PRE-CAMBRIAN FORMATIONS— ONTARIO AND MANITOBA 467 



Eastward across the divide on the headwaters of the Hayes 

 River, early pre-Cambrian rocks are exposed at Knee Lake and 

 Oxford Lake.^ A lower, dominantly sedimentary part, consists of 

 rusty weathering garnetiferous biotite gneiss, impure quartzite, 

 slate, conglomerate, tuffaceous rocks, and some interbedded flows. 

 The thickness is probably several thousand feet. Above the domi- 

 nantly sedimentary group are flows of ellipsoidal weathering lavas 

 together with a few bands of iron formation. These groups are 

 apparently merely parts of a great continuous series. The sedi- 

 ments are in great part typical continental deposits. 



COMPARISON OF THE FORMATIONS 



From the descriptions of the various areas quoted it is clear 

 that there are two distinct types of rocks in the early formations: 

 (i) volcanic flows now altered to greenstone and chlorite schist, 

 and (2) sedimentary rocks consisting largely of gneiss but also in 

 places including slate, quartzite, and minor amounts of conglomer- 

 ate. The gneisses retain evidences of bedding although in many 

 occurrences metamorphism has destroyed some of the original 

 texture. Analyses of specimens of these old gneisses are comparable 

 to analyses of typical sediments. The slates commonly show the 

 original bedding as color variations at slight angles to the fissility. 

 Some arkosic rocks still retain the cross bedding and ripple- 

 markings of the original sands and conglomerate, even though 

 the matrix may be thoroughly schistose, with complete recrystalli- 

 zation of the constituent minerals, and are still recognizable as 

 water-laid clastic rocks. 



The peculiarities of all the sedimentary formations of this 

 early period are the coiflparatively small amount of conglomerate 

 and the complete lack of limestone. The sediments found in 

 Manitoba are continental deposits probably formed under deltaic 

 or piedmont conditions. The lack of any large amount of coarse 

 material is evidence that no high land masses existed near the 

 site of deposition, but the ripple-marking and cross-bedding of 

 some of the rocks indicate shallow-water conditions during the 

 formation of some of the beds. The early sediments in other areas 



^ E. L. Bruce, Summary Report, Geol. Survey of Canada, Part D (1919). 



