Page:The Journal of geology (IA journalofgeology11893univers).pdf/103

Superior region is not known. The same is true of other regions. For this reason, the various terms, Huronian, Keweenawan, Vishnu, Chuar, etc., which have been used to designate definite parts of the group, will still be retained, for in the absence of criteria for the satisfactory correlation of the subdivisions of the group in the various regions where they occur, these parts must continue to bear local names.

The group is so extensive as to be comparable in thickness to the Paleozoic, Mesozoic and Cenozoic combined, and inferentially to represent an equal lapse of time. It contains great systems, separated by great unconformities. Concerning the two unconformities in the systems in the Lake Superior region, those between the Lower and Upper Huronian and between the latter and the Keweenawan, Professor Van Hise says: "Each represents an interval of time sufficiently long to raise the land above the sea, to fold the rocks, to carry away thousands of feet of sediments, and to depress the land again below the sea. That is, each represents an amount of time which is perhaps as long as any of the periods of depositions themselves." In parts of the region the Lower Huronian is known to be unconformable on the Archean. In other parts the relations are unknown. This statement of the case gives some idea of the thickness of the group, as well as of its complexity and importance.

The delimitation of the Algonkian is theoretically easy, after the definitions of the Archean and Cambrian. It includes all pre-Cambrian sedimentary rocks, and their igneous equivalents. Although a great unconformity generally separates the two groups, helping to render their distinction clear, it is not always easy of recognition. Locally parts of the Algonkian have undergone such profound metamorphism at the hands of dynamic forces which affected the Archean as well, that they seem to be structurally one. In such cases it is believed that the apparent conformity is in reality apparent only, the original structural relations being obscured or even obliterated by the structures superinduced by dynamic forces on both series involved. Even where there is a common structure in rocks regarded as Archean and Algonkian, there is sometimes inherent evidence that one part of the rocks concerned is clastic, while similar evidence is wanting in the other.

Not the least instructive part of the volume is the discussion of the principles applicable to Algonkian stratigraphy. It would be useless