STUDIES FOR STUDENTS. 851 



Any acceptable theory of the drift must account for the 

 divergent systems of striae, for the local deflections from the 

 general direction within these systems, and for the association of 

 such deflections with local topographic features. It must also 

 account for the occurrence of striae on steep slopes, on vertical 

 faces, on the under side of overhanging rock masses, and on the 

 lower, upper, and interior surfaces of horizontal grooves in vertical 

 cliff faces. It must be consistent with the absence of striae and 

 polishing on the rock surface at many points within the drift- 

 covered territory, and must be mindful of the absence of the 

 same sort of markings on the surface of the rock formations out- 

 side the drift. The phenomena of the striae show that the drift 

 agent or agents, or some of them, must have been such as could, 

 under some circumstances, work with a large measure of inde- 

 pendence of topography, at the same time that they were, 

 under other conditions, locally influenced or even largely con- 

 trolled by it. They must have been such as could have some- 

 times operated at great altitudes, as in the mountains of the 

 west, without affecting the lower levels surrounding. 



The shape of the rock hills projecti7ig through the drift. In many 



places within the drift-covered area, but not everywhere, the 



rock knobs which project through the drift, or which are but 



slightly covered by it, possess certain characteristics which are 



not without significance. Such bosses of rock frequently have 



smoothed and rounded forms which are so distinctive that they 



have received a special name, roches moutonnees. In some regions 



each rock-prominence seems to be a roche moutonnee. In such 



cases, one side of each boss seems to be worn more than the 



others, and in any limited area it is generally the same side of each 



hill which shows the greatest wear. This fact must be taken into 



account in any attempt to explain the drift, for it is a phenomenon 



about as widely distributed as the drift itself, though not by any 



means to be seen at all points. The sides which are most worn 



are those facing the direction from which the drift forces moved, 



as shown by the direction of striae, and by the direction in 



which material has been transported. „ _ ~ 



1 Rollin D. Salisbury. 



(To be continued.) 



