272 ALCOCK AND BRUCE PRECAMBRIAN ROCKS OF MANITOBA 



East of Cliff Lake, rocks of the Amisk volcanic group are intruded by 

 granite-porphyry, a pink-colored rock mottled with pale blue to lavender- 

 colored phenocrysts of quartz. This is regarded as older than the sedi- 

 mentary series described under the term Missian, on account of the pres- 

 ence in the sediments of grains of peculiar bluish quartz similar to the 

 quartz of the granite-porphyry and of pebbles of granite with striking 

 graphic intergrowths similar to textures displayed by the rocks of the 

 Cliff Lake boss. 



The Missian sediments are tj^pically developed on Missi Island, at the 

 northern end of Amisk Lake. They are divided into two groups, de- 

 scribed respectively as the Lower and the Upper Missi. The Lower Missi 

 consists of conglomerate, quartzite, slate, graywacke, and carbonate rocks. 

 The greater part of the series consists of a great thickness of quartzite 

 and slate. The slates are extensively drag folded, and consequently their 

 thickness may be much less than it appears. 



The lithology of the rocks of the Lower Missi is sufficiently different 

 from those of the Amisk group to justify differentiation from the vol- 

 canics, and, moreover, there is structural evidence of an unconformity 

 between the two series. In most places the slates and quartzites dip very 

 steeply or are vertical, and the position of the axes of the folds can be 

 inferred only by interpretation of the minor folds. Along the southern 

 limb of the syncline lying just north of the bay into which Sturgeon AVeir 

 Eiver empties the fold has not been so closely compressed. There the 

 slates of the Missi group strike nearly west and dip northward at angles 

 of 30 degrees or less. The general strike of the schistosity of the volcanic 

 rocks in this vicinity is north to northeast. Hence, since the compression, 

 which threw the sediments into close folds, developed a northwesterly 

 trending structure, it is evident that -the slates overlie the volcanics 

 unconformably and were deposited after some schistosity had already 

 developed. 



The character of the sediments indicates a period of rather unstable 

 conditions, with alternations of well sorted siliceous material now form- 

 ing quartzites, clayey beds now slates, and less well sorted debris whicli 

 has consolidated as graywacke. Certain periods were sufficiently undis- 

 turbed to produce thin bands of carbonate rocks, but even in these there 

 is a large proportion of detritus. iVpparently the period of sedimentation 

 was one of many changes, and all of the rocks belong to shore or near- 

 shore types. 



The Upper Missi rocks consist of arkose and conglomerate which in 

 places rest unconformably on the Amisk volcanics. No contact was found 

 with the Lower Missi, but the presence in the conglomerate-arkose of the 



