3 1 o PREHISTORIC E UR OPE. 



All the shells belong to species still living in Lombardy (Val- 

 vata piscinalis and Planorhis complanatus). 



Stopanni is of opinion that the lignite and its associated 

 deposits were accumulated at the very time that the Serio was 

 engaged in piling up the great embankment across the mouth of 

 the Gandino or Leffe valley. He thus relegates the growth of 

 the lignites to a glacial epoch, which is of course consistent with 

 his views of the origin of the lacustrine deposits of the Val 

 Borlezza. Eeferring to the lignite of the principal seam, he 

 says, " I am of opinion that this wood is nothing but drifted 

 wood deposited in the bosom of the lake. How could this be 

 if all the mountains were very precipitous and desolately bare ? 

 I think that the lacustrine plain would be found sometimes 

 either entirely or in certain places converted into a marsh, 

 where the forests of walnuts (I speak of the principal con- 

 stituents) would be condensed perhaps for ages, until they were 

 submerged by the waters, which would gradually rise on all 

 sides as the cdne de dejection which dammed the lake continued 

 to increase in height." The final filling up of the basin he 

 attributes to the action of torrents carrying down shingle and 

 gravel from the surrounding hills, and thus gradually pushing 

 out their deltas, until little by little they gradually filled up the 

 hollow, and converted it into a wide alluvial plain. 



The overlying conglomerate is no doubt of torrential origin, 

 but I do not believe it has been formed in the manner suggested 

 by Professor Stopanni. It is disposed in broad terraces, the 

 upper surfaces of which are inclined outwards or down the Val 

 Gandino to the gorge of the Eomna, which now separates the 

 great bar from the mountain-slope, against which, according to 

 Stopanni, it formerly abutted. The whole surface of the basin 

 in fact forms an inclined plane that rises gently towards the foot 

 of the mountains east of Gandino. This plane is deeply cut 

 and trenched by the numerous feeders of the Eomna, in which 

 capital sections of the overlying conglomerate are exposed, and 

 the beds are there seen to have the same low dip towards the 

 mouth of the valley. The whole of these ancient shingle- and 



