jaESUMfi OF AECHEAK 49 



therefore seem to have been due to the action of percolating water, without 

 special mechanical influence. Hence we may date the intrusion of this 

 particular dike after the orogenic movements which affected the granite 

 core, rendering portions of it schistose, and crushing all of it to a greater 

 or less extent. These movements are presumed to have taken place just 

 prior to or during Keweenawan time; and therefore the age of this dike is 

 Keweenawan or post-Keweenawan.^ 



In the above-described granite massif we have a rock of pre-Huronian 

 age, as shown by its relations to the overlying sedimentaries. It possesses 

 in general a coarse granular, and in places porphyritic textu:re. Along its 

 border it contains portions which are much finer grained, darker than the 

 rest of the mass, and very well banded. The boundaries between the 

 banded rock and the granite at times are sharp, but frequently are very 

 indefinite. This banded schistose portion is found to be due to pressure, 

 causing the gradual passage from the granular granite to the gneissoid, 

 schistose granite. 



One instance of undoubted inclusion of gneissoid granite by a true 

 granite was observed. If the gneissoid granite was derived by pressure 

 from the Archeau granite, then the particular granite dike which includes 

 the fragments must be of later age than the great mass of granite of the 

 Archean area. 



The Archean is cut by basic dikes of two ages. The earlier ones were 

 rendered schistose, and the production of this secondary structure was 

 accompanied by a total obliteration of the primary igneous texture and 

 the production of a large amount of mica and hornblende. All the dikes 

 were probably injected at the time of the volcanic activity when the vol- 

 canics of the higher series were ejected, but no proof of their connection 

 can be produced. They were, however, injected before the folding of the 

 area took place, as shown by their having been rendered schistose by it. 



A single dike belonging to the later series was studied. It is massive, 

 and therefore was irrupted after the folding which produced the schistosity 

 in the earlier series of dikes. It belongs probably to a Keweenawan or 

 post- Keweenawan period of eruption. 



' For a discussion of the orogenic movements which alit'eoted the Cry.stal Falls district, the reader 

 is referred to p. 158 et seq. 



MON XSXVI 4 



