Page:The Journal of geology (IA journalofgeology11893univ).pdf/141
This great volcanic period was doubtless one of unstable equilibrium, the lithosphere falling here and rising there. One of the final movements was the production of the Lake Superior synclinal. This synclinal movement affects not only the Keweenawan rocks, but the lower series, and in areas in which the unconformity between the Upper Huronian and the Keweenawan is not great, there is such a likeness in strike and dip of the two series as to suggest, at first, that the two are conformable. It is only as the contacts between them are followed for some distance, and the Keweenawan is seen to be now in contact with one member of the Upper Huronian, and now with another, that it is perceived that between the two there is an unconformity.
What proportion of the Keweenawan had accumulated before this Lake Superior synclinal began it is impossible to say. Possibly somewhere near the center of the Lake Superior basin were the larger foci, from which the great extrusions of lava occurred, and here a simultaneous sinking went on, such as is usual as a result of the upbuilding of a mountainous mass of volcanic material. This suggestion, if true, would also partly explain the apparent absence of volcanic fragmental material which naturally would accumulate near these foci.
Nowhere are the Keweenawan rocks so closely folded as to give them a schistose structure or a metamorphic character. Their induration is almost wholly a process of cementation.
The Cambrian Transgression.—At the close of Keweenawan deposition the Lake Superior region was again raised above the sea, and the pre-Cambrian erosion continued until the enormous thickness of Keweenawan deposits was wholly truncated. What must have been mighty mountains were reduced to mere stumps, or to base level. Following this denudation, the sea once more transgressed upon the land, and the horizontal Lake Superior sandstone was deposited. It now occupies many of the bays about Lake Superior. It once was much thicker, and perhaps covered all but the highest points of land. Certainly it or an overlying formation once was at least one thousand feet higher