246 J. AV. GOLDTHWAIT^ ALGOKQUIN AKD IROQUOIS BEACHES 



the old rocks are laid bare by erosion and the surrounding lands thickly cov- 

 ered with younger sediment. The limit of the Baltic shield, where it has been 

 directly observed, and perhaps everywhere, is marked by great faults. Now the 

 isobase for zero, or the boundary for the uplifted area, seems all the way a 

 little outside of the above-named limit, and follows very conspicuously its con- 

 vexities and concavities. Likewise all the other isobases point to a close 

 connection between the upheaval and the geological . . . structure of the 

 land."^" 



In the same paper, after reviewing the evidence then available for 

 North America, De Geer writes : 



"The conformity between ice load and subsidence seems to have been still 

 greater here than in Scandinavia ; and in this respect it will be very inter- 

 esting to see what will result from a continued investigation of the warped 

 beaches in the lake basin with its marked ice-lobes. . . . The connection 

 between the subsidence and the geological structure of the earth's crust is per- 

 haps not quite so striking as in Scandinavia. Still it seems probable that the 

 Canadian Azoic or Archean region has changed its level more than the sur- 

 rounding tracts. . . . The general conformity between the ice covering and 

 the old Azoic plateau makes it difficult, in the present state of our knowledge, 

 in many cases to discern between the influence of these two circumstances." '" 



The question thus raised is too difficult to be answered satisfactorily 

 even yet, because too little is known of the course of isobases outside of 

 the Michigan-Huron-Ontario basins. An examination of the isobases 

 for even this limited portion of the Great Lake region, however, is not 

 without interest. 



In the first place, the recent detailed studies of the Algonquin plane 

 have indicated in every district that local irregularities, if present, are 

 immeasurably small. The uplift around such depressions as Green Bay, 

 Wisconsin; Grand Traverse Bay and Saginaw Bay, Michigan, and Geor- 

 gian Bay, Ontario, show accordance — not discordance — with the broader, 

 deeper lake basins adjoining them. The wye-level surveys have discov- 

 ered no looping of the Algonquin isobases where they cross the lakes, 

 such as one might expect from the greater thickness of the ice-lobes in 

 those basins which are bordered by concentric moraines. If the melting 

 away of the ice-sheet was the immediate cause of the uplifts of the region, 

 the irregular load of these late, lobate stages must have been a very small 

 factor in the equation, perhaps because the ice-sheet at that time pos- 

 sessed but a small fraction of its original thickness. The isobases run in 

 a general way parallel to the boundary of maximum glaciation, but they 



88 De Geer : op. cit., pp. 458-459. 



89 Idem, pp. 473-474. 



