44 THE ROYAL SOCIETY OF CANADA 



to sea level, abundant plant growth, and leisurely drainage are the 

 conditions which gov^ern the formation, transportation, and accumu- 

 lation of high grade clays and sands. 



Superficial kaolinization was probably general in Cretaceous and 

 Jurassic times over many parts of the area of pre-Cambrian rocks, 

 but the effects of kaolinization do not appear to have extended to any 

 great depth. There was a dearth of residual clay on the pre-Cambrian 

 upland during the period immediately preceding the Pleistocene 

 glaciation, as shown by the almost complete lack of clay in the com- 

 position of glacial drift which has been derived solely from pre- 

 Cambrian rock surfaces. 



Mr. J. G. Cross made an examination of the pre-Cambrian rocks 

 on Mattagami and Abitibi rivers for the Ontario Bureau of Mines in 

 1919. He reported the occurrence of kaolinized garnet gneiss on 

 Mattagami river in the vicinity of Long Portage. The kaolinized 

 zone is about 400 feet wide, and is of a whitish colour. The feldspars 

 are entirely altered to kaolin, but unaltered garnet and mica still 

 remain. A pegmatite dike which cuts the gneiss at this point is also 

 semi-kaolinized. 



A sample of the material, submitted for examination, is rather 

 too hard to be called a residual clay, but it probably represents a zone 

 of gradation between the unaltered rock and the overlying residual 

 clay, which has been removed. It is precisely the kind of material 

 which, worked over by water, would produce high grade plastic clays 

 such as those found a few miles farther down the river. 



This is the only remnant of kaolin resulting from weathering, 

 J^nown to the writer, in the pre-Cambrian area of Canada, but it is 

 not unlik,ely that other similar deposits are concealed beneath the 

 widespread glacial drift. 



The rocks exposed on Missinaibi river, north of the railway, 

 belong principally to the pre-Cambrian schist complex. The pre- 

 vailing kind of rock is a stratiform, biotite gneiss which, while not 

 necessarily the exact equivalent of the Marshall lake series, bears a 

 strong resemblance to it in appearance and geological relations. 

 Well-foliated, hornblende gneisses of igneous origin are also exposed 

 at intervals. The greatest interest from the point of view of the 

 present investigation lies in the bedrock exposed along the canyon 

 walls at Long Portage. This rock is a binary granite, and probably 

 is an acid differentiate on a large scale from an igneous mass. It is 

 exposed for about a quarter of a mile along the canyon, and is flanked 

 on the north and south by foliated hornblende gneisses. The granite 

 is friable and appears to be softened by incipient kaolinization, and 



