CHARACTER AND RELATION OF THE LAURENTIAN ROCKS. 337 



the younger, sometimes the older and sometimes a contemporaneous rock ; 

 while it is not probable that there is any great difference in their respect- 

 ive ages. In this connection it is interesting to note that in Finland Dr 

 J. J. Sedesholm * finds that there are two main series of granites, of which 

 the earliest plageoclastic and hornblendic are eruptive in their relations 

 to the Archean schists and younger than them; the other series, red 

 garnetiferous muscovite microcline granites, the coarser varieties of which 

 merge into a pegmatite, are the latest eruptive rocks of the Archean com- 

 plex. On Hunters island there is a considerable development of red 

 garnetiferous muscovite granite, very coarse-grained in places, the rela- 

 tions of which are not so clear. 



Frequently the granites of Rainy River district are almost devoid of 

 bisilicate, merging into red felsites and compact, massive gray feldspathic 

 rocks approaching quartzites, which a well-marked system of cleavage 

 planes sometimes cuts into regular rhomboidal blocks. 



Again the bisilicate is frequently altered to chlorite. The granites 

 sometimes exhibit a distinct porphyritic structure, evinced by the large 

 crystals of feldspar in a finer-grained groundmass. These porphyritic 

 granites have often a distinct gneissic foliation. 



For convenience of reference the separate areas of granite in this region 

 have received distinctive geographic designations in the reports of the 

 Canadian Geological Survey. One of them, which in the forthcoming- 

 report on the Seine River district will be named the Seine area, is re- 

 markable for the predominance of plagioclase in the rocks of its south- 

 western portion; this, in association with chlorite, which is probably 

 derived from hornblende, characterizes the rock rather as a quartz-diorite 

 than a granite. The microscopic examination of the rocks of this area 

 is not yet complete, but in the field there seems to be a gradual passage 

 of the quartz diorite or chloritic plagioclase granite into the ordinary 

 orthoclase granite; certainly the writer has so far been unable to find 

 anywhere a sharp line between them. Plagioclase in greater or less 

 proportion occurs in main' of the granites of the whole region. 



RELATION OF CONTCHICIIING AND KEEWATIN SERIES TO LAURENTIAN 



GRANITES. 



Before proceeding to a consideration of the rocks of the Contchiching 

 and Keewatin series, it would be well to refer briefly to the relations 

 which the Laurentian granites bear to them. 



Contact Phenomena Criterion- of Relative Age. — In the absence of pale- 

 ontologic evidence the most important criterion that remains for the 

 determination of the relative age of contiguous rock series arc the features 



*The Archaean Eruptive Rocks of Finland, by Dr J. J. Sedesholm. 



