EARLIER DIVISIONS OF THE LAURENTIAN. 351 



by the work of Lawson around the Lake of the Woods and b} r that of 

 Bell and Barlow in the Sudbury district that a very considerable portion 

 of the more syenitic mass must now be regarded as truly of eruptive 

 character. These masses, which are sometimes syenitic or granitic, in 

 places assume a gneissic structure, a peculiarity also sometimes observed 

 in the more recent syenitic and granitic masses of eastern Quebec and 

 New Brunswick. The same conclusions as to the origin of much of the 

 so-called syenitic gneiss will apply to large portions of the rock in the 

 area north of the Ottawa more particularly under consideration. 



Earlier Divisions of the Laurentian. 



Tn the earlier reports and on the great geologic map of Canada, 1866, 

 the Laurentian was divided into two portions — a lower, comprising the 

 gneiss and limestones, and an upper, which embraced the great areas of 

 anorthosite or labradorite rocks, found more particularly at that date to 

 the northwest of Montreal, near Saint Jerome.* No attempt was at that 

 time made to separate the calcareous portion from the gneissic, and in 

 fact the former was then regarded as an integral part of the whole, 

 occurring as regularly inters tratified beds often of great thickness, at 

 different points in the column representing the entire thickness of the 

 Laurentian rocks. 



Scope of the Present Paper. 



In this paper we propose to reconsider the typical section upon which 

 the great thickness of the supposed sediments, comprising a total of over 

 32,000 feet, was based, and to show that, in the light of the explorations 

 carried out during the last fifteen years, certain modifications of the 

 arrangement of strata as there laid down must be made. 



The Laurentian Limestone and Gneiss. 



The development and distribution of the limestones of the Laurentian 

 plays a very important part in the determination of the structure and 

 thickness of the whole, since in some places the calcareous bunds are 

 exceedingly limited, having a thickness of not move than live to ten feet, 

 or even less, while in other places this thickness increases to several 

 hundreds of feet. Thus, in a section published by Logan in 1845 of the 

 High falls of the Madawaska, representing a, thickness of 1,350 feet, the 

 limestone is noted as occurring in seven bands, the thickness of which 

 varies from less than one foot to nineteen, in which latter, however, 



*Se.e map published in atlas, Geol. Canada, 1863. 



