REVIEWS. 529 



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 satis- 

 factory 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, sepa- 

 rated 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, Pro- 

 fessor Van Hise says: "Each represents an interval of time suffi- 

 ciently 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 unconform- 

 able 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-Cam- 

 brian 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 meta- 

 morphism 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 



