424 REVIEWS 



Geology of the Matachewan District, Northern Ontario. By H. C. 

 Cooke, Memoir 115, Canada Department of Mines, Geological 

 Survey, Ottawa, 1919. Pp. 60 (including index), with map, 

 figs. 5. 



The area described lies in the district of Timiskaming and includes 

 about 430 square miles. Topographically two elements are recognizable 

 ■ — first, pre-glacial erosion, chiefly of the Cobalt series, which frequently 

 outlines structural features in the pre-Cambrian rocks, and second, drift 

 features, changing with the character of glacial erosion and deposition. 

 The bed rocks are entirely pre-Cambrian, ranging from Keewatin ( ?) to 

 Keweenawan. The Keewatin ( ?) rocks are volcanics — extrusives, for 

 the most part, but with small masses of peridotite intrusive into them. 

 The peridotites may prove to be of commercial importance, since 

 asbestos of good quality has been found in them; the rock is highly 

 metamorphosed — kaoHnized, or altered to talc, or sericitized. The 

 rhyolites are slightly less quartzose than those of northern Quebec, and 

 the basalts of the Keewatin frequently show pillow structure. All these 

 rocks are highly altered and closely folded and faulted, the folding 

 probably following the deposition of the overlying Kiask series which 

 are dominantly metasedimentaries of many types. From their character, 

 it is thought that the Kiask sediments were laid down rapidly, without 

 much weathering, on an uneven surface. 



Kiask sedimentation was succeeded by granitic intrusions and later 

 by a period of basic intrusion, marked by diabase dikes. The overlying 

 Cobalt series is divided into the Gowganda formation (basal conglom- 

 erates and very coarse elastics) and the Lorraine quartzite, following 

 Collins. Faulting and some gently folding have been developed here also. 

 Deposits of asbestos and small deposits of barite, fluorite, and 

 hematite have been found. The asbestos occurs as veinlets in small 

 masses of serpentinized peridotite. The barite, fluorite, and hematite 

 occur in veins. By far the most important mineral however is gold, 

 which has been known in this district since 1917. The gold is closely 

 associated with intrusive granite porphyry; solutions thought to have 

 come from the granite porphyry magma have mineralized the volcanic 

 country rock with the deposition of auriferous pyrite. The gold is in 

 narrow veins of quartz intersecting the granite porphyry or in lenticular 

 ore bodies in the tuff and schist, varying in size up to 75 feet, with their 

 long dimensions parallel to the bedding planes of the tuff and schists. 



A geologic map makes the work complete, but the absence upon it 

 of topographic contours is regrettable. 



