STRUCTURAL FEATURES OF THE PRE-CAMBRIAN 69 



names of Subdury series, Timiskaming series, and Hastings series. 

 These may eventually be correlated, altogether or in part, with the 

 Mattagami series of the region under discussion; but lack of knowl- 

 edge makes this as yet no more than an interesting possibility. 



The basal lavas of this region and of the Timiskaming district 

 have almost universally been termed "Keewatin" by various 

 writers, but there is little good evidence for so correlating them. 

 Like the Keewatin of Lake Superior, they occupy a basal position 

 in the geologic column, appear to be of pre-Huronian age, and are 

 prevailingly basaltic and andesitic in composition; but more precise 

 evidence of identity is lacking. 



In connection with the occurrence in the northern Quebec region 

 of a series of sediments which appear to be of Grenville age, a brief 

 statement of the present status of the Grenville series is necessary. 

 Rocks of the Grenville series were first described by Logan in 1847, 

 and since that time he and many other workers have studied the 

 series and extended our knowledge of it. A summary of the differ- 

 ent investigations is given in Van Hise and Leith's '^Pre-Cambrian 

 Geology of North America.'" The series.has been definitely recog- 

 nized only in eastern Ontario, Quebec, the Adirondacks, and the 

 little-known area of Baffin Land; and singularly enough, in these 

 areas other surficial formations of the pre-Cambrian are lacking, 

 excepting the comparatively small development of the Hastings 

 series, together with certain basic lavas. It was therefore impos- 

 sible definitely to correlate the Grenville with any part of the pre- 

 Cambrian developed in the region of the Great Lakes. Recognizing 

 this, the International Committee in 1907 adopted the following 

 simple succession for this region: 



Cambrian: Potsdam sandstone, etc. 



Unconformity 

 Pre-Cambrian: Grenville series 



Intrusive contact 

 Laurentian 



In the same year Miller and Knight, of the Ontario Bureau of 

 Mines, pubhshed a brief description of work done by them in the 



' U.S. Geol. Surv., Bull. No. 360, p. 448. 



