44 6 BOTANICAL GAZETTE [juxe 



It seems, therefore, that some other factor must be sought. Plane- 

 table maps were made of a number of bogs and the distribution of 

 plants was carefully plotted. Contour maps of the bottoms of the 

 original post-glacial lakes were then made. In making these maps 

 a drill was made of gas pipe, cut in four-foot (i.2 m ) lengths. To 

 the end section was welded a carpenter's auger. With this drill, 

 soundings were made through the peat to the clay bottom of the 

 original post-glacial lake. A base line was run taking the longest 

 diameter of the bog. Along this and at right angles to it, cross lines 

 were drawn every hundred feet (30. 48™) in large lakes and every 

 fifty feet (15.24™) in small lakes. Borings were made at the inter- 

 sections of these lines and every hundred or fifty feet on the cross lines. 

 Near the shores it was necessary to make the borings closer, often 

 every five feet (i-5 m ). In bogs with open water it was necessary 

 to make all borings through the ice, because it was impossible to hold 

 a boat steady enough to bore through the peat without breaking the 

 drill. 



First Sister Lake. 2 — This bog is situated a short distance west 

 of Ann Arbor and has already been described by other writers (8, 

 9). It is surrounded on all sides by hills, except for three breaks. 

 To the north was a small post-glacial lake; on the southwest corner 

 it connected, by a narrow channel, with Second Sister Lake. The 

 whole formed a very irregular chain of lakes draining into the Huron 

 River. 



The contour map of the bottom of the original post-glacial lake 

 is shown by red lines in Jig. i. The heavy line indicates the margin of 

 the original lake as indicated by the peat deposit. The shores rise 

 somewhat abruptly on all sides. The lake had one basin. The 

 deepest part was east of the center, the deepest point being fifty-four 

 feet (16. 5 m ). From this point the bottom sloped gradually to the 

 eastern shore. On the opposite side the forty-foot (12 . 17™) contour 

 made a wide divergence to the west; from this contour the bottom 

 sloped gradually to the shore. 



No attempt is made to show the flora of this early time. The black 

 of fig. 1 shows the arrangement of things as they were last summer. 

 The open water occupies a very small area compared to what it did 



2 Borings were made for the most part by Mr. Harold Steele. 



