416 C. K. LEITH 



grained gray gneiss, which often becomes granitic and coarser grained 

 in texture. The Huronian rocks are most commonly massive diorites 

 and diabases, and hornblende and chlorite-schists ; less commonly, they 

 are slates, felsites, quartzites, and sericite-slates. The Laurentian is 

 frequently in eruptive contact with the Huronian. 



In two areas, the Nipigon or Keweenawan rocks overlie the 

 Huronian and Laurentian rocks. These areas are one two miles north 

 of Cape Choyye and one on the peninsula of Gargantua. 



Walker, 1 in 1897, describes the stratigraphy and petrology of the 

 Sudbury nickel district of Canada. The oldest rocks of the district 

 are gneisses of various kinds, which are regarded as of Laurentian 

 age. Next in age to the gneisses is a belt of rocks consisting of 

 quartzite, graywacke, amphibolite, mica-schist, phyllite, clay-slate, and 

 altered volcanic breccia, which extends from the north shore of Lake 

 Huron northeastward to Lake Mistassini, in the neighborhood of Sud- 

 bury, the belt being about twenty-five miles wide. The rocks of this 

 belt are believed to be of Huronian age. The Huronian rocks have 

 suffered severe metamorphism, and the original character of many of 

 them cannot be made out. In and adjoining the Huronian belt are 

 elliptical areas of later eruptive greenstone, in places intimately 

 associated with and genetically inseparable from gneissoid and micropeg- 

 matitic granites. The nickel ore, principally pyrrhotite, occurs 

 intimately intermingled with the eruptives, and is regarded as a con- 

 centration by differentiation from the eruptive magma. Cutting both 

 the Huronian and the included nickel-bearing eruptives are masses of 

 fine grained pinkish biotite-granite, sending apophyses into the sur- 

 rounding rocks. This granite is found to have been intruded in two 

 eruptions. The youngest rocks of the Sudbury district are olivine- 

 diabases, which occur in dikes, cutting all the other rocks of the 

 district. 



Bell 2 reports on the geology of the French river sheet, which 

 represents the country around the north end of Georgian Bay. 

 Huronian rocks occupy the northwest corner of the sheet, and Lauren- 

 tian rocks all the area to the southeast. 



1 Geological and petrographical studies of the Sudbury nickel district of Canada, 

 by T. L. Walker, Q. J. G. S., Vol. LIII, 1897, pp. 40-66. With geol. map. 



2 Report on the geology of the French river sheet, Ontario, by Robert Bell : Ann. 

 Report of the Geol. Surv. of Canada for 1896, Vol. IX, 1898, Part 1, pp. 29. With 

 geol. map. 



