728 ABSTRACTS OF I>APERS 



in the title of Faircliild's paper ("Glacial erosion a fallacy"). The paper 

 aims to set forth the differences of conditions which caused the deep erosion 

 of Alpine valleys and the slight erosion of the Great Lakes basins. 



Discussion 



Dr. C. A. Davis : In the course of some years' work for the Michigan Survey, 

 many cases of slight glacial erosion in the direct lines of ice-movement, as 

 shown by striae, etcetera, were discovered. In several cases apparent deep 

 grooving and scratching were found on examination to be due to the cleaning 

 out of weathered joint and other similar cracks, even when the eroded material 

 was soft schist. In one locality in the northern peninsula of Michigan, stauro- 

 lite crystals included in a schist were weathered out of the schist preglacially, 

 and had been passed over so lightly that, although the direction of ice-move- 

 ment could be determined by the clear-cut striae on the truncated surface of 

 the crystals down to the general surface of the rock only a half inch below, 

 the freshness of the striae on the crystals and the well known resistance of 

 the schist to weathering preclude the hypothesis that the weathering is post- 

 glacial. 



Prof. R. D. Salisbury : The phenomena cited by Mr. Taylor are interesting 

 as an illustration of the position which his paper sets forth — namely, ineflS- 

 ciency of ice erosion in much of the lower peninsula of Michigan — but I think 

 it would be easy to assemble phenomena from other parts of the broader 

 region of which this peninsula is part which point with equal clearness to 

 the opposite conclusion. This leads to the general comment that the ice 

 seems to have acted very differently in different places, and that while the 

 phenomena of some areas seem to indicate inefficiency of glacial erosion just 

 there, the phenomena of other areas indicate with equal clearness efficiency 

 of glacial erosion. The general fact that in the broad region of which the 

 lower peninsula of Michigan may be said to be a part the great body of drift 

 is made up of fresh, unweathered rock material, much of which has been 

 transported but a short distance, seems to me to be a final argument against 

 inefficiency of ice erosion in these latitudes. 



Dr. J. B. Tyrrell: I am very much interested in the remarks which have 

 just been presented by Mr. Taylor, and I would like to say a word on the 

 probable amount of material that was swept from northern Canada by the 

 glaciers of the different Glacial periods. 



To begin with, it is quite clear that all the decomposed rock which existed 

 on or covered the hard rocks of northern Canada in pre-Pleistocene times was 

 cleared off and carried away by one of the ice-sheets, and probably by the 

 first one. 



At the present time there are very few places in the north which show the 

 amount of decomposed material which covered the rocks, but there is one 

 place — namely, in the Klondike district of the Yukon territory — which was 

 not overridden by the ice-sheet of the Glacial period, and which, therefore, 

 shows the normal conditions of weathering on the surface. 



In that district the rock in the bottoms of the valleys is more or less fresh 

 and undecomposed, but on the higher parts of the hills these rocks, often 

 biotite schists, similar to those so common in northern Ontario, are weathered 



