200 THE ICE AGE IN NORTH AMERICA. 



more than one hundred miles from the southwestern limit 

 of the glaciated area in Montana), the continental glacier, or 

 the glacial sea, according to which one of the theories of 

 transportation is adopted, had a maximum depth of two 

 thousand feet. This he determines by the height to which 

 he found glacial deposits resting upon the Cypress Hills. It 

 is the necessity of accounting for such an elevation of bowl- 

 ders in glacier-ice which has made the Canadian geologists 

 hesitate about accepting the glacial theory. It seemed to them 

 at first more probable that there had been a depression of 

 about four thousand feet in the Rocky Mountain region, and 

 that these bowlders were transported from the Laurentian 

 axis by floating ice. We think, however, that such facts as 

 are illustrated in the diagram of Professor Lesley's on page 

 196, as well as other facts yet to be stated concerning the 

 elevation of bowlders in ice, go far to remove the objections 

 to the glacial theory urged by the Canadian geologists, and 

 we therefore speak with considerable confidence of the great 

 depth of the ice over the Laurentian axis. The glacial the- 

 ory, moreover, as Dr. Dawson frankly and early admitted, 

 relieves them of many difficulties in accounting for the 

 noticeable absence of other indications of subsidence in the 

 region under consideration. For example, there is, first, ac- 

 cording to Dr. Dawson,* a complete absence of any marine 

 animal remains in the drift over that region ; and, secondly, 

 " the yielding, scarcely solidified " sediments over this vast 

 region bear slight evidence of any such great change in ele- 

 vation. 



The great depth of the ice over the lake-region during 

 the Glacial period is also evident from the second mode of 

 calculation, namely, that based upon the distance over which 

 bowlders are known to have been transported by the direct 

 movement of the ice. The fluidity and plasticity of ice are so 

 slight that, where we find it moving hundreds of miles over 

 a level country, the thickness at the starting-point can scarcely 



* "Report on the Forty-ninth Parallel," as above, pp. 216, 244, 260, et a!. 



