SOME PRE-C AM BRIAN LITERATURE OF NORTH AMERICA 465 



mineralogically similar and are gradational. Bowen believes that 

 the granophyre may be quite different in origin from the red 

 spots scattered through the sills, despite their similarity. A 

 somewhat similar phenomenon in the Cobalt district, the separa- 

 tion of diabase and aplite, has been ascribed to differentiation by 

 Collins and Hore, Collins showing that the separation depended on 

 the rate of cooling, being entirely absent in diabase, which was 

 rapidly chilled. A magmatic separation of soda-rich aplite and 

 gabbro in the intrusive of Mt. Bohemia, Mich., has been described 

 by F. E. Wright. Here there is no evidence that the aplite was 

 associated with slate. 



Bowen^ presents chemical analyses and descriptions of the 

 diabase and aplite of the Cobalt silver area. 



Bowen^ describes the rocks of the Thunder Bay district as 

 Upper Huronian argillites, gray quartzite and gray slates inter- 

 stratified, black slate, and iron formation, all unconformable above 

 the Laurentian granite complex, and intruded by Keweenawan 

 diabase dikes and sills. Silver veins are found in steeply dipping 

 fault zones cutting the black slate in an east-west direction. The 

 ore minerals are principally native silver and argentite, associated 

 with calcite, quartz, fiuorite, barite, witherite, and other minerals. 



Coleman^ states that the rocks of the Black Sturgeon Lake 

 district southwest of Lake Nipigon are Keewatin green schists 

 intruded by granite gneiss, and slightly disturbed Keweenawan 

 diabase, shale, and sandstone. Hematite veins are found in fault 

 zones of the Keewatin. 



Coleman'' reports that the Alexo nickel deposit near Matheson, 

 northern Ontario, consists of a pyrrhotiferous nickel-bearing rock 

 of unknown extent, grading upward into an ultra basic rock, 

 peridotite altered to serpentine, and resting on footwall of andesite. 



^N. L. Bowen, "Diabase and Aplite of the Cobalt-Silver Area," Jotir. Can.Min. 

 Inst., XII (1909), 517-28. 



^N. L. Bowen, "Silver in Thunder Bay District," Out. Bur. Mines, 20th Ann. 

 Rept., pp. 119-32. 



3 A. P. Coleman, "The Black Sturgeon Lake District," Out. Bur. Mines, i8th 

 Ann. Rept., Pt. i, pp. 163-179. 



'•A. P. Coleman, "The Alexo Nickel [Deposits," Fxon. Geo/., V, No. 4 (1910), 

 Pt. 4, 372-76. 



