ISOBASES AND PRE-CAMBRIAN BOUNDARY 245 



right position for a "liinge line/' we might move the "100 plus/' "200 

 plus/' and "300 plus" lines southward to correspond, and we could then 

 estimate the tilt rates more correctly. x\ll these rates would^ of course, 

 be smaller after this correction than they appear in the table, because of 

 the southward flattening of the plane; but the rate nearest the zero line 

 would be affected most. The result of decreasing the tilt rates of the 

 Iroquois plane in the table and of decreasing especially the rate of 2.24: 

 feet per mile for the stretch between the and 100 isobases, would be to 

 make these tilt rates agree even more closely than before with the rates 

 for corresponding stretches of the Algonquin plane. 



The bearing of this similarity of attitude of the Iroquois and Algon- 

 quin water-planes upon the question of their relative ages is obvious. If, 

 as Gilbert's observations seem to require, the Iroquois beach is older than 

 the Algonquin, it can not be much older, for it has been tilted only a 

 little more than the Algonquin. Uplifts are known to have been going 

 on for some time previous to the two outlet stages of Lake Algonquin, 

 and to have continued for some time after it. Yet the Iroquois beach 

 seems scarcely to have been affected by those uplifts which preceded the 

 two-outlet stage. The two beaches appear, therefore, to be almost syn- 

 chronous. In other words, it appears that the Ontario ice-lobe had only 

 recently withdrawn from the northwest slope of the Adirondacks when 

 the Algonquin Eiver ceased running and the Algonquin beach began to 

 rise above the 600-foot plane. 



The Isobases and the pre-Cambrtan Boundary 



In his paper on "Quaternary changes of level in Scandinavia,"^^ in 

 1891, De Geer said: 



"The isanabases of Sweden were found to conform with the limits of the 

 Scandinavian Azoic territory, and, according to the very latest determinations, 

 not only in a general way, but also in many details. . . . The coincidence 

 between the area of upheaval and the Azoic territory may possibly be ex- 

 plained by assuming that this territory, which is an old tract of erosion, has 

 also been one of continental upheaval, which subsided during the ice age, for 

 the greater part perhaps in consequence of the considerable ice-load, again 

 rising after the release from the latter, though not to its former altitude." 



Again, in his paper of the following year,^^ De Geer emphasized 



"the coincidence of the uplifted area with the Scandinavian Azoic region, or 

 what Suess has called 'the Baltic shield' ... a well-defined tract where 



3* De Geer : Bulletin of the Geological Society of America, vol. 3, 1892. pp. 65-60. 

 ^ De Geer : On Pleistocene changes of level in eastern North America. Proceedings 

 of the Boston Society of Natural History, vol. 25, 1892, pp. 454-477. 



