SUMMARY 247 



do not appear to conform at all to the more strongly lobate boundaries of 

 later stages. 



Comparing the Algonquin and Iroquois isobases with the pre-Cambrian 

 boundary of Ontario (marked on plate 5 by a line of small crosses), we 

 find a general parallelism. The pre-Cambrian boundary, to be sure, is 

 much more irregular than the isobases; but some of its sinuosities may 

 be ignored, since they indicate the presence merely of thin outliers of 

 the sediments or of protruding inliers of the crystallines. The direction 

 of the border of the crystallines seems not to depart for any considerable 

 distance or to any considerable degree from the direction of the isobases. 



Whether the Adirondack oldland in the east and the pre-Cambrian 

 area of northern Wisconsin in the west were also central areas of uplift 

 or not, around which the isobases of the Iroquois and Algonquin planes if 

 extended would turn, is a question which can not be answered from data 

 now at hand. Such measurements as are available give no indication 

 of it. 



In agreement with De Geer^s statement, therefore, it may be said that 

 the parallelism of isobases to the border of the pre-Cambrian oldland in 

 Ontario is as suggestive as the parallelism of isobases to the border of 

 the glaciated area; that a causal connection between the area of long- 

 continued erosion, the area of glaciation, and the area of post-glacial 

 uplift may fairly be inferred from the most recently collected facts. 



Summary 



Detailed surveys of the Algonquin beach around Lake Michigan, Lake 

 Huron, and Georgian Bay have recently yielded a body of accurate data 

 from which a set of isobases can be constructed on the upwarped plane 

 of the Algonquin beacli at intervals of 50 feet. Incidentally a datum 

 plane appears to have been estal)lished, from which one can estimate the 

 amount of uplift in more northerly parts of the Great Lakes region since 

 Algonquin time. 



Fewer measurements of altitude of the Iroquois beach around Lake 

 Ontario furnish ground for the construction of a set of isobases at 100- 

 foot intervals on the Iroquois plane. 



The similarity between the two sets of isobases is remarkable, both as 

 regards the direction and the amount of tilt indicated for corresponding 

 parts of the planes. Accepting Gilbert's conclusion from physiographic 

 evidence that the Iroquois beach is of earlier date than the Algonquin, 

 we are led by this comparison of the two tilted planes to conclude that 

 the difference in age between them is probably verv slight. 



