20 Schuchert and Barrett — Revised Geologic 



invasion. Batholithic mountains and regional metamorphism 

 were the result. 



The greatest additional advance in recent years regarding 

 the pre-Cambrian classification is doubtless the differentiation 

 of the Laurentian igneous invasion and crustal disruption into 

 two distinct crustal revolutions separated by a long era of 

 erosion and sedimentation. Such a difference in age in the 

 Laurentian base is discussed by Yan Hise and Leith (op. cit., 

 p. 28), part of the fundamental granites being regarded as 

 intrusive in the Algonkian. The term Laurentian they recom- 

 mend to be restricted to the older granites intrusive into the 

 Keewatin, but not into younger rocks, and they point out the 

 confusion which results when the determination of the age of 

 the granite is neglected. 



When in 1889 it was proposed by the LTnited States Geo- 

 logical Survey to classify the pre-Cambrian rocks into two 

 great divisions, the Archean and Algonkian, it was not known 

 that the granitic base held any important masses of sediments ; 

 and the Archean, represented mostly by Keewatin basaltic 

 lavas and Laurentian intrusive granites, both in altered forms, 

 that is, greenstones and gneisses, was looked upon as a primal 

 igneous eon. The Algonkian in contrast was thought to con- 

 sist dominantly of sediments, though including much igneous 

 material. Later work has largely broken down this distinc- 

 tion. The enormously thick Grenville series and the Sud- 

 burian were once widely spread, but have been mostly swal- 

 lowed in the rising granites. Important intrusions cut also 

 the Huronian and Animikian. In view of this intermeshing 

 of what were once thought to be two kinds of dominant ter- 

 restrial activity, distinctly separated in time, the term Algon- 

 kian has largely lost its usefulness. If used at all, it should 

 apparently be restricted to the rocks laid down after the 

 second of the wide-spread granitic invasions which disrupted 

 the foundations of the Canadian Shield. 



The first of the granites, Lawson in his recent paper con- 

 tinues to call the Laurentian, the second great invasion he has 

 named the Algoman, and he places it after the Huronian. 

 Other writers, however, place the second before the Huronian; 

 Coleman, moreover, considers it probable that the Laurentian 

 of Logan's original area belongs to the later invasion, not the 

 earlier, and the name refers more properly therefore to the 

 later of the two. Such a conclusion leaves the earlier granite 

 and gneiss nameless. In order to avoid confusion until more 

 definite knowledge is attained, the writer proposes to call 

 the older the Paleolaurentian, the later the Neolaxirentian. 

 Probably much of the fundamental granite gneiss for a long 

 time, if ever, cannot be classified positively into either the one 



