Page:The Journal of geology (IA journalofgeology11893univ).pdf/142

than the level of Lake Superior, but it has since been almost completely removed, so that it occurs only in patches within the depressions of the older rocks.

Since Cambrian time no important orographic movements nor outbursts of volcanic material have occurred in the Lake Superior region, consequently the rocks have received little subsequent alteration. To these facts is due the possibility of outlining the pre-Cambrian history of this area with greater fullness than has been done in areas in which later disturbances have obscured the early history.