the phenomenon of the crystallization of a magma super- 

 saturated with alumina, the excess of which has separated 

 out in the form of corundum, forming deposits of this 

 mineral of economic value, which have been extensively 

 worked. These nepheline rocks occur almost without 

 exception along the border of the granitic batholiths 

 when the latter come against the limestone series. 



HISTORY OF GEOLOGICAL EXPLORATION IN 



THE LAURENTIAN OF EASTERN 



CANADA. 



When Mr. W. E., afterwards (1856) Sir William, 

 Logan, of the Geological Survey of Canada, made an 

 examination in 1844 of the region bordering the Ottawa 

 river, he found great areas underlain by very ancient 

 foliated crystalline rocks. These seemed to him, on 

 further study, capable of subdivision into two conform- 

 able series, which he subsequently (1853) called the 

 "Laurentian series." This designation was proposed by 

 the fact that these rocks constitute the bulk of the "Lauren- 

 tide Mountains," a name suggested by F. X. Garneau, 

 the historian of Quebec, for that great stretch of rocky 

 country which forms the highlands to the north of the 

 River and Gulf of St. Lawrence. This sharply defined 

 series of elevations is not strictly a mountain range, but 

 merely the steep margin of the great rocky plateau of the 

 Canadian Shield. 



Logan's lower or older group consisted exclusively of 

 "syenitic gneiss showing no end to the diversity of 

 arrangement in which the minerals and the colours will be 

 observed, but there is a never-failing constancy in respect 

 to their parallelism. But this, though never absent, is 

 sometimes obscure." These rocks were supposed by 

 Logan to form a low anticlinal arch in the region extending 

 from Mattawa river to the vicinity of Montreal river on 

 Lake Temiskaming. The upper group is stated to crop 

 out in the district south of Mattawa and Ottawa rivers 

 and to be characterized "by the presence of important 

 bands of limestone which have undergone extensive crystal- 

 lization as a result of extreme metamorphism," while the 

 different gneissic rocks which separate the various bands of 



