20 NATURAL SCIENCE. Jan., 



to fifty years. Since this is a periodicity of more than a generation, 

 history may be consulted with advantage ; but at the same time one 

 must not forget to study the parallel meteorology of the district. It 

 may, for instance, be enquired for how many years after a snowy 

 winter, or a sequence of such, glaciers show increase in their lower 

 portions ; or how far local and prevalent warm winds may mask 

 glacial advance in a valley. Without being able to give precise figures, 

 we suspect that in several cases the advance or retreat of glacier snouts 

 has had too great an importance attached to it. 



The evidence as to the pace of the same glacier at different 

 heights is none too copious, whilst the motion of the neve and of the 

 snow-masses which feed the ice is very little known. Professor Forel's 

 action in causing lead plates, engraved with the date, to be buried near 

 the summit of Mont Blanc, is one that might be followed elsewhere. 



The comparative amounts of rocks and detritus resting upon 

 glaciers, and of that which has found its way beneath, as well as the 

 amount removed by torrents from under the ice, are difficult points on 

 which some good work has recently been done, but on which still 

 more is required. 



A few other subjects, as well as methods, of investigation are 

 summed up in the circular by Mr. Freshfield, to which allusion has 

 already been made, and which runs as follows. 



" The recent movements of glaciers may be noted by the follow- 

 ing signs : — 



" When the ice is advancing the glaciers generally have a more 

 convex outline, the icefalls are more broken into towers and spires, 

 and piles of fresh rubbish are found shot over the grass of the 

 lower moraines. Moraines which have been comparatively recently 

 deposited by advancing ice are disturbed, show cracks, and are 

 obviously being pushed forward or aside by the glacier. 



" When the ice is in retreat, the marks of its further recent ex- 

 tension are seen fringing the glacier both at the ends and sides in 

 their lower portions, the glacier fails to fill its former bed, and bare 

 stony tracts, often interspersed with pools or lakelets, lie between the 

 end of the glacier and the mounds of recent terminal moraines. 



" Where a glacier has retreated to any considerable extent, care- 

 ful observations of the form of its bed are of value. W T hat is the nature 

 of the rock surfaces exposed — convex or concave ; are they rubbed 

 smooth on their lee-sides ; how far have the contours of the cliffs or 

 slopes, or the sides of any gorge, been modified where they have 

 been subjected to ice-friction ? Is there any evidence that the ice 

 has flowed over large boulders, or loose soils, such as gravel, without 

 disturbing them ? How has it affected rocks of different hardness, 

 for instance, veins of quartz in a less hard rock ? Generally, do the 

 appearances indicate that the glacier has excavated, or only abraded 

 and polished its bed ; that it has scooped out new rock-basins, or 

 only cleaned out, scratched, and preserved from filling up by alluvial 

 deposits or earthslips, existing basins ? What is the general character 

 of the valley bottom and slopes above and below the most conspicuous 

 ancient moraines ? 



" The depth of mountain lakes and the position of the point 



