270 ASSINNIBOINE AND SASKATCHEWAN EXPEDITION. 



covers a considerable space between the Lakes Ontario 

 and Champlain, and constitutes the Adirondack Moun- 

 tains. With this exception, and perhaps also a small 

 exposure in Arkansas and another near the sources of the 

 Mississippi, this formation is not found to the south of 

 the St. Lawrence, and as it belongs especially to the valley 

 of this river and constitutes the Laurentide Mountains, 

 the Geological Commission of Canada has distinguished 

 it by the name of the Laurentian system." 



THE LAURENTIAN SYSTEM. 



" The rocks of this system are, almost without excep- 

 tion, ancient sedimentary strata, which have become 

 highly crystalline. They have been very much disturbed 

 and form ranges of hills, having a direction nearly north- 

 east and south-west, rising to the height of 2,000 or 

 3,000 feet and even higher. The rocks of this formation 

 are the most ancient known on the American continent, 

 and correspond probably to the oldest gneiss of Finland 

 and Scandinavia and to some similar rocks in the North 

 of Scotland. 



" The rocks of the Laurentian formation are in great 

 part crystalline schists, for the most part gneissoid or 

 hornblendic. Associated with these schists, are found 

 large stratified masses of a crystalline rock, which is 

 composed almost entirely of a lime and soda felspar. 

 The rock is sometimes fine grained, but more often por- 

 phyritic, and contains cleavable masses of felspar, some- 

 times several inches in diameter ; these felspars are 

 triclinic, and have ordinarily the composition of andesine, 

 labradorite, anorthite, or of intermediate varieties. Their 

 colours are various, but the cleavable felspars are gene- 

 rally bluish or reddish, and often give coloured reflections. 



