278 ALCOC'K AND BRUCE PRECAMBRIAN ROCKS OF MANITOBA 



in origin. Some feldspar, botli plagioclase and orthoclase, is present and 

 is replaced to an even greater extent than is the quartz. A little green 

 amphibole is present in some sections and some chlorite. Pyrite occurs 

 in most specimens and there is some evidence of the introduction of sec- 

 ondary quartz. In some of the gneiss garnets have developed rather 

 abundantly, and in certain bands mark the original bedding planes. The 

 massive mica rock differs from the gneisses only in the lack of distinct 

 foliation. Under the microscope the biotite shows no parallel orientation. 

 More hornblende is present than in the foliated rocks, a greater propor- 

 tion of plagioclase to orthoclase, and a somewhat greater variety of min- 

 erals, including considerable epidote. 



Associated with the gneissic rock are well banded, fine-grained, impure 

 quartzites, fine-grained slates, and a dark green weathering, slaty rock 

 that is in places conglomeratic. There are also thick beds of fragmental 

 rocks which are, at least in part, conglomeratic, but which may also con- 

 tain pyroclastic fragments. The pebbles consist of rocks of many types, 

 quartz, granite, granite-i^orphyry, quartzite, and rocks that are very 

 similar to the gneissic rocks and to the greenstones. 



The lava flows interbedded with this complex of gneisses and sediments 

 do not show typical ellipsoidal weathering, but massive greenstone bands 

 occur, and it is probably from these that the greenstone fragments in the 

 conglomerates are derived. The lavas are comparatively fresh and vary 

 from andesite to basalt in composition. Some of them are fairly coarse 

 grained and show typical diabasic texture. 



Pre-Granite Complex, Upper Part 



The rocks of the upper part of the pre-granite complex are lavas and 

 associated volcanic rocks, with some sedimentary members of minor im- 

 portance. Most of the lavas are now ellipsoidal weathering greenstones 

 which had originally the composition of andesite. There is the usual 

 development of amygdaloidal and autoclastic rocks and some ash beds. 



The sedimentary rocks grouped with these are of two types: (1) A 

 chert conglomerate, and (2) iron formation with associated black slate. 

 The conglomerate contains rounded to subangular fragments of chert 

 and greenstone cemented by chert. With the conglomerate occur well 

 banded chert layers, some of which are truncated by later laminations. 

 The iron formation consists of thin laminations of granular quartz and 

 magnetite. Well banded black slate occurs along with the true iron for- 

 mation and possibly represents a change in character along the strike of 

 the same bed. All of these sedimentary bands are thin. 



