552 KIRTLEY F. MATHER 



resultant changes in sea-level. Daly 1 has presented the requisite 

 data for estimating the area, ice-covered during the Pleistocene, 

 which is now freed from its glacial burden. If the ice averaged 

 5,000 feet in thickness over this area, the sea-level would have been 

 lowered about 235 feet by abstraction of water. In comparing 

 this figure with the 136 feet of elevation at the border of the assumed 

 Labradorian ice sheet it should be remembered that a considerable 

 body of Pleistocene ice existed at distances of more than 4 l 5° from 

 its border and would have exerted an attraction in the opposite 

 direction. 



It seems safe to conclude that at no time or place was the 

 gravitative attraction of a Pleistocene ice cap powerful enough to 

 elevate sea-water at its border sufficiently to overcome the lowering 

 of sea-level due to withdrawal of water from the sea. This means 

 that so far as these two factors are concerned the movement of the 

 strand line was everywhere negative during the time of advancing 

 ice and positive during the waning stages. 2 Near the ice borders 

 the movement was in each case less in amount than in lower lati- 

 tudes, but its direction was the same. Disregarding secular adjust- 

 ment within the earth, the retreat of the Labradorian ice front from 

 Covey Hill northward must have resulted in a progressively more 

 extensive submergence of the St. Lawrence-Ontario Valley. 



For present purposes it is unnecessary to make any inquiry 

 into the causes of the differential uplift of northern lands which is 

 known to have occurred since retreat of the Labradorian ice sheet 

 began. It is, however, essential to know the time relations between 

 the withdrawal of the ice and the readjustments which so convin- 

 cingly appear to be of an isostatic nature. Did the land mass 

 respond so quickly that secular movement entirely compensated 

 the rising sea-level and thus maintained a stationary, or even a 

 retreating, strand-line ? Or did crustal deformation lag behind 

 unloading of the ice burden sufficiently to permit an upward move- 

 ment of the sea-level to precede the upward movement of the land ? 



1 R. A. Daly, "The Glacial-Control Theory of Coral Reefs," Proc. Amer. Acad. 

 Arts and Sci., L (1915), 172. 



2 Cf. C. Schuchert, "The Problem of Continental Fracturing and Diastrophism 

 in Oceanica," Amer. Jour. Sci. (4), XLIII (1916), 92, 93. 



