112 J. W. DAWSON — SOME RECENT DISCUSSIONS IN GEOLOGY. 



ing some exceptional features. 4. The Muir glacier of Alaska, also a 

 local glacier, but perhaps, like the Malaspina, showing some features 

 illustrative of local Pleistocene glaciers. 



In the " conferences and comparisons," however, the facts detailed in 

 the earlier part of the .paper are placed in comparison with postulates 

 respecting the Pleistocene which are incapable of proof: 1. It is taken 

 for granted that the upper limits of giaciation in the mountain ranges of 

 America indicate the thickness of a continental ice-sheet. They probably 

 indicate only the upper limit of the abrasion of local glaciers. 2. Hence 

 it is computed that the thickness of a continental glacier floAving radially 

 outward in all directions from the Laurentian highlands of Canada 

 amounted to two miles, and in connection with this it is stated that the 

 maximxim thickness of the great Cordilleran glacier of the west in the 

 Pleistocene age has been estimated to be about 7,000 feet, an entirely 

 different thing and referring to the maximum depth of a local glacier 

 traversing deep valleys. 3. It is admitted that the assumed continental 

 glacier could not move without an elevation of the Laurentian highlands 

 to the height of several thousand feet, of which we have no evidence, for 

 the cutting of the deep fiords referred to in this connection must have 

 taken place in the time of Pliocene elevation of the continents before the 

 Glacial period. 4. The upper and lower boAvlder-drift, so different in 

 their characters, are accounted for on the supposition that the former 

 comes from material suspended in the ice at some height above its base, 

 the other from that in the bottom of the ice. In like manner the widely 

 distributed interglacial beds holding remains of land-plants of north 

 temperate character are attributed to such small local occurrences of trees 

 on or under moraines as appear in the Alaska glaciers. 5. The rapid 

 disappearance of the ice is connected with a supposed subsidence of the 

 land under its weight, though from other considerations we know that if 

 this was dependent on such a cause it must have been going on from the 

 first gathering of the ice, so that the required high land could not have 

 existed. All the evidence, however, points to subsidence and elevation 

 owing to other and purely terrestrial causes, and producing, not produced 

 by, the glaciers of the Pleistocene. 



The question of erosion by glaciers is still agitated. My own conclu- 

 sions, formed from the study of the Savoy glaciers in 1865, is that glaciers 

 are never important eroding agents, that in valleys they protect the rock 

 from the greater denuding action of streams, and that the mud and sand 

 which they produce are derived not from the rocks in which they slide, 

 but from the material that falls upon the glacier. The bottom rock is 

 merely the nether millstone. 



One of the most experienced of alpine geologists. Professor Bonney, in 



