348 ON MOUNTAINS AND MANKIND. 



The Alps are (listiiioiiislied by their subalpine hikes — 



Aiiiic laciis tniitos; te, Lari niaxime, teque 

 Fhictilius (»t freniitu as>;urgtMis, Benace, inarino? 



of \'ii-<>:ih But perhaps even more interesting to the student are the 

 lake basins that have been filled up, 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 sweeps out, a hill 500 feet high, 

 known as '" La Serra," from the base of the Alps 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 

 distribution 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 acquainted 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 subaerial denudation in these districts, 

 and consequently 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 I'ocks 

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

 rarity of tarns in the highlands behind Kangchenjunga calls, per- 

 haps, foi- explanation. AVe came ui)on many basins, but, whether 

 formed by moi'aines or true rock basins, they had for the most part 

 been filled up by alluvial deposits. 



In my opinion, the presence of tarns must be taken as an indica- 

 tion that the ])orti()n of the range where they are found has, until a 

 c()nq)aratively recent date, been under snow or ice. The foi-mer the- 

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

 rock. I believe that it simi)ly kept scoured preexisting basins. The 

 ice removed and the surroiniding slopes left bare, streams on the one 

 hand lilled the basins with sediment, oi". on the other, tapped them by 

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

 facts I have observed, and I may point out that the actual process of 

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

 our eyes in numy places, notably in the glens of the Adainello group. 

 Professor (Jarwood has lately em])]()yed his holidays in sounding 

 many of the tarns of the St. (Jottliard groiij). and his results, I under- 

 stand, tend to corroborate the conchisioiis stated. 



