468 JAMES WALTER GOLDTHWAIT 



Trent pass up to the level of the one at Port Huron would have raised 

 this old plane (within this region of deformation) well above any 

 subsequent level of the waters. No such plane above the Algonquin 

 beach of the "two-outlet stage" is known, unless it be the Toleston 

 beach. As already noted, this beach is lost, north of Ludington, 

 and no beach is known in the Huron basin which connects with this in 

 the northern part of the lower peninsula of Michigan, 



CONSTRUCTION OF A PROFILE OF WATER PLANES 



The altitudes of the Algonquin beach at about fifty localities in 

 the northern part of the Michigan basin is shown in Fig. 3. This is 

 a miniature copy of a large scale chart (composed of sheets of the 

 U, S. Lake Survey) on which the data collected in 1905 and 1907 were 

 plotted for final inspection. Through and among these points, a 

 series of isobases was drawn with a vertical interval of ten feet. 

 These were found to be parallel to each other, and to be in harmony 

 with the similar lines across the Huron basin, as shown in Fig. i. 

 Across the isobases was then drawn a line in the direction of maximum 

 incHnation — a gentle curve that changes gradually from N. 15° E. 

 near the Straits of Mackinac to N, 5° E. south of Grand Traverse 

 Bay. With this map as a basis for locating points, a profile of the 

 Algonquin beach and the complex series of shore lines below it was 

 then drawn, as follows: Upon the line of maximum inclination was 

 plotted the position of each station on the east side of the lake where 

 measurements had been made. Each one was then transferred 

 directly to a sheet of co-ordinate paper, on which distances from left 

 to right represented distances from south to north. A horizontal line 

 at the base was taken to represent the present level of Lake Michigan 

 (581.5 feet A. T. in July and August, 1907). With a vertical scale 

 of 20 feet to the inch (500 times as large as the horizontal) the altitude 

 of every beach and bench was recorded by an ordinate. These 

 ordinates served to reconstruct the water plane in profile. A reduced 

 copy of this profile constitutes Plate I. In it, between Hessel and 

 Onekama, a distance of 125 miles, over 25 different lines of levels at 

 as many localities along the east side of Lake Michigan are repre- 

 sented. In other words, the stations are on the average 5 miles apart. 

 About 190 measurements are on well-formed beaches or benches, and 



