44 Proceedings of the Royal Physical Society. 



dilatation or regelatioii of the ice, or otlier minute physical 

 experiments requiring such accuracy of observation as to pre- 

 occupy the mind from a broad view of surrounding pheno- 

 mena. In M. Violet Le Due's "Mont Blanc" we have a 

 comprehensive survey of the work of the glacier, with its 

 accessories. And there it is laid down that torrents, not 

 glaciers, are the immediate cause of the dreaded avalanche of 

 the Alpine villagers, as well as the distribution of great 

 beds of boulders, sand, gravel, or clay. The glaciers, indeed, 

 are only reservoirs of potential water, rendered so when 

 loosened from their icy covering by melting spring or autumn 

 suns. The glacier does comparatively little in altering the 

 surface-contour of the mountains. Its course is marked by a 

 long gently-smoothed plain. The side moraines will be there, 

 so, too, a thickish deposit of mud ; but the rocks will retain 

 their sculpturing they had before being covered by its icy 

 sheet. Change, indeed, is only effected when the water from 

 its hard sides is melted and flows into the joints and fissures, 

 when mountain masses are reft in obedience to hydraulic 

 laws. At the same time, the ancient glaciers of the Alps, in 

 their recession from a level which was probably at least 6000 

 feet below the present ice limit, have left moraines, which 

 serve as dams to the descending torrents, till they gain such 

 an impetus, when bursting them, to carry the huge boulders 

 down to the plains below. The deposits consequent on such 

 action are funnel-shaped. The stream drops first the large 

 boulders ; next, gravel, great and small ; afterwards, clay and 

 sand, as its carrying-power decreases. But it may happen 

 that a torrent, instead of bursting an old moraine, may only 

 have sufficient energy to cast up the sand and gravel it has 

 hitherto carried around its sides, so forming a mountain lake. 

 This will become a permanent scenic feature all the more, if 

 fed by natural springs. That it has an outlet in nowise 

 denies its original mode of forming. We have thus to take, 

 as concomitants of torrent action, the more ancient exten- 

 sion of the glaciers, and surely, also, accompanied with a 

 more powerful torrent energy : the breaking through and 

 carrying away of ancient moraines, a power evidenced by the 

 existence of mountain lakes where it is only half effected ; 



