274 ALCOCK AND BRUC^E PREOAMBRIAN ROCKS OF MANITOBA 



Acid volcanics, including quartz-porpliyry, rliyolitc, and derived seri- 



citic schists. 

 Basic volcanics (greenstones), including andesite, basalt, diorite, and 



derived chlorite, mica and hornblende schists. 



The volcanic rocks nowhere occupy large areas in the region, for the 

 most part forming fringes along the lakes. The basic or greenstone 

 varieties are the most abundant, but a zone of dominantly acid rocks 

 occurs along the east coast of Wekusko Lake and is economically impor- 

 tant because it contains the larger quartz veins of the area. The rocks 

 consist of flows, tuffs, breccias, and intrusives, with schists derived from 

 these types. In some of the massive greenstone definite evidence of flow 

 origin, such as amygdaloidal and ellipsoidal structure, is observed; in 

 other massive types the rock is coarser-grained and in thin section pre- 

 sents the character of diorite, which may represent either the interior 

 portions of thick flows or intrusives. Some of the flow rocks have the 

 composition of basalts; intermediate types, however, of the composition 

 of andesites and dacites are more common. 



The acid volcanic members are light-colored rocks, in places massive, 

 in places sheared into sericite schists. They vary in composition from 

 rhyolite to dacite. Some are porphyritic, with phenocrysts of quartz and 

 feldspar. Acid flow breccias and finely banded, gray tuffs are also asso- 

 ciated with the flows. 



The sedimentary rocks of the Wekusko group consist of garnet gneisses, 

 mica schists, and bands of arkose and conglomerate. The dominant rock 

 type is a finely banded gneiss varying from light gray to dark gray in 

 color. Nearly everywhere the rock is garnetiferous and in places garnets 

 are very abundant. On the weathered surface, lines of original bedding 

 and cross-bedding may locally be distinguished. In thin section the rock 

 is seen to consist of quartz, orthoclase and plagioclase feldspar, biotite, 

 and muscovite, with accessory iron oxide and apatite. The mica shreds 

 are parallel and the quartz grains are fresh, with interlocking outlines 

 showing that the rock has suffered recrystallization. 



The mica gneisses pass by loss of feldspar into mica schists. Of these 

 there are several varieties, depending on the character of the secondary 

 silicates developed in them. Some contain many well crystallized garnets, 

 a few of which reach a diameter of over two • inches. In other types 

 staurolite is the characteristic mineral, with crystals from one to four 

 inches in length, which stand out prominently on Aveathered surfaces. 

 The matrix is a biotite schist containing small red garnets. In certain 

 outcrops small staurolite crystals lie in bands which apparently mark the 

 original bedding planes. In one locality a cyanite-bearing schist is inter- 



