482 R. W. ELLS — -MICA DEPOSITS OF THE OTTAWA DISTRICT. 



through extreme processes of metamorphism have been changed from 

 their original condition of limestone, sandstone and possibly shale into 

 marble, quartzite, slate and gneiss. Many of these beds are regularly 

 interstratified, and this peculiarity can be readih^ seen in that portion 

 styled by Logan " the Grenville series." In many places throughout the 

 area occupied by these rocks a re,2;ular and gradual passage is traceable 

 upward from the silicious into the calcareous beds, as pointed out in my 

 paper on the Laurentian read before this Society last year. 



Intrusives. — There are, however, great rock masses throughout the sys- 

 .tem which present very different aspects from the strata just mentioned, 

 not only in their macroscopic and microscopic characters but in their 

 relations to the stratified portions as well, and as has been very clearly 

 pointed out by the observers in the more western areas, these masses, 

 often of large extent, are clearly intrusive, and consequently of more 

 recent date than the presumably sedimentary portion with which they 

 are associated. 



This peculiarity of intrusion is not confined to any particular area, 

 but is as readily recognized in the district north of the Ottawa river as 

 in the country north of lakes Superior and Huron, or further east along 

 the shores of Labrador, though over some areas the intrusive masses 

 have a much greater extent than in others. 



Former Errors in Identification. — The intrusive character of some of 

 these rocks was clearly recognized by Sir William Logan and other early 

 workers in this difficult field. Certain other masses, however, more par- 

 ticularly associated with the development of the economic minerals, 

 such as mica, apatite, graphite, etcetera, were held to be of sedimentary 

 origin. These included most of the pyroxenic rocks, the quartz-felspars 

 and certain of the syenites, together with the anorthosites, in many of 

 which a certain gneissic structure is developed through the agencies 

 which have produced the general metamorphism of the sj^'stem. Hence 

 many of the mineral deposits which occur in these intrusives were sup- 

 posed to occur as beds and to have their origin in the same causes 

 which tended to develop veins of mineral matter in what were known 

 to be true sedimentary rocks. The confusion of gneissic sedimentary 

 with volcanic rocks and the placing of them in the same group were 

 very natural errors, and the metamorphic origin of many of the granites 

 and syenites is still maintained by many earnest workers on the subject. 

 This view applies not only to those masses found in the Laurentian, but 

 to others occurring in Quebec, New Brunswick and Nova Scotia, which 

 have generally been regarded as of the age of the Devonian or the close 

 of the Upper Silurian, and in which a gneissic structure — a structure 

 clearly induced by pressure — is also frequently developed, more particu- 



