70 THE POST-PLIOCENE PERIOD. 



America, such as would at present convert all the plains of Canada 

 and New York and New England into sea, would determine the 

 course of the Arctic current over this submerged land from north-east 

 to south-west ; and as the current would move up a slope, the ice 

 which it bore would tend to ground, and to grind the bottom as it 

 passed into shallower water; for it must be observed that the character 

 of slope which enables a glacier to grind the surface may prevent ice 

 borne by a current from doing so, and vice versa. 



Now we know that in the Post-pliocene period eastern America was 

 submerged, and consequently the striation at once comes into harmony 

 with other geological facts. We have, of course, to suppose that the 

 striation took place during submergence, and that the process was slow 

 and gradual, beginning near the sea and at the lower levels, and 

 carried upwards to the higher grounds in successive centuries, while 

 the portions previously striated were covered with deposits swept 

 down from the sinking land or dropped from melting ice. It would 

 be easy to show that this view corresponds with many of the 

 minor facts. 



Farther, the theory thus stated accounts for the excavation of the 

 deep and land-locked basins of our great American lakes. Ocean 

 currents, if cold, and clinging to the bottom, must cut out pot-holes, 

 just as rivers do, though geologists are too apt to limit their function 

 to the throwing up of banks. The course of the present Arctic current 

 along the American coast has its deep hollows as well as its sand- 

 banks. Our American lake-basins are cut out deeply into the softer 

 strata. Running water on the land would not have done this, for it 

 eonld have no outlet ; nor could this result be effected by breakers. 

 Glaciers could not have effected it ; for even if the climatal conditions 

 for these were admitted, there is no height of land to give them 

 momentum. But if we suppose the land submerged so that the Arctic 

 current, flowing from the north-east, should pour over the Laurentian 

 rocks on the north side of Lake Superior and Lake Huron, it would 

 necessarily cut out of the softer Silurian strata just such basins, drifting 

 their materials to the south-west. At the same time, the lower strata 

 of the current would be powerfully determined through the strait 

 between the Adirondac and Laurentide Hills, and, flowing over the 

 ridge of hard rock which connects them at the Thousand Islands, 

 would cut out the long basin of Lake Ontario, heaping up at the same 

 time, in the lee of the Laurentian ridge, the great mass of boulder 

 clay which intervenes between Lake Ontario and Georgian Bay. 

 Lake Erie may have been cut by the flow of the upper layers of water 

 over the Middle Silurian escarpment; and Lake Michigan, though 



