of Sea-level in the Ottawa Valley. 33 



a large body of ice still in existence, that the most rapid return of 

 water to the ocean is to be expected. 



2. Brogger has clearly established that the isostatic recovery 

 progressed with a wave-like motion from south to north along the 

 Cattegat, following up the retreating ice-margin. This seems to 

 indicate that the recovery takes some time to get under way, and 

 does not attain its maximum rate until the neighbourhood is altogether 

 clear of ice. 



At this particular period of the retreat, therefore, it would be 

 natural to expect that the rise of the ocean level might be, for a time, 

 faster than the isostatic recovery, and submergence would result. 

 . .a^er, when the isostatic recovery had gathered pace and the amount 

 of water returning to the ocean from the waning ice-sheets had 

 become gradually less, we might expect the isostatic recovery to 

 attain the upper hand and give us progressive emergence. 



Now the best test of the validity of this theory is its applicability 

 to the isostatically affected areas of the British Isles and North 

 America. Unfortunately, in the British Isles the highest late-glacial 

 shoreline is only 100 feet above the present sea-level, and within this 

 small vertical range evidence of the kind utilized by Brogger is not 

 to be expected. In North America, until the appearance of the 

 paper under review, no investigation, such as would bring to light 

 a relation of this nature, appears to have been placed on record. 

 Johnston now brings forward evidence, of a nature similar to that 

 adduced by Brogger, to show that the late-glacial changes of sea-level 

 in the Ottawa Valley were precisely the same as those established 

 for the Christiania region, namely, that the sea first rose on the land 

 as the glaciers retreated, and that it was not until a later date that 

 emergence supervened. Moreover, he makes a further point of 

 great importance in establishing the isostatic theory on a firm basis. 

 This point, for which there was no direct evidence in the Norwegian 

 case, is to the effect that the tilting of the Great Lakes region was in 

 progress before and during the rise of the sea in the Ottawa Valley, 

 for, presumably from a consideration of contemporaneous ice-margins, 

 it is concluded that " the Ottawa Valley must have been, in part at 

 least, occupied by the ice-sheet during the existence of Lakes Iroquois 

 and Algonquin, and at least a small amount of uplift affected the 

 region at the foot of Lake Ontario during the life of Lake Iroquois. 

 Uplift also affected the northern portion of the Great Lakes region, 

 and probably included the upper portion of the Ottawa Valley near 

 Mattawa during the existence of Lake Algonquin, and while the 

 ice-sheet still occupied the upper portion of the Ottawa Valley". 

 Further, it is not a case of alternating elevation and depression, 

 "for the results of investigations by numerous geologists, of the 

 raised beaches of the Great Lakes region, has shown that differential 

 uplift took place almost continuously as the ice withdrew." 



"We thus have direct proof that a district which was rising 

 relatively to those around it was nevertheless undergoing submergence 

 beneath the level of the sea, that in fact the two factors invoked to 

 explain the late-glacial changes of level were in action simultaneously 

 in the same region. 



DECADE VI. — VOL. IV. — NO. I. 3 



