44 THE FLOOR OF THE OCEAN 



Apparently contemporaneous with the last Fennoscandian 

 ice-cap was a much more extensive and probably thicker ice- 

 cap in North America. Here too there was deep basining of 

 the earth's crust under the load of ice, followed by recoil which 

 still continues, as shown by water-gages at harbors of the Great 

 Lakes. Here too the recoil occurs in an area of deficient mass. 

 An illustration of this last fact is furnished by the latest gravity 

 observations in a part of southern Canada. The region con- 

 cerned, 700 miles long from east to west and in average about 

 200 miles wide, lies between 75° West Longitude and 90 West 

 Longitude. It is well inside the southern limit of the glaciated 

 tract, though everywhere more than 600 miles from the point 

 where the ice was thickest. In this region, the average isostatic 

 anomaly for 26 stations, reported by Mr. A. H. Miller of the 

 Dominion Observatory, is — 17 milligals, a quantity identical 

 with the average anomaly in Belt II of Finland. 10 Near the 

 center of glaciation, in Hudson Bay, the tide-gage is said to 

 have shown current uplift even faster than at the Fennoscan- 

 dian center. Thus, in spite of the difficulties of travel and the 

 making of adequate observations in Labrador, northern On- 

 tario, and the Northwest Territories of Canada, it seems safe 

 to assume that the negative load at the Hudson Bay area is 

 comparable with that at the analogous center near the Gulf of 

 Bothnia. If this is true, we have a second proof of the ex- 

 tremely small strength of a layer beginning at the depth of not 

 more than 50 miles below a continental surface. 



Manifestly ice-caps cannot grow on the floor of the deep 

 sea; here, then, we cannot use Nature's experiments with ice- 

 caps for judging the distribution of strength below the ocean. 

 Yet the indirect testimony of Fennoscandia and eastern Canada 

 is seen to have value when we remember that the cause of 

 subcrustal weakness is high temperature, and that, in spite of 

 difference of radioactivity, a continental sector of the earth is 



