﻿308 Canadian Becord of Science. 



stitnting the essential part of the Grenville series. They 

 usually, however, form but a very small proportion of the 

 rocky complex in the areas in which they occur, and 

 which, owing to their presence, is referred to the Grenville 

 series. They are associated with and often enclosed by 

 much greater volumes of gneissic rocks, identical in 

 character with the Fundamental Gneiss. The limestones 

 are also almost invariably penetrated by masses of coarse 

 pegmatite, and occasionally large masses of the limestone 

 are found embedded in what would otherwise be supposed 

 to be the Fundamental Gneiss. The whole thus presents 

 a series of sedimentary rocks, chiefly limestones, invaded 

 by great masses of the so-called Fundamental Gneiss, and 

 in which, possibly, some varieties of the gneissic rocks 

 present may owe their origin to the partial commingliug 

 of the sedimentary material with the igneous rocks by 

 actual fusion. There is, however, no reason to believe- 

 from the evidence at present available, that any consider- 

 able proportion of the series has originated in the last 

 mentioned manner. 



It will be readily seen that an exact delimitation 

 of areas of the Grenville series is thus sometimes a matter 

 of great difficulty, as they often appear to shade away 

 into the Fundamental Gneiss, and it has hitherto been 

 difficult in the case of the Grenville series to account for 

 the existence of such a comparatively small proportion of 

 sedimentary strata, intimately associated with such great 

 volumes of igneous gneisses. 



The relations of the two series, as determined by the 

 investigations of the last two seasons, throws new light 

 upon the subject, and indicates the probable explanation 

 of the difficulty. 



The north-western half of the more restricted area 

 at present under consideration is underlain by Funda- 

 mental Gneiss, presenting the characters described above. 

 A smaller area of the same gneiss occurs at the south- 



