44 MICHIGAN SURVEY, 1905. 



The conditions and changes that are going on in this region at the present 

 time have been given in the discussion of the different stations, but, owing 

 to the fact that the relation between environmental changes and the succes- 

 sion of societies has also prevailed in the past, the historic factor in biotic 

 interpretation, the present conditions will not alone explain the present 

 biotic conditions. It is therefore necessary to take into account the condi- 

 tions that have prevailed in the past. This may be done by reversing the order 

 followed in the discussion and by considering the past in the light of the 

 action of the present processes. 



The historical geology of the Lake Superior region has been worked out 

 by Van Hise ('04), and his results are the basis of the following account. 

 During the earliest period the oldest rocks were formed of which we have 

 any knowledge; the crystalline schists, gneisses and granites comprising 

 the Basement Complex (the Kewatin and Laurentian) . They form in North 

 America the broad old land area, extending, according to Wilson ('03, p. 617), 

 from Coronation Gulf in the extreme northwest of Canada, southward around 

 Hudson Bay, and northward through Labrador to Baffin Bay and beyond. 

 South of Lake Superior, in Michigan and Wisconsin, there is a continuation 

 of this area, largely buried in Michigan under later deposits but exposed 

 in a large area in Central Wisconsin. There were at least three series of 

 rocks formed upon the Basement Complex before the Keweenawan, but the 

 mountains formed by the elevation and folding of these rocks were reduced 

 to a peneplain before the Keweenawan rocks were laid down. During the 

 period of unstable equilibrium that followed the Huronian Epoch, this 

 peneplain was submerged, and the sediments of the Keweenawan Series were 

 laid down. During their formation, these sediments were covered from 

 time to time by great sheets of volcanic lavas, the products of fissure erup- 

 tions. At the close of the Keweenawan Epoch, the land was again elevated 

 and the strata tilted to form great mountains, as is shown by the inclination 

 of the strata in the cross section. Fig. 2. During the subsequent cycle of 

 erosion, the entire thickness of the Keweenawan Series was greatly eroded, 

 and the mountains together with the whole pre-Cambrian area (Wilson, '03, 

 and Weidman, '03) were reduced nearly to sea level. 



During Mesozoic times, there was a particularly well marked period of 

 baseleveling that removed the overlying Palaeozoic sediments from the 

 Lake Superior region and reduced the topography to a peneplain. This 

 peneplain, the Jura-Cretaceous, extended over much of the existing land 

 area of North America and has been recognized in Canada (Wilson, '03, p. 

 658), in the Ozarks (Hershey, '01, pp. 22-24), eastern (Davis, '89, p. 197) 

 and western United States (Woodworth, '94, p. 221). At this time eastern 

 and western North America were separated by the Cretaceous Mediterranean 

 Sea. At the close of the Cretaceous Period, the continent was unified by 

 an elevation of the interior which banished the Mediterranean Sea and put 

 an end to the Jura-Cretaceous peneplain; but in the Tertiary Period that 

 followed, another cycle of erosion occurred which, although not complete 

 in the mountainous regions of North America, sufficed to reduce the northern 

 part of the continent nearly to baselevel (Upham, '04). The surface of the 

 earth in the Great Lakes region at this time probably became covered by a 

 thick mantle of residual soil, owing to the enormous length of time during 

 which it was subjected to disintergrating processes. 



The present elevations in the Porcupine Mountain region are evidently 

 formed by the projecting edges of the more resistant basic and acidic lava 

 sheets, while the valleys of Carp and Union rivers are sunk by erosion into 



