i8q 5 . THE STUDY OF EXISTING GLACIERS. 21 



of greatest depth should be ascertained wherever possible. The 

 marginal rock-structure of lake basins, particularly near their outlet, 

 is of much importance with a view to ascertaining whether they are 

 true rock basins, or whether they are reservoirs formed by ancient 

 moraines, earthslips, or alluvial deposits. 



" The traveller or surveyor should, if possible, paint a mark and 

 date on any conspicuous rock in situ parallel with the termination of 

 the glacier at the time of his visit, marking the distance in yards of 

 the. ice from it. The next visitor will then be able to measure the 

 movement that has taken place since his predecessor's visit. Leaving 

 out of question elaborate trigonometrical methods, such, for instance, 

 as have been carried out on the Rhone Glacier in Switzerland, the 

 following plan gives very valuable results, and demands no other 

 instruments than a small jar of paint, a brush, a measuring tape, 

 and a pocket compass. To ascertain the recent retreat of a glacier, 

 measure the distance from the end of the ice in front of the longi- 

 tudinal axis of the glacier to the most advanced terminal moraine, 

 where vegetation first shows itself. The bare ground recently left by 

 glaciers is easily recognisable. The diminution of volume is best 

 measured by ascertaining the height of bare soil left on the sides of 

 the lateral moraines in the portion of the glacier within the zone 

 of vegetation. All photographic representations of the glacier-end, 

 and of the ground which has been freed from the glacier-ice, are of 

 great value. Those will be of most service that show the position 

 of the glacier-snout with relation to some conspicuous rock or other 

 feature in the local scenery. Each photograph should be dated, and 

 the bearings and distance of the camera with reference to any such 

 feature accurately noted. 



"It is very important to investigate the state of various glaciers 

 as regards advance or retreat. Neighbouring glaciers often furnish 

 very different results in this respect, owing to the fact that steep 

 glaciers anticipate in their oscillations those of which the beds are 

 less inclined. To ascertain the oscillations of glaciers, it is necessary 

 to fix the actual position of the ice-snout at the end of the glacier 

 with the greatest accuracy. Two methods can be employed for this 

 purpose, either of which may be selected according to circumstances. 



" Paint some signs on large boulders, not too far from the end of 

 the glacier, and measure their distance from it by a tape (Richter's 

 system), or build a low wall of stones of a few yards in length, 

 and, say 15 to 20 inches in height, some distance from the ice-end, 

 and measure this distance (Gosset's system). It is to be recom- 

 mended that the stones of these walls should also be painted. If the 

 traveller himself returns after some interval — even after only two or 

 three weeks — he will be able to judge of the movement of the glacier, 

 and he will have laid down a basis for future observations by future 

 travellers. 



" One of the results most to be desired is an exact knowledge 

 of the dates: (i.) Of the maximum extension of the ice. (ii.) Of the 

 commencement of retreat, (iii.) Of the minimum, (iv.) Of the 

 commencement of fresh increase. In dealing with a mountain group, 

 therefore, the traveller should note (where he can get the information 

 as to the past) the date of the commencement of the actual 

 movement of each glaciev, and in all cases whether the ice is in 

 advance, or retreat, or stationary. Of course the rate of forward 

 movement, or velocity of the ice, and the oscillations in the extension 

 of the ice must be kept carefully distinct. 



