280 ALCOCK AND BRUCE— PRECAMBRIAN ROCKS OF MANITOBA 



that the elastic material filled up the crevices 

 formed. Hence the top of the bed lies toward the 

 greenstone. This snppositio]i is corroborated by 

 the gradation in the basic flows from coarse-tex- 

 tured to finer-textured rock from the conglomerate 

 northward. 



A gradation from the banded rocks of the lower 

 group to ellipsoidal weathering rocks of the upper 

 grouj) is shown in exposures three miles farther 

 west. At this place the sediments dip northward 

 at an angle of 70 degrees and are succeeded by 

 greenstone. Near the contact the banding in the 

 siliceous slates of the lower group becomes some- 

 what indistinct and the rocks become the nearly 

 massive biotitic variety. Above these are bands of 

 biotite in elliptical forms, and finally the typical 

 ellipsoidal greenstone. It is suggested that the 

 lava may have been poured out on a still uncon- 

 solidated mud into which the ellipsoids sank, 

 squeezing the oozy material into the openings be- 

 tween tliem. This mud now forms the biotitic 

 margins of the lower ellipsoids. 



From these characteristics it seems reasonable 

 to assume that there is in this district an immense 

 sedimentary series with some volcanic rocks inter- 

 calated. This series may be largely terrestrial in 

 origin, the conglomerate beds being deposited in 

 torrential streams. Periods of local erosion no 

 doubt occurred, but without any important or 

 widespread hiatus. Later, volcanic activity became 

 more intense and conditions probably changed 

 from land deposition to submarine, so that the 

 upper part of the complex consists of ellipsoidal 

 greenstones with only very minor amounts of 

 clastic material. After the consolidation of these 

 sedimentary and volcanic rocks and their very 

 severe dynamic metamorphism, probably at great 

 depth, batholiths of granite were intruded. Long 

 periods of erosion have now uncovered these batho- 

 liths and have removed all but the deeper folds of 

 the pre-granite complex. 



.1 



:ii 



s. o 



Point 



'"•*'«» 



