276 ALCOCK AND BRUCE PRECAMBRIAN ROCKS OF MANITOBA 



flows, in places showing good ellipsoidal structure. In thin section they 

 are all seen to he highW altered, consisting of a mass of secondary min- 

 erals, chiefly light green hornhlende, sericite, epidote, and carbonate with 

 iron oxide in varying amounts. The rocks grade into schists, of which 

 the chief varieties are those containing chlorite and hornblende. Horn- 

 blende schist is a common phase near the contacts with granitic intru- 

 sives and in places it is garnetiferous. An outcrop of a white anorthosite 

 on an island in Pipestone Lake near the mouth of Nelson River consists 

 almost entirely of labradorite. From its association with the greenstone 

 volcanics, it is considered to be a differentiate from them. Other fine- 

 grained, dark rocks with banded structure associated in places with the 

 more massive volcanics are thought to be tuffs. With the massive, dark- 

 colored flow rocks, light-colored acid types occur locally, commonly as 

 parallel bands alternating with the darker-colored varieties. In places 

 they have the appearance of being interbanded flow rocks, and in others 

 they have dike relationships cutting the basic rocks. Thin sections show 

 that they are fine-grained rocks, consisting of quartz and feldspar with 

 phenocrysts of orthoclase, acid plagioclase, and quartz. 



The sedimentary rocks of the complex are exposed chiefiy between 

 Cross and Pipestone lakes. The succession is from greenstone through 

 finely banded, dark, tuffaceous beds to true sediments which become 

 coarsely conglomeratic. The sediments stand vertically or with steep 

 dips. They are poorly sorted and show great irregularities, both along 

 and across their strike. The conglomerate la^^ers contain boulders of 

 granite, acid and basic volcanic rocks, and vein quartz. The granite and 

 quartz pebbles are well rounded, whereas most of those of volcanic origin 

 are subangular. The series is coarsely cross-bedded in a manner sug- 

 gestive of torrential river deposits. A band of greenstone 200 feet in 

 thickness lies between two beds of conglomerate. 



Two other areas of sediments occur in the region. The rocks of Indian 

 Reservation and adjacent islands are quartzite and sedimentary gneiss 

 with conglomeratic horizons. On the northwest shore of Indian Reserva- 

 tion Island a band of greenstone lies between two beds of conglomeratic 

 garnet gneiss. The lower bed is coarsely conglomeratic, with large 

 boulders of granite which have been squeezed out parallel to the contact 

 with the adjoining granite. The third area consists of a strip of the 

 mainland near the outlet of Cross Lake. The dominant rock type is 

 finely banded gneiss which is nearly everywhere garnetiferous. Locally, 

 certain bands contain rounded pebbles and boulders, for the most part of 

 granite. The beds nearly everywhere are vertical. Locally, a hybrid rock 

 has been produced by lit-par-lit injection of granite along the bedding 

 planes of the sediments. 



