288 ALCOCK AND BRUCE PRECAMBRIAN ROCKS OF MANITOBA 



types of volcanic rocks appear in most of the areas and in many of them 

 somewhat similar sedimentary formations occur. The resemblances 

 which are displayed by the types from the various areas are due to two 

 causes: First, original similarity, and, secondly, induced similarity, due 

 to metamorphism under similar conditions. Contact and regional meta- 

 morphism produce schists and gneisses which may resemble each other 

 very much and yet the similarity may have no age significance. Under 

 extreme conditions, metamorphic processes may even produce from dis- 

 similar original rocks similar derivatives. In spite of these facts, there 

 are, however, certain characteristics of the sedimentary complex from 

 which its original nature can be inferred. The chief of these is the domi- 

 nance of unassorted clastic sediments. Limestone is entirely lacking. 

 Shale, the predominant rock of a normal marine series, is represented 

 only by a relatively small amount of slate and by the garnet, staurolite- 

 and cyanite-bearing mica schists, analyses of which show a proportion of 

 the oxides such as is characteristic of shale. The dominant sediments, 

 however, in all the areas are paragneisses and arkosic quartzites with con- 

 glomeratic bands. 



The rock succession also presents features which serve as a basis of 

 comparison. In nearly all the areas the oldest rocks are greenstones 

 associated with more acid volcanics and interbanded with sediments. 

 Conformably overlying these rocks are sedimentary members with inter- 

 banded volcanics. This is the succession as worked out in the Cross Lake 

 area and the Eeed-Wekusko area and is apparently the succession in the 

 Eice-Wanipigow and Star Lake areas. In the Knee-Oxford Lake area 

 there is a reversal of conditions, with a lower division, consisting domi- 

 nantly of sediments with interbanded volcanics and an upper division of 

 dominant volcanics with subordinate amounts of sediments. The rock 

 types, however, are much alike in all the areas. In two areas, the Atha- 

 papuskow and Churchill River, there are pre-Cambrian series which are 

 apparently younger than the main pre-granitic complex. The Missi 

 series and the Churchill quartzite may represent equivalent series older 

 than the regional granite, but younger than the volcanic-sedimentary 

 complex. They are, however, both of local extent and are lithologically 

 dissimilar, both to each other and to the pre-granite complex of the other 

 areas. 



k 



Origin of the pre-Granite Complex 



In considering the probable origin of the pre-granite complex the fol- 

 lowing facts concerning it should be noted : 



