SPECIFIC DISCUSSION 21 



differences of condition does not justify any claim for extreme erosion. 

 Such claim must be proved by direct evidence. 



(4) The correspondence between rivers and glaciers has been exagger- 

 ated. It was a valuable idea in the early study of glaciers, while it was 

 necessary to prove that alpine glaciers had motion. To the extent that 

 the glacier has apparent viscosity and flow, the differential movement 

 simulates that of streams; but there the correspondence ends, except 

 that the glacier is in general an aqueous agent, conveying rock rubbish 

 from higher to lower levels. In its manner of erosion, transportation, 

 and deposition, the glacier is quite unlike a river. Its transporting power 

 seems to be equal to any imposed burden ; in other words, the glacier is 

 never full-loaded. But the heavy loads of glaciers are not derived from 

 erosion by the ice itself, but are contributed by weathering effects above 

 the glacier, either at the valley head or alongside the valley by atmos- 

 pheric action on the valley walls. To the extent that glacier-flow obeys 

 the same law as river-flow, the rate of motion at sides and bottom is re- 

 duced, and becomes so slow as to be an inconsequential element of erosion 

 during the life of any glacier. In another respect the glacier is unlike 

 the river. In the bed of the latter chemical decay is effective and the 

 riverbed is subject to disintegration ; and the coarse burden of the river 

 is all carried at the bottom. In the bed of the glacier the disintegrating 

 forces are at a minimum. The work of subglacial streams and the fur- 

 ther general discussion will be found later. 



(5) The fine rock-dust which gives the milky color to the water from 

 the glacier — the " Gletschermilch " — is admittedly the product of the 

 glacier mill, but it is only partly derived from the abrasion of the bed- 

 rock. It is also produced by the grinding of the transported rock rub- 

 bish, as is shown by the worn and striated character of the boulders and 

 pebbles in the till. It is optically conspicuous, not for its volume, but 

 because it is so fine that it floats in the water. The argument for rapid 

 glacial erosion, made by Helland, based on the gletschermilch, has been 

 shown by Heim * to be fallacious, and that the large estimates are mis- 

 leading. He shows that the amount of detritus washed annually from 

 the alpine glaciers is equal to only a small fraction of the amount of 

 detritus carried by the non-glacial streams of the same region. As com- 

 pared with streams in equal time, glaciers are weak in erosive power. 

 Properly interpreted, the milkiness of the glacier is a proof of its slight 

 abrading effect. 



The estimates of the erosion by Muir glacier based on the amount of 

 sediment held in the tidal waters of the inlet can be of little value f for 



♦Albrecht Heim : "Handbuch der Gletscherkunde." 



fH. F. Reid : "Glacier bay aud its glaciers." Sixteenth Ann. Rep. U. S. Geol. Survey, 1896, pp. 

 154-458. 



G. F. Wright: " The ice age in North America," 4th ed., p. f.4. 

 IV— Bull. Geol. Soc. Am., Vol. 16, 1904 



