12 SANDUSKY FLORA. 



sided until what are now the islands appeared above 

 its surface. This view is doubtless correct, but there is 

 now much evidence to show that it continued to 

 subside until the islands formed part of the mainland 

 and afterward rose and isolated them again, and is 

 still rising and likely to submerge them again. The old 

 beaches which may be traced for long distances 

 running nearly parallel to the present shores of the 

 Great Lakes, must have been level at the time they 

 were formed, but they are not now level, and there has 

 therefore been a tilting of that part of the earths crust 

 which includes the basins of the Great Lakes, as there 

 has been of many other parts. These beaches all have 

 gentle slopes, toward the south and south-west, indi- 

 cating that in this part of North America, there has 

 been an uplifting of the land toward the north 

 north-east or a depression toward the south 

 south-west or both. The effect of this tilting 

 of the basins of these lakes has been to 

 raise the water on the south and west as 

 compared with that on the opposite sides, just as the 

 tipping of a saucer partly filled with water would do. 

 The fluctuation of the water due to variable winds and 

 rainfall make such comparisons difficult, but Mr. G. K. 

 Gilbert found b}-^ comparing the heights above the 

 normal level of Lake Erie in 1895, of a certain point in 

 Cleveland, and a certain point at the head of the Wel- 

 land canal with the heights of the same two points as 

 carefully determined in 1858, that the point near the 

 north-east end of the lake rose 0239 foot as compared 

 with the point in Cleveland. This is a small amount 

 and in view of the difficulty of determining the normal 

 level and measuring the exact height of any point on 

 the land above it even by measurements many times 

 repeated, it might well be attributed to some inaccur- 

 acy in the measurements if it were an isolated case, but 

 it is not. Similar comparison of points on Lake 



