28 THE ROYAL SOCIETY OF CANADA 



The most conspicuous feature of the region is the northern margin 

 of the Pre-Cambrian rocks, which rises from 150 to 200 feet above the 

 southern fringe of the Palaeozoic plain. 



The rivers have cut post-glacial gorges or canyons in this margin, 

 which are generally longer and steeper than those in any other part 

 of their courses. 



The surface of the drift is a gently undulating plain, of the swell 

 and sag type of topography. The depressions are generally shallo.w 

 marshes or muskegs supporting little or no tree growth, but the higher 

 portions are thickly wooded. 



Lakes are almost absent over the greater part of the plain under- 

 lain by thick drift, but are numerous farther south, where the drift 

 cover on the rocks is thinner. 



The deposits of clay, sand, and lignite, which are described in 

 this paper, lie below the glacial drift, near the northern margin of the 

 Pre-Cambrian rocks. Their distance from the nearest points on the 

 National Transcontinental railway line is 40 to 50 miles. 



Geological Outline 



The surface of the crystalline rocks of the great Pre-Cambrian 

 upland slopes steeply to the north, and at Long Portage on the 

 Missinaibi and Mattagami and at Coral Portage on Abitibi river 

 they finally disappear beneath younger deposits. Sedimentary 

 formations — shale, limestone, sandstone, and gypsum — then appear 

 wherever bedrock is exposed on the rivers. Most of the sedimentary 

 rocks here are of Palaeozoic age, and include Devonian, Silurian and, 

 perhaps, Ordovician sediments.^ 



The Palaeozoic rocks for the most part are flat-lying and show 

 no indication whatever of metamorphism. Some of the clay layers 

 included in the Upper Devonian rocks are as soft and plastic as many 

 recent clays. 



At certain isolated localities along the rivers north of the border 

 of Pre-Cambrian rocks bright coloured clays and sands with lignite 

 seams occur below the universal covering of glacial drift. These 

 rocks are the subject of this paper, and are supposed to be of Lower 

 Cretaceous age. The basal beds of these deposits have not been seen, 

 but presumably they lie unconformably upon black and green shales 

 of the Portage formation^ because these rocks are found outcropping 

 a few miles below the Cretaceous beds on Mattagami river. 



1 Palaeozoic Rocks in Northern Ontario, M. Y. Williams, Summaty Report 

 Geol. Surv., 1919, Part G. 



