346 T. F. Jamieson — Changes of Levelin the Glacial Period. 



water-lines at which the surface of the lake formeiiy stood, we have 

 here a very interesting question presented to us. How is it that 

 these old beaches are not now horizontal, but rise gradually as we 

 follow them to the north ? Has the ground risen to the northward 

 since the lake disappeared, or did the surface of the lake form a 

 plane which was not horizontal, while the land remained all along 

 stationary as it is ? The latter is the explanation Mr. Upham adopts. 



According to the law of gravitation, matter of all kinds exerts an 

 attractive force in proportion to its mass, and varying inversely as 

 the square of its distance from the body it attracts. Mr. Upham, 

 therefore, supposes that the glacier, by virtue of this attractive force, 

 drew the water towards it, so that the lake rose to a higher level 

 near the glacier than it did farther off. The northern barrier of the 

 lake, he says,^ " was the receding Continental glacier. All the 

 differences of the once (?) level lines of Lake Agassiz from our present 

 level- line would be produced by the gravitation of the water of the 

 lake towards this ice-sheet. At first this attraction had a large effect 

 upon the lake-level, because of the nearness of a great depth of ice 

 on the east in Northern Minnesota, and northward in British America ; 

 but it was gradually diminished to a comparatively small influence 

 when these ice-masses had been melted and the attracting force pro- 

 ceeded from the region far north between Lake "Winnipeg and Hudson 

 Bay." 



Whether we can agree with Mr. Upham in this explanation or not, 

 we are certainly much indebted to him for the discovery of a very 

 remarkable and interesting fact. Another explanation, however, 

 may be offered. 



If, as some geologists believe,^ the outer crust of the earth reposes 

 at no great depth upon a stratum of mineral matter in a state of 

 fusion, we may without violence suppose that the addition of a heavy 

 load upon the surface would cause the crust to press deeper down 

 into this soft stratum and drive part of it away to where the pressure 

 was less. Then, if the load was afterwards removed, and the pressure 

 on the surface thereby relieved, the subterranean fluid or semifluid 

 matter would tend to return towards the place it formerly occupied,, 

 until an equilibrium of pressure was restored, thus causing the 

 surface of the depressed tract to rise again. In accordance, there- 

 fore, with this notion we may conceive that the weight of the great 

 glacier had caused a depression of the northern region on which it 

 lay, and that afterwards, when the ice melted, the land, being relieved 

 from its load, gradually rose again, so that the old beach, which was 

 formerly horizontal, now presents an upward slope to the north, such 

 as Mr. Upham has fonnd.^ 



Both explanations would account for the facts. In either case the 

 result would be that the old shore of the lake would now rise 

 towards the place where the glacier formerly existed. To the one 

 theory it may be objected that it overestimates the attractive force of 



1 he. cit. p. 313. 



^ See 0. Fisher, Physics of the Earth's Crust, p. 223, and elsewhere. 



3 See T. F. Jamieson, Geol. Mag. Sept. 1882. 



