200 GEOLOGY. 



involved in the Huronian and Animikean systems, their heights would 

 perhaps have surpassed any elevation now existing on the surface of 

 the earth. It is not probable, however, that such enormous elevations 

 existed at any time. It is more probable that as erosion proceeded, 

 the land itself reacted by rising slowly, or that further arching took 

 place, or that the sea bottom sank, drawing off the waters and leaving 

 the land relatively higher. In this way, degradation and elevation 

 may have been in progress at the same time, and the one process may 

 never have got far ahead of the other. It is even believed by many 

 geologists that the removal of sediments in large quantities from the 

 land would allow it to rise, and that their deposition on the sea bot- 

 tom would cause that to sink. This doctrine is known as isostasy. 

 Succession of events. — Reviewing the succession of events in the 

 Lake Superior region, we find (1) that the land was high enough after 

 the Archean rocks came into position for its surface to suffer erosion, 

 and that the period of erosion was long, but that the sites of the earliest 

 sedimentation are unknown. (2) The Archean land then sank or 

 was so eroded or deformed as to permit the deposition of the Lower 

 Huronian (p. 161) sediments on portions of its eroded surface. (3) 

 The area of the former Archean land, together with the Lower Huronian 

 beds about it, was again brought into such an attitude, presumably 

 by crustal warping, that it was subject to a long period of erosion, with 

 contemporaneous sedimentation elsewhere. During the time of the 

 deformation, the rocks involved were metamorphosed. (4) Again the 

 land seems to have sunk, allowing the sea (or at any rate, conditions 

 for deposition) to cover a large part of the territory which had been 

 above sea level, and to deposit upon its eroded surface the sediments of 

 the Middle Huronian system. (5) After this period of sedimentation, 

 the land seems to have emerged, exposing the landward border of 

 the Middle Huronian system, and all the older rocks not covered by it, 

 to erosion. (6) Submergence of the eroded surface of the Middle 

 Huronian was followed by the deposition of the Animikean system. 

 (7) This newly deposited system then emerged over great areas, and 

 was exposed to erosion. This emergence was accompanied by some 

 deformation and metamorphism. At various stages in the progress 

 of these changes there were intrusions and extrusions of lava, on such 

 a scale that the aggregate amount of igneous rock associated with 

 the sedimentary is very great. The igneous activity also aided in 



