308 PREHISTORIC E UR OPE. 



sents a steep face now to the Val Seriana, for it has been under- 

 cut by the Serio in the process of lowering its bed. But its 

 upper surface still slopes in from the Val Seriana towards the 

 basin of Gandino, and I could notice here and there that the 

 coarse shingle of which it is made up had the same general 

 inclination. 



Viewed from the top of the embankment, the basin appears 

 like a concave plain, bounded on every side by rough, steep 

 mountains ; it has all the aspect, indeed, of an old lake-bottom, 

 drained by the Eomna, which now flows in a rocky glen at a 

 depth of over 300 feet below the summit-level of the bar, down 

 through which it has cut its way. 



The whole surface of the basin appears to be covered with 

 shingle and gravel, now for the most part hardened into con- 

 glomerate, consisting of rocks which are all of strictly local 

 origin, derived in fact from the limestone and porphyry of the 

 adjacent mountains. This conglomerate is well exposed in all 

 the numerous ravines and gorges which the streams have cut 

 out in the bed of the old lake, and appears to attain a very con- 

 siderable thickness. In the neighbourhood of Leffe, which is 

 near the lower end of the basin, I should say it must exceed 

 150 feet. 



Underneath the conglomerate come beds of fine lacustrine 

 clay, silt, and shell-marl, with one or more seams of brown coal 

 or lignite, of which Stopanni gives the following section, dis- 

 closed in one of the pits near Leffe : — 



52-=about 170 feet. 



