52 MICHIGAN SURVEY, 1908. 



the island would tend to prevent extensive ponding, combined with 

 the fact that the valleys extended in the same general direction as the 

 uplift and not across it. It therefore appears that many processes 

 have tended to increase the land habitats at the expense of the aquatic, 

 such as the falling of the lake level, the encroachment of organic re- 

 mains on the depressions, the perfecting of drainage lines and the 

 tilting of the surface. 



With the advent of the forest a habitat differentiation developed 

 in contrast with the natural openings. These openings were originally 

 due to the lack of soil, as on the ridges, wave action, as on the beach, 

 or an excess of water as in the depressions. With the accumulation 

 of soil, the downward migration of the waves, and the filling up or 

 draining of the depressions, the range of the forest has been extending, 

 and is tending to completely cover the surface. 



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Bigelow, F. H. 



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