ON ICE AS AN AGENT OF GEOLOGIC CHANGE. 173 



places, for the lateral moraines d and e. Then ascertain the rates of the 

 onward movement of the glacier, according to circumstances, m various por- 

 tions of its length, and at various seasons of the year ; and by these means 

 wlU be ascertained to a great extent (but not precisely) the quantity of 

 matter carried annually on the surface of the glacier to its termination, and 

 this matter will represent a very large part of the waste of the sides of the 

 mountains that bound the snow and glacier basins o, p, q, and the sides of 

 the mountains that bound the glacier lower down towards its terminal 

 moraine. 



Thirdly. The chief part of the remainder of the rocky and earthy matter 

 that is carried from the mountains to the level of the glacier will pass under 

 it at its sides, and mingle with the material that finds its way to the bottom 

 of the glacier through the means of crevasses and moulins, and also with 

 that which is the product of the erosive action of the glacier exerted on 

 its bed and on the stone blocks imprisoned at the bottom of the ice. A 

 small part of the above-named remainder may also be caught in the ice and 

 imprisoned in rejoined crevasses. 



Fourthly. We see no way of precisely estimating the amount of erosion 

 produced by the weight and movement of the glacier — that is to say, the 

 rate at which any given glacier may deepen and widen its valley by pure 

 wearing action, owing to the circumstance that the sediments discharged 

 along with the water that flows from the end of a glacier do not represent 

 the amount produced by mere erosive force, for the reason stated under 

 head 3. But it is essential to the main question that correct estimates 

 should be made of the amount of solid matter brought from under the glacier 

 by the help of running water, and also of the amount carried away by the 

 continual wasting by streams of the terminal moraine. 



As regards the matter in suspension in the river, and also that forced 

 along its bottom, it should be estimated, if possible, at a point r, just below 

 where the various streams unite that flow from the ends of most great gla- 

 ciers. Where there is only one stream (as in the Aletsch glacier), the closer 

 to the glacier the better. The operation would be very laborious ; for, unless 

 frost and snow prevented it, it would requii'e to be done for every daj' in a 

 year or years, and several times each day, at least in summer and autumn, 

 and probably in spring and winter also. For example, in summer the quan- 

 tity of water varies largely, according to the heat of various periods of the 

 day ; and it would probably be necessary to make an observation every day 

 before sunrise, another some time before noon, another between four and six 

 o'clock in the afternoon, and another after nightfall ; in fact sufficiently 

 often to obtain an average for each day in the year. 



AVith rgard to the transport of heavier matter from the terminal moraine 

 (which forms a portion of this part of the subject) by the glacier-streams 

 that waste it, an index to the amount may approximately be obtained by 

 means of the estimates indicated under head 2, assuming that all terminal 

 moraines are formed chiefly from matter transported on the sm-face of the 

 glacier. 



Other methods involving special study on the spot would be required for 

 the terminal and lower side-moraines of such glaciers as those of La Brenva 

 and Miage, which on the sides that face up the valley towards the Lake of 

 Comballe are still growing. 



Fifthly. If the foregoing methods are correct, they might afterwards be 

 applied to all the glaciers of the Alps, and the rate of waste and transport 

 by glacier- action might be approximately determined ; and in like manner 



