50 MICHIGAN SURVEY, 1908. 



lake currents. Since the location of outlets, prevailing winds, topo- 

 graphy of the basin and rotation of the earth all influence lake cur- 

 rents,' it is evident that any important change in these conditions will 

 cause a modification in the currents. By means of these criteria then 

 we may infer what currents are likely to have existed under certain 

 conditions. Some of these conditions have had a very permanent value 

 in the Superior basin, because the general form of the southern shore 

 (except Keweenaw Peninsula), the earth's rotational deflection to the 

 right, and the prevailing westerly winds, made, relatively definite condi- 

 tions. Thus the early Glacial lakes in this basin, which had south- 

 western outlets, must have had different currents, perhaps more or 

 less against the prevailing westerly winds, and the absence of large 

 islands would be favorable to uniformity. Later at the Algonquin stage. 

 Fig. 52, there must have been a very complicated system of lake cur- 

 rents, perhaps a rough outline of those of the present Great Lakes, at 

 least in the deflection toward the right shores on account of the rota- 

 tion of the earth, and to the eastward on account of the prevailing 

 westerly winds and the eastern outlets. The broad connection between 

 the Superior and the Huron basins perhaps also favored a north shore 

 return whirl, while at the Nipissing stage. Fig. 54, in the Superior 

 basin the currents were in general quite similar to those of the present 

 lake, but more simplified in detail by the greater depth of the lake. 



If such general relations as these obtained, it will be seen that the 

 north shore return whirl may have been of considerable duration, and 

 that the opportunity for these currents to carry life from the south 

 shore must have been 'constantly less favorable than the chances for 

 them to effect transportation from the north shore of the Superior basin. 

 In this basin then it seems that the currents were first relatively simple, 

 became quite complex at the Algonquin stage and were simplified at 

 the Nipissing stage. A detailed study of the beach lines such as those 

 of the Nipissing, might add much positive information as to these 

 ancient lake currents and their biological relations. 



f. The Origin of the Habitats. Isle Eoyale is about 45 miles in length, 

 has an average width of about seven or eight miles and an area of about 

 210 square miles. The shallow soil, rock ridges, forested swamps, lakes, 

 small streams, rocky coast, and harbors provide a variety of 

 conditions and furnish play for such a variety of processes that many 

 diverse habitats are produced. Generally speaking, the island is covered 

 with a stunted coniferous forest growth. Attention has already been 

 called to some of the conditions \ and processes which have produced 

 the major environmental regions and the general topography of the 

 surface.. If Isle Eoyale had high mountains and greater extent, very 

 different habitats would be expected. 



As we have seen, the entire surface of the island has been beach, 

 and previous to that it had been a reef in the lake, so that the beacli 

 represents the original land habitat upon the island. Generally speak- 

 ing this habitat has migrated from the crest of the Greenstone Range 

 downward for about 550 feet to the present lake level. With this 

 progressive downward movement, there has been an increasing area 

 exposed to subaerial processes of erosion. The origin of the harbors 

 has been a part of the beach problem, but that of the protected beach, 



