﻿Yol. 64.] THE LA.UREJfTIA]Sr SYSTEM IN EISTEEN CANABA. 127 



7. On the Structure and Relations of the Lafrentian System in 

 Eastern Canada. By Frank Dawson Adams, D.Sc, F.E.S., 

 F.G.S., Logan Professor of Geology in McGill University^ 

 Montreal. (Communicated by permission of the Director of 

 the Geological Survey of Canada. Read November 6th, 

 1907.) 



[Plates XI-XIII.] 



Contents. 



Page 



I. Introduction 127 



II, Structure of the Area 130 



III. The Invading Bathyliths 130 



IV. The Rocks of Sedimentary Origin 136 



V. The Amphibolites 137 



VI. The Gabbros and the Nepheline-Syenites 139 



VII. Contact-Phenomena about the Borders of the Granite-Bathyliths . 141 



VIII. Distribution and Thickness of the Grenville Series 142 



IX. Relation of the Grenville Series to other Pre-Oambrian Series 144 



X. Summary " 145 



I. Introduction. 



When Sir William Logan, in the early years of the Geological 

 Survey of Canada, was gradually unravelling the stratigraphical 

 succession as displayed in the Dominion, he found that at the base 

 of the whole column lay the crystalline rocks of the Laurentian 

 Mountains. The name ' Laurentian Mountains ' had been previously 

 given to that great stretch of iron-bound coast which lies along 

 the north of the Gulf of St. Lawrence and is not, properly 

 speaking, a mountain-range at all, but is merely the margin of a 

 great rock -plateau — the Laurentain peneplain — which forms a 

 portion of the northern protaxis of the North American continent. 

 Logan's studies in the Province of Quebec led him to believe that 

 the Laurentian System, as he termed it, consists of a series of 

 highly-crystalline limestones interstratified with quartzites and 

 gneisses, which in their turn overlie a great thickness of foliated 

 orthoclase-gneiss, the foliation of the latter being regarded by him 

 as the survival of an almost obliterated bedding. 



He subsequently found in Eastern Ontario a series of rocks which 

 he considered in all probability to represent the Grenville Series in 

 a less altered state, and to this he gave the name of the ' Hastings 

 Series.' Later investigations showed that the anorthosite of the 

 Laurentian System is of intrusive origin ; but little further light was 

 thrown upon the relations of the underlying limestone-series (which 

 is now termed the Grenville Series) to the Lower or Fundamental 

 Gneiss. The relation of the Grenville and the Hastings Series also 

 remained uncertain. 



Q. J. G. S. No. 254. k 



