210 George M. Dawson — Erratics at Sigh Levels. 



arisen, probably from my omission to explain the circumstances at 

 sufficient length. As Mr. Geikie's work is deservedly referred to as 

 an authority on the Glacial Epoch, I would here offer a few remarks 

 in additional explanation of the point in question. 



In the vicinity of the forty-ninth parallel, boulders from the 

 Laurentian Axis, a great highland plateau, are distributed westward 

 or south-westward across the whole breadth of the Great Plains to 

 within twenty-five miles of the Kock^? Mountains, where the country 

 has an elevation of about 4,200 feet. Debris from the Eocky Moun- 

 tains at the same time passes far eastward across the plains, the drifts 

 of diverse origin overlapping to the extent of several hundred miles. 

 The general inclination of the plain is eastward, or north-eastward, 

 toward the foot of the Laurentian Axis, along which Winnipeg and 

 associated great lakes lie, with an elevation of about 700 feet only. 

 The higher parts of the Laurentian Axis have an average height of 

 about 1600 feet. 



If it be supposed that the western boulders were deposited in 

 their present position by glacier-ice formed on the Laurentian Axis, 

 or flowing over it from the north (as Mr. Geikie thinks most pro- 

 bable), the ice-sheet must have been pushed up-hill for a distance — 

 measured from the Laurentian Axis — of about 700 miles. Many 

 facts stated in the paper above referred to, but which cannot hei'e 

 be detailed, lead me to believe that the boulders in question reached 

 their present position attached to sea-borne ice. Mr. Geikie, in 

 objecting to this hypothesis, writes: — "When we remember, how- 

 ever, that the maximum height of the latter (Laurentian Axis) is 

 only some 1600 feet, we may well ask how these boulders could 

 possibly have been carried by floating-ice ; for, when the sea stood 

 at the level of 4200 feet, the Laurentian Axis, from which the 

 boulders have come, must have been drowned to a depth of 2,400 

 feet, at least ! " ^ 



This difficulty, though at first sight a very grave one, was not 

 ignored. I believe the evidence to be conclusive, that the western 

 portion at least, of the Laurentian region, was covered by a con- 

 fluent glacier, guided in the direction of its motion by that of the 

 general slope of the surface, but impelled chiefly by the pressure 

 resulting from the continual addition of snow and ice to its central 

 and higher portion. Pre-supposing this, I have written : ^ " The 

 occurrence of Laurentian fragments at a stage in the subsidence, 

 when, making every allowance for subsequent degradation, the 

 Laurentian Axis must have been far below water, would tend to 

 show that the weight and mass of the ice-cap was such as to enable 

 it to remain as a glacier, till submergence was very deep." By this 

 it was intended to suggest that a ponderous ice-cap, several thousand 

 feet in thickness, continually reinforced by abundant snow-fall, 

 might continue to act as a glacier till the surrounding water gained 

 on it to such an extent as to float it bodily away. To this I might 

 have added as an additional, and perhaps more probable suggestion, 



1 Great Ice Age, p. 472. 



2 Quart. Journ. Geol. Soc, 1875, p. 622. 



