STRUCTURAL FEATURES OF THE PRE-CAMBRIAN 373 



characteristics so definite that its identification can scarcely be 

 doubted. The dotted patches are those in which the amount of 

 Gren\dlle present, relative to that of the granite which has intruded 

 it, is small, so that its identification is more open to argument. 

 Even in these cases, however, the effort has been made, as stated, 

 to weed out the doubtful cases and include none but those in which 

 identification is fairly certain. The results show the presence of 

 the Grenville over the whole interior of the Labrador peninsula, 

 wherever exploration has been carried, as well as in Bafhn Land 

 and the long-known districts to the south. 



The study of the older reports, together with the writer's own 

 field work, has brought to light a peculiarity in the distribution of 

 the limestone member of the series. Limestone is found in Baffin 

 Land, in places along the Labrador coast, and throughout southern 

 Quebec and Ontario, as well as in the Adirondack region; but little 

 or no limestone has been described throughout the interior plateau 

 of northern Quebec. It has been considered by certain authors that 

 the lack of limestone in this region is due to its solution by the intru- 

 sive granite, where the granite is present in sufficiently large amount. 

 This opinion does not appear to be well founded. In the section 

 northward along the Gatineau River, above the town of Maniwaki, 

 the writer found that the limestone member decreases in relative 

 amount, with corresponding increase of the clastic members of the 

 series, until it disappears altogether, without any notable increase 

 in the amount of intrusive granite which is present throughout 

 in considerable amount. In the Nemenjish area, there are at least 

 several hundred feet of elastics, entirely free from intrusive granite. 

 With them there are interbedded only two bands of limestone, each 

 about a foot in thickness. These facts seem explicable only on the 

 hypothesis that the limestone never was deposited in the localities 

 mentioned; in other words, the lack of limestone in certain areas 

 is due mainly to a primary difference of sedimentation rather than 

 to removal after deposition by the granite. A preliminary attempt 

 has been made, from a study of the reports, to determine the position 

 throughout the Labrador-Quebec region at which this change in 

 sedimentation took place, and to indicate it on the map (Fig. 13) by 

 a heavy black line. On the Hudson Bay side of this line little or 



