ALGONQUIN WATER-PLANE 229 



incomplete to justify the drawing of isobases, our efforts ought to be 

 redoubled to accumulate data so accurate and so abundant that a set of 

 isobases shall provoke no more skepticism than so many well-executed 

 contour lines. Then, and not until then, can we hope to do much in 

 analyzing the great upwarpings of late glacial time. 



Good isobasic maps are valuable in two ways : First, as aids to fixing 

 the relative ages of extinct lakes in different parts of a region; and, sec- 

 ond, as indicators of the extent, nature, and cause of epeirogenic move- 

 ments. The present paper touches both sides of the subject. Isobases 

 for the Iroquois beach and the Algonquin beach will be compared, with a 

 view to answering the question whether these two critical stages of the 

 extinct lakes, Iroquois and Algonquin, are synchronous or not. The 

 question of the nature and cause of the differential uplifts will be treated 

 very briefly, because that is soon to be taken up by Mr. F. B. Taylor in a 

 monograph which is now in preparation. 



The data here used are from several sources, as references in every 

 case will show. Most of the measurements of altitude of the Algonquin 

 beach have been made during the last five years by the geological surveys 

 of Wisconsin and Michigan, the U. S. Geological Survey, and the Geo- 

 logical Survey of Canada. These measurements are especially reliable, 

 because they were made with the wye-level, rather than the hand-level or 

 the aneroid barometer. 



The Algonquin Water-plane ^ 



stage recorded by the algonquin beach 



The Algonquin beach marks a critical stage in the history of Lake 

 Algonquin. For some time previous the discharge of the ice-dammed 

 lake had been entirely through the Trent Yalley into the waters of the 

 Lake Ontario basin along the "Algonquin Eiver"^ (see figure 1). Dif- 

 ferential uplifts, however, had been lifting this region with respect to 

 more southerly districts, and the rising waters of the lake had been ad- 

 vancing on the shores in the Michigan and Huron basins. When the 

 head of the Trent Yalley, at Kirkfield, Ontario, had been lifted to an 

 altitude as high as the pass at the south end of Lake Huron, that pass 

 began to receive a share of the overflow which ran down the Saint Clair 

 and Detroit rivers into Lake Erie. This two-outlet stasfe of the lake is 



5 The names "Lake Algonquin," "Algonquin beach," and "Algonquin River" were first 

 used by .1. W. Spencer : Notes on the origin and history of the Great Lakes of North 

 America. Proceedings of the American Association for the Advancement of Science, 

 vol. 37, 1889, p. 199. 



