464 EDWARD STEIDTMANN 



by Lauren tian granite and syenite; Huronian quartzite, arkose, 

 conglomerate, slate, and breccia, unconformably above the 

 Keewatin and Lauren tian; and post-Huronian diabase intrusives. 

 Quartz and calcite veins bearing native silver, smaltite, and 

 niceolite have been found near contacts between the post-Huronian 

 diabase intrusives, and Keewatin basic igneous rocks. 



Burrows^ reports that the pre-Cambrian succession in the 

 Gowganda and Miller Lake silver area between Sudbury and Cobalt 

 is Keewatin altered basic igneous rocks and acid porphyries, 

 intruded by Laurentian granite, syenite, and gneiss; unconformity; 

 Huronian quartzite, arkose, graywacke, conglomerate, and slate; 

 intrusive into all the preceding, post-Middle-Huronian diabase. 

 The diabase contains silver-bearing veins whose value has not been 

 determined. 



Burrows^ finds that the compact rocks of the Porcupine district 

 100 miles northwest of the Cobalt district, are all pre-Cambrian, 

 principally Keewatin. Keewatin and Huronian rocks contain 

 irregular gold-bearing veins, in which quartz is the dominant 

 mineral. 



Bowen^ reports that sills and dikes of diabase cut older forma- 

 tions in the Gowganda Lake district. The dikes, some of them 250 

 feet wide, show no evidence of differentiation. Red spots composed 

 of an intergrowth of quartz and soda-rich plagioclase feldspar occur 

 in the diabase of the sills. At the contact of sills and slate, a 

 granophyre consisting of soda-rich plagioclase, quartz, and . cces- 

 sory minerals including calcite grades into a slate adinole developed 

 by contact metamorphism. The granophyre and adinole are both 

 ascribed to hydrothermal actions, the granophyre being regarded as 

 an extreme phase of the alteration of the slate, a recrystallization 

 from a state of aqueous fusion. 



To the reviewer, it seems that the red spots in the sills, and 

 the granophyre, may be due to the same causes, since they are 



^ A. G. Burrows, "The Gowganda and Miller Lakes Silver Area," 0«i.5«r.ilf«ze5, 

 i8th Ann. Rept., XVIII (1908), Pt. 2, pp. 1-20, 20 figs., 3 maps. 



^ A. G. Burrows, "Porcupine Gold Area," Quart. Bull. Canadian Min. Inst., No. 

 16, 1911, pp. 59-62. 



5 Norman L. Bowen, "Diabase and Granophyre of the Gowganda Lake District, 

 Ontario," Jour. Geol., XVIII, No. 7 (1910), 658-74. 



