﻿Problems ill Quebec Geology. 481 



strata or rock masses there found were originally 

 sedimentary deposits; and that by subsequent meta- 

 morphism these passed into the condition of crystalline 

 rocks such as gneiss, limestone and quartzite as we 

 now find them displayed. In this category were also 

 included the great masses of anorthosite rocks which 

 have a very considerable development to the north of the 

 city of Montreal: and these were supposed, from their 

 presumed relations to the crystalline limestones, to 

 represent the- upper member of the Laurentian system. 

 The Laurentian was, therefore, practically divisible into 

 three parts, viz., a lower fundamental gneiss, a middle 

 gneiss and limestone series, and an upper or anorthosite 

 division, all of which were held to be originally of 

 sedimentary origin. These were, however, usually 

 described under two heads, a lower and an upper, and the 

 whole of the rocks pertaining to the system beneath 

 the anorthosite was placed in the lower division of 

 the Laurentian. The syenites of Grenville and certain 

 dykes of trappean or diabase rock were regarded as 

 of igneous origin. 



As early as 1877-78 the study of the anorthosites 

 by Vennor led to the expression of the. opinion on his part 

 that these, in part at least, were intrusive in the crystalline 

 limestones, and this view was shortly after supported by 

 Dr. Selwyn. Subsequent study of the granitic and 

 anorthosite masses by McConnell and Adams showed that 

 this new view was the correct one, and that all these 

 rocks should be removed from the position they had long 

 held as tlie upper portion of the Laurentian, as an altered 

 sedimentary series, and that they were clearly intrusive 

 and newer than the limestones and quartzites with which 

 they were associated. The series of limestones and grey 

 gneisses became, therefore, the upper member of the 

 Laurentian. The sedimentary origin of these has also 

 been called in question by some geologists, but the recent 



