268 E. C. COOKE 



the name "Mattagami series" will be used until the doubt is 

 cleared up and the term "Pontiac" re-defined, if necessary. In 

 the remainder of this paper the term ''Mattagami series" will 

 therefore be used to include the original Mattagami series, the 

 Broadback, Lucky Strike, and Brock series, and the conglomerate- 

 arkose portion of the Pontiac series to the south. 



Age. — The Mattagami series has been shown to consist of 

 scattered patches of sediments, all of which are older than a great 

 orogenic movement and a period of granitic intrusion. In the 

 southwest corner of the region under discussion, the Mattagami 

 series is overlain by the flat-lying Cobalt series, which rests on a 

 peneplain that bevels the Mattagami series, the intrusive batho- 

 liths of granite, and the older lavas indiscriminately. A very 

 long period of erosion must have ensued therefore between the 

 end of the deposition of the Mattagami series and the commence- 

 ment of the deposition of the Cobalt series. The Cobalt series 

 has recently been shown by W. H. Collins^ to be of the same age 

 as the upper part of the original Huronian series of the north 

 shore of Lake Huron. It is separated there from the lower part 

 of the Huronian, which Collins has termed the Bruce series, by 

 an unconformity, represented by the erosion of some i,6oo feet 

 of sediments and a. gentle folding. The Bruce series has been 

 shown by Collins^ to be very similar in the character and succes- 

 sion of its members to the Lower Huronian of the Marquette 

 region. The erosion plane dividing the Cobalt series and the 

 older rocks in northern Quebec passes southward beneath the 

 Cobalt series in the Cobalt district, and beneath the Bruce series 

 in the Lake Huron district. It may safely be stated, therefore, 

 that the Mattagami series is of pre-Bruce age, and therefore prob- 

 ably pre-Lower Huronian, and that it antedates the Bruce by an 

 interval sufficient for orogenic forces to raise a mountain range 

 of unknown height, and for the forces of erosion to reduce this 

 range toward its southern side at least where the Bruce series 

 was deposited, nearly to base-level. 



'W. H. Collins, Geol. Stirv. Can., Museum Bulletin No. 8, 1914. 

 ' W. H. Collins, memoir in preparation. 



