74 H. C. COOKE 



Younger than the granite just mentioned, and intrusive into it 

 and the Grenville series, are great bodies of anorthosite and gabbro. 

 Several of these are found in the area under discussion. 



A few small patches of a younger sedimentary formation are 

 found here and there throughout the region. These have hitherto 

 been known by various local names, such as the Pontiac, Mattagami, 

 and Broadback series. They include mainly conglomerate, arkose, 

 and greywacke, and lie unconformably on all the older rocks, with 

 the possible exception of the anorthosites. They are highly folded 

 and metamorphosed, so that even the most competent of the rocks 

 are converted into schists. 



Following the deposition of these sediments came an intrusion 

 of lamprophyre dykes. They are relatively few in number, but 

 their distribution appears fairly general. Next in age are the 

 enormous batholithic intrusions of granite, which now underlie 

 about one-half of the region. It is possible that these may be of 

 different ages, but no evidence of this has as yet been found. The 

 intrusion of the batholiths probably followed closely the orogenic 

 movements which folded the older rocks. They seem to have 

 removed by stoping and digestion enormous thicknesses of the 

 older rocks, including all of the ancient floor on which these were 

 laid down, and undoubtedly also had a certain metamorphosing 

 action on the adjacent rocks. 



The Cobalt series, of Huronian age, rests with great uncon- 

 formity on the surface of the complex described above. It is 

 definitely recognized only in the southwest corner of the region, but 

 a few small areas of similar rocks occur on Chibougamau Lake, and 

 in the report of the Chibougamau Mining Commission were corre- 

 lated with the Cobalt series. 



Dykes of diabase petrographically similar to the Cobalt diabase 

 cut the Cobalt series and all the older rocks. They are found 

 sparsely distributed throughout the entire region. The diabase 

 here never forms sills as in the fiat-bedded rocks of the Cobalt 

 region; hence the chance of mineralization by it is slight. 



The youngest consolidated formation in the area is supposedly 

 the fiat-lying Mistassini limestone, which outcrops around Lake 

 Mistassini and has been described by A. P. Low and the Chibouga- 



