POWERFUL AND PERSISTENT GLACIAL EROSION. 295 



dispersion remains to the present day in the fact that the general isothermal 

 lines appear to circle round them as the areas of greatest cold. As the ice- 

 sheet increased or diminished there would, no doubt, be great local variations, 

 and immediately to the south of the general Laurentian outline there was 

 at one period a strong movement to the southwest up the St. Lawrence, from 

 near Montreal, through the basins of Lakes Erie and Ontario aud over the 

 peninsula between the latter and Lake Huron. A similar movement took 

 place from Lake Superior westward, carrying the debris of the red rocks of 

 the Nipigon formation up over the Laurentian plateau towards the valley of 

 Red river, as pointed out by the writer many years ago. 



I do not like to offer any explanation of the above general facts ; but it would 

 appear that they indicate a greater elevation of the land than at present 

 exists in the north and east. In addition to the aid afforded by gravitation, 

 the movement of the ice was probably largely due to its continual accumu- 

 lation in certain regions and its constant thaw in others, the latter being 

 due not only to the heat of the sun but also to the influence of the warm 

 water of the ocean in the direction towards which the ice traveled. A fur- 

 ther cause of the southward tendency of the ice, which I have not seen 

 referred to by other writers, would be the tangential component of the 

 centrifugal force due to the rotation of the earth on its axis. 



The Archean country is thoroughly denuded of everything down to the 

 bare rock. The eroding force must have been most powerful and long con- 

 tinued. As a rule, not only is all the decayed rock gone, but even the 

 crushed or loosened portions, leaving a smooth and sometimes polished surface, 

 well calculated to resist the denuding agencies of the present period. The 

 general form of the rocky domes which remain has been shaped by the same 

 force. The longer diameter of each, as a rule, is parallel to the direction of 

 the striation of the locality and the stoss or crag end is steeper than the tail 

 or lee extremity. The rock of the stoss side, which had been long exposed 

 to the stream of ice, like the upper side of a pier in a river, is more solid 

 and free from joints and flaws than that of the tail, showing deeper erosion. 

 In confirmation of this, it was found in constructing the Canadian Pacific 

 railway north and west of Lake Superior that it was more difficult to remove 

 rock on northward than southward slopes. The general bearing of the 

 striae gives us the line of the ice movement ; but it is not always safe to 

 assume that it came from the side indicated by any preconceived theory, 

 and we have in the above circumstances one of the best means of determin- 

 ing the actual direction from which the force came. Another guide to the 

 direction of the movement is this: The grooves are frequently found to radi- 

 ate, sometimes at considerable angles, on reaching a certain point, as on 

 meeting with some obstruction or with a change in the grade, especially when 

 tlye slope is steep. I have observed the same thing happen to previously 



XXXIX— Bull. Geol. Soc. Am., Vol. 1, 1889. 



