PRE-CAMBRIAN LITERATURE OF NORTH AMERICA 649 



The base of the succession exposed in the Gowganda' dis- 

 trict consists of Keewatin greenstones mostly of igneous origin 

 associated with some iron formation. They are intruded by batho- 

 liths of Laurentian granite. Overlying the granites and green- 

 stones with well-marked unconformity are Huronian sediments 

 which are separated into two members by a faint unconformity. 

 The lower group from 500 to 1,000 feet thick consists of con- 

 glomerates, arkose, graywacke, and slates, showing poor assort- 

 ment, variable bedding, and till-like character in the coarse phases. 

 Locally, the beds are associated with rhyolitic extrusions. The 

 upper is a single quartzite formation 600 or more feet thick, ran- 

 ging from arkose to pure, well-bedded quartzite. 



Intruded into the preceding are Keweenawan diabase sills and 

 dikes. 



Selected areas between the original Huronian and the Cobalt 

 and Sudbury districts were examined by Collins^ with the view of 

 correlation. The Bruce, Blind River, Whiskey Lake, Espanola, 

 and Round Lake areas were selected, the widest gap between them 

 being about 28 miles. 



Collins recognizes two major stratigraphic divisions, the pre- 

 Huronian and the Huronian. They are separated by the most 

 conspicuous unconformity of the region, characterized by a strong 

 basal conglomerate, great differences in structure, metamorphism, 

 igneous intrusions, and general lithologic character of the two 

 groups. The pre-Huronian consists of basic schists and gneisses 

 mostly of igneous origin, granite batholiths of more than one period 

 of intrusion and highly metamorphosed slates and quartzites. 

 The pre-Huronian has not been completely subdivided into strati- 

 graphic units and its various members have not been traced and 

 correlated over the entire region. 



The Huronian is separated into two units by an unconformity 

 far less pronounced than the one at the base of the Huronian. 

 Individual beds of both divisions have been traced successfully 

 from district to district. The lower division, called the Bruce 



'W. H. Collins, "The Geologj^ of Gowganda INIining Division," Canada Geol. 

 Surv. Mem. No. 33 (1913), 121 pp., 4 pls., 5 Sgs- 



= W. H. Collins, "The Huronian Formations of Tuniskaming Region, Canada," 



Canada Geol. Surv. Mus. Bull. No. 8 (1914), 27 pp., 2 maps, i fig., i pi. 



