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OUTLINFs <>l I II 



offer such temptation^ to thc-ir study as tin- smoothed ami 

 striated surfaces left upon rocks by the passap 

 of ice across them. They occur in such unlooked-for 

 I, and among surroundings now sugge , Imost 



any other physical condition than that of an Arctic ice- 

 sheet. When the field geologist has once seen this kind 

 of surface, he is not likely to confound it with any other. 

 The only one for which it sometimes might be mistaken 

 is that termed s/ickensides, where the two walls or faces of 



Fie. 83. Ice-worn hummocks of rock, the arrow pointing in the direction of the 

 ice-movement. 



a joint or fault have slid upon one another so that 

 side is rubbed smooth, polished, and grooved. But a 

 little practice and the study of good examples will give 

 the observer such confidence in discriminating between 

 them as he cannot acquire in any other way. The glacial 

 striation is a merely superficial marking. The ruts are 

 often paler than the rest of the rock, as scratches on a 

 fresh rock-surface are, and though marked in each case 

 by one prevalent direction, are found often to cross each 

 other obliquely. There is generally great variety in the 

 size and depth of the striae ; some being such fine lines 



