468 PLEISTOCENE OF INDIANA AND MICHIGAN. 



differential elevation or tilting for several of the more northerly stations as compared with sta- 

 tions farther south. The mean result on the assumptions made indicates an uplift of 0.42 foot 

 per 100 miles in a century on a line running N. 27° E. 



The stations chosen were Milwaukee and Escanaba on Lake Michigan, Port Austin on 

 Lake Huron, Cleveland and Port Colborne on Lake Erie, and Charlotte and Sacketts Harbor 

 on Lake Ontario. 



Unfortunately, when Gilbert finished his investigations (1896) the position and attitude 

 of the Nipissing plane in the southern parts of the basins of Lakes Huron and Michigan had not 

 been correctly determined. It was then supposed that both the Algonquin and Nipissing 

 beaches passed under the present lake level southward, the Nipissing being about 20 feet beneath 

 it at Port Huron and about 100 feet beneath it at Chicago. These, however, were estimates 

 based chiefly on the planes of the beaches produced from the north and partly on erroneous 

 determinations of the places where the planes cut the lake surface. The responsibility for 

 these errors is shared by Spencer and the writer. 



Since that time it has been found that, instead of passing under the lake surface southward 

 both beaches become horizontal, the Algonquin at about 25 feet above the present lake level 

 and the Nipissing at about 15 feet. The hinge fines, where the planes pass from an inclined to a 

 horizontal attitude, are thought to be very nearly coincident across the southern peninsula of 

 Michigan. (See fig. 8, p. 439, and fig. 9, p. 461.) 



These newly found facts throw fresh light on the problem of recent earth movement in 

 the Great Lakes region, introducing important changes in Gilbert's assumptions and modify- 

 ing his interpretation of some of the data. The relations of his chosen stations to the area of 

 horizontafity becomes a matter of much importance. Of these, Chicago and Cleveland are far 

 within the area of horizontafity in which no recent earth movement is known. Port Austin and 

 Port Colborne are both close to the hinge line of latest uplift and maybe on it or slightly south of it; 

 at most they are not far north of it and have been affected very little by recent uplifts. Only Esca- 

 naba, Charlotte, and Sacketts Harbor are well within the area of uplift. Of the four pairs of 

 stations considered only the pair on Lake Ontario is wholly within the deformed region. This 

 makes it necessary to modify Gilbert's result for three of the pairs, and leaves it unchanged for 

 only one — Charlotte and Sacketts Harbor. 



If the movement indicated between Milwaukee and Escanaba is really caused by uplift 

 at Escanaba, its rate is more than twice that obtained by Gilbert, if his determination is numer- 

 ically accurate, for the hinge line of the movement is probably more than halfway north from 

 Milwaukee to Escanaba. The last movement recorded by the beaches appears to have hinged 

 on this line. 



It seems doubtful whether Gfibert's investigation proves that uplift of the land is now in 

 progress. But, on the other hand, a demonstration that Gilbert's conclusions are not well 

 founded does not constitute proof of the truth of the contrary proposition, namely, that uplift of 

 tfie land is not now in progress. The author is inclined to the belief that the movements of uplift 

 are spasmodic in character and occur rather suddenly at intervals separated by long periods of 

 rest, during which movements do not occur. It may be doubted whether a measurable move- 

 ment of uplift has occurred within the last 100 or 150 years. The old beaches in many places 

 suggest sudden uplift, and the place in the Niagara gorge which marks the last increase of vol- 

 ume (just above the railroad bridges) shows a change in gorge characters so abrupt as to be 

 consistent only with a relatively sudden great increase of volume. This change was due to 

 uplift at North Bay, Ontario, closing the outlet there and sending the discharge back to Niagara. 



The lack of water-gage stations on the northern shores, especially on those of Georgian 

 Bay and Lake Superior, seems to necessitate postponement of further efforts to solve this 

 problem by this method until observations shall have been collected for many years from 

 stations that still remain to be established. The need of such stations is urgent and immediate. 

 The most desirable places are at the mouth of French River at the north end of Georgian Bay, 

 and at Peninsula Harbor at the extreme northeast angle of Lake Superior. Stations at these 

 two places would afford the longest possible lines in the direction of recent tilting. 



