290 ALCOCK AND BRUCE PRECAMBRIAN ROCKS OF MANITOBA 



dred feet. The matrix is eommo]]lY garnetiferous and is exactly like the 

 sedimentary gneisses which contain no boulders or pebbles. The frag- 

 ments for the most part are well rounded, but in places they are squeezed 

 and elongated parallel to granite contacts. Only locally have garnets 

 been developed in them. No evidence has been found that any of the 

 pebbles or boulders consist of rock which had been rendered schistose 

 before its erosion and deposition in the sediment. The boulders consist 

 of granite, porphyry, acid and basic volcanics, and vein quartz. The 

 granite and quartz pebbles are usually the better rounded, suggesting 

 longer transportation. In places pebbles of volcanics are subangular, 

 suggesting that they had a local origin. There is a lack of sorting, with 

 large and small fragments intermingled and with great variations in the 

 amount of the matrix associated with the boulders. 



CROSS-BEDDING 



On the weathered surface of many of the gneisses cross-bedding is well 

 shown, and in places it is easily possible to determine by this means the 

 top and bottom of the beds. Locally, as in the Cross Lake area, cross- 

 bedding on a large scale, suggestive of torrential deposits, is excellently 

 displayed. On an island in Cross Lake the width of a cross-bedded hori- 

 zon between the parallel beds of the main structure is nine feet. The 

 oblique strata in this bed also show alternating coarse and fine layers 

 with conglomeratic bands up to 14 inches in width and finer-grained 

 bands up to 16 inches in width. 



CONCLUSION 



Summarized briefly, the pre-granite complex is dominantly clastic, 

 with great variations in thickness and in the order of succession of the 

 various lithological elements. A large part of the sediments are feld- 

 spathic and only a small part consist of well sorted, argillaceous and 

 quartzose types. In addition, there are numerous conglomerate horizons. 

 Both conglomeratic and arkosic bands show cross-bedding on a large 

 scale. In the igneous rocks of the complex, ellipsoidal structure is 

 common. 



It is concluded from these facts that the complex is chiefly of terres- 

 trial origin — in part deposited as outwash fans, in part as deltas, along 

 the continental margin. The lavas were apparently poured out under 

 water during periods of submergence or in the part of the deltas which 

 were below sealevel. The granite pebbles in the conglomerates of this 

 ancient series may have been derived from early granites which were 

 intruded into rocks, of which no record is left — rocks which have been 



