314 TerfAary. 



tion of the current was from north to south, although locally changed 

 or directed by the pre existing inequalities of the surface. 



The drift is succeeded on the Mississippi by a narrow belt, called 

 the Bluff Group, but in other parts of the State the drift is covered by 

 a yellow loam, which also succeeds the Bluff. The second bottom, or 

 Hommock deposits, and the alluvial, are yet more recent in their char- 

 acter. 



Drift materials* are strewn over a great part of the surface of Michi- 

 gan. At East Saginaw these materials are from 90 to 100 feet thick, 

 and at Detroit 130 feet thick. Wherever large surfaces of the under- 

 lying rocks are exposed, they are found to be more or less smoothed 

 and striated. The island of Mackinac shows the most indubitable evi- 

 dence of the former height of the water, 250 feet above the level of the 

 lake. The trunks of white cedar trees are not uncommon in the drift, 

 and on the north shore of Grand Traverse bay there is a bed of lignite. 



In 1862, Prof. J. D. Whitney pointed out,f approximately, the terri- 

 tory in northern Illinois, western Wisconsin, northeastern Iowa, and 

 eastern Minnesota, that is destitute of drift. . This tract is several 

 hundred miles in length, and from 100 to 200 miles in width. There 

 is an entire absence of bowlders or pebbles, or any rolled and water- 

 worn materials, which by their nature would indicate that the region 

 in question had been exposed to the action of those causes by which 

 the drift phenomena were produced. The surface of the rock is uneven 

 and irregular, bearing the marks of chemical rather than of mechanical 

 erosion, and there are no furrows, stria? or drift scratches, such as may 

 be observed on many of the rocks over which the drift has been moved. 



He concluded: 



1st. That there has existed, ever since the period of the deposition 

 of the Upper Silurian, a considerable area, chiefly in Wisconsin and 

 near the Mississippi river, which has never been sunk below the level 

 of the ocean, or covered by any extensive and permanent body of 

 water, and which, consequently^ has not only not received any newer 

 deposit than the Upper Silurian, but has also entirely escaped the in- 

 vasion of the drift, which took place over so vast an extent of the 

 northern hemisphere. 



2d. That the extensive denudation, which can be shown to have 

 taken place in this region, as witnessed by the outliers of rock still re- 

 maining, and the general outline of the surface, has not been occasioned 



* Geo.Sur. of Mich.. 1861. 

 i Geo. Sur. of Wisconsin. 



