna escaped into the Val Seriana may have 



1 Ueber Pliocen unci Eisperiode ctuf beiden Seiten der Alpen. 



