348 ON MOUNTAINS AND MANKIND. 



The Alps are distinguished by their subalpine lakes — 



Anne laciis tantos ; te, Lai-i inaxime, teque 

 Flnctiltus et froiuitu assiirKciis. Henat'e, luarino? 



of ^"iroil. I^ut perhaps even more interesting to the student are the 

 lake basins that have been filled uj), and thus suggest how similar 

 lakes may have vanished at the base of other ranges. 



I know no more striking walk to any one interested in the past 

 doings of glaciers than that along the ridge of the mighty moraine of 

 the old glacier of Val d'Aosta, which s\veei)s out, a hill 500 feet high, 

 known as " T^a Serra,"' from the base of the Ali)s near Ivrea into the 

 plain of Piedmont. Inclosed in its folds still lies the Lago di Viv- 

 erone; but the Dora has long ago cut a gap in the rampart and 

 drained the rest of the inclosed space, filling it up with the alluvial 

 deposit of centuries. 



It is, however, the tarns rather than the great lakes of the Alps 

 which have been the chief subjects of scientific disputation. Their 

 distril)ution is curious. They are found in great quantity in the Alps 

 and Pyrenees, hardly at all in the Caucasus, and comparatively rarely 

 in the part of the Himalaya I am acMjuainted with. 



A large-scale map will show that where tarns are most thickly 

 dotted over the uplands the peaks rise to no great height above the 

 ridges that connect them. This would seem to indicate that there 

 has been comparatively little siibacrial denudation in these districts, 

 and conse(iuently less material has been brought down to fill the hol- 

 lows. Again, it is in gneiss and granitic regions that we find tarns 

 most abundant — that is, where the harder and more compact rocks 

 make the work of streams in tapping the basins more lengthy. The 

 rarity of tarns in the highlands behind Kangchenjunga calls, i)er- 

 hai)s, foi- exi)lanation. We came ii|)on many basins, but, Avhether 

 formed by moraines or true rock basins, they had for the most i)ai-t 

 been filled up by alluvial deposits. 



In my opinion, the ])resence of tai-ns nnist be taken as an indica- 

 tion that the jjortion of the range where they are found has, until a 

 comparatively recent date, been under snow or ice. The former the- 

 ory, still held, was that the ice scooped out their basins from the solid 

 I'ock. I believe that it simply kept scoured preexisting basins. TIk'* 

 ice removed and the surrounding slopes left l)are, streams on the on.* 

 hand filled the basins with sediment, or, on the other, tapjM'd tluMu by 

 cutting clefts in their rims. This theory meets, at any rate, all the 

 facts I have observed, and I nuiy i)()int out that the actual ])rocess of 

 the destruction of tarns by such action nuiy be seen going on under 

 our eyes in many places, notably in the glens of the Adamello group. 

 Professor (larwood has lately employed his holidays in sounding 

 many of the tarns of the St. (iotthard grouj). and his I'esults, I under- 

 stand, tend to corroborate the conclusious stated. 



