PRE- CA MBRIA N NOR TH A ME RICA N LITER A TURE 745 



Laurentian are chiefly hornblende-schists, but in other localities are 

 chlorite-schists. Between these two are numerous transitions. Gray- 

 wackes occur at several localities, and agglomerates and conglomerates 

 are plentiful. The upper acid division includes felsites, sericite-schists, 

 and quartz-porphyries. These are apparently younger than the green 

 schists and massive rocks of the Keewatin, but the two divisions are 

 conformable, as is also the whole of the Keewatin to the Coutchiching. 

 The rocks included in the Laurentian are, for the most part at least, 

 intrusive in the Coutchiching and Keewatin. In the Keewatin are 

 various intrusive granite areas. Cutting all of the previous series are 

 dike rocks, which may be divided into an acid division, including 

 granite and pegmatite, and a basic division, including diabase and 

 quartz-diabase. 



Many details are given as to particular occurrences of the various 

 rock series. The occurrence of gold in Ontario is described, and 

 incidentally the rock succession in the Hastings district is summarized. 



Winchell and Grant' give a preliminary account of the Rainy Lake 

 gold region. Following Lawson, the rocks there found are separated into 

 four distinct groups. Beginning with the lowest these are : (i) Lauren- 

 tian, composed of granites and granitoid gneisses and allied rocks ; 

 (2) Coutchiching, composed of mica-schists grading into fine-grained 

 gneisses ; (3) Keewatin, composed of hornblendic, greenish, and sericitic 

 schists, conglomerates, graywackes, etc.; (4) Diabase dikes, more recent 

 than and cutting all the others. The Coutchiching mica-schists have 

 in many places rapid alternations in bands from an inch to several feet 

 in width of slightly different mineralogical composition, structure, or 

 color. The position of these bands gives the strike and dip of the 

 rock, and when they are lacking the schistose structure is taken as giv- 

 ing the strike and dip, as this seems to be parallel with the banding 

 when the two are seen together. On account of basal conglomerate 

 beds in places in the Keewatin resting on the Coutchiching, while an 

 unconformity between the two is not proven, it seems quite probable. 



COMMENTS. 



The banding of the Coutchiching mica-schists in many places 

 described by Coleman, Winchell, and Grant is just such as has been 



'Preliminary Report on the Rainy Lake Gold Region, bv H. V. Winchell and 

 U. S. Grant. 23d Ann. Rept. Geol. and Nat. Hist. Sur. of Minn., for 1S94, Part 

 II, 1895, pp. 36-105. 



