THE TERM LAURENTIAN. 339 



Huronian, and the writer is not aware of any general contradiction that 

 has been given to the assumption by more recent writers on Quebec and 

 eastern Ontario. For this reason the application of the term Laurentian 

 to the irrnptive granite gneisses west of Lake Superior is perhaps un- 

 fortunate, as involving a hasty and undetermined correlation. 



Consideration* affecting the Status of the Term Laurentian. — Admitting 

 for the moment that the Laurentian rocks of Quebec are of sedimentary 

 origin, and that the granite gneisses west of Lake Superior are irruptive 

 in their character, it is still possible, nay, probable, that the two are con- 

 temporaneous in age and identical in primal origin on the assumption 

 that these latter rocks but represent metamorphism carried to the ex- 

 treme of fusion (as a consequence of the relatively higher local elevation 

 of the isotherms), resulting in their irruption into the overlying Huronian 

 strata and their recrystallization in the form of consolidated magma. 

 The question, then, of the most appropriate name for these western 

 Ontario granites becomes a question of the era of our chronology. 



Shall we date them from the time of their intrusion into the overlying 

 strata, or shall we go further back into their obscure history and date 

 them from the time when in all probability they formed the solid floor 

 on winch the Contchiching and Keewatin rocks were laid down ? In 

 other words, shall we call them Huronian (the term is here used to in- 

 clude all the rocks between the fundamental granites and the Animikie) 

 because they are intrusive into Huronian strata, or Lurentian on the 

 above assumption of their genetic identity with the Laurentian gneisses 

 of the east? If we can regard the irruptive origin of the present relations 

 of these granites to the Upper Archean rocks as indubitably established, 

 it would seem unwise to go behind this fact into the uncertain realm of 

 theory to justify for them the name " Laurentian," as the term ' : Huronian 

 granite " embodies a more precise statement of our conclusions. 



But the passage of the granitic phases into the gneissic is so gradual, 

 the lithologic similarity between the gneisses of the east and of the west 

 is so marked, and their geographic continuity so highly probable, if not 

 an established fact, that it is difficult to conceive of any great genetic dif- 

 ference or of any considerable geologic interval between their respective 

 ages. The applicability of the term Laurentian, as applied to the gran- 

 ites of Rainy River district, is also supported by the fact that as the 

 Laurentian of Quebec is being reexamined in the light of modern 

 knowledge, the opinion is gaining ground that at least the "Lower 

 Laurentian" rocks present characters precisely analogous to these of the 

 west. Perhaps some of the rocks that have hitherto been called Upper 

 Laurentian in the east are the equivalents of the Contchiching series of 

 Lawson, although they differ from them in some lithologic characters. 



