378 



is a brecciated one of no great width, the Laurentian is 

 granitoid and the adjoining Keewatin is a dark finely 

 crystalline hornblende schist or diorite. 



LATER GRANITE. 



Several of the above mentioned outcrops of granite, 

 supposed to be later than the Lauientian, have been 

 minutely described by Dr. Lawson, but the distinctive 

 characters by which they may be distinguished from the 

 Laurentian granites are apparently lacking in the exposures 

 to be visited. 



KEWEENAWAN. 



A remarkable series of dykes crosses Lake of the Woods 

 and Shoal lake in a general northwest and southeast 

 direction. The continuation of some of these in Rainy 

 Lake region gives a length of about ioo miles (161 km.) 

 to some of the better developed dykes. 



These dykes are essentially a coarse grained quartz 

 diabase with a porphyritic border. In the original des- 

 cription (2), garnet is mentioned as one of the prominent 

 minerals in the central portion, but this has not been found 

 in the material secured by the writer from outcrops of Lake 

 of the Woods. The other minerals observed as well as 

 the characteristic texture of the rock, agree with the des- 

 cription given by Dr. Lawson. In the northern part of 

 Lake of the Woods four of these dykes are known, while 

 in Welcome channel a fifth dyke, that has been altered to 

 serpentine, may possibly upon further study be correlated 

 with these. 



The adjacent Keewatin rocks usually show marked 

 metamorphism for 20 to 30 feet (6 to 9 m.) away from the 

 dykes. This is more evident in sericite schists con- 

 taining ferro-dolomite, though is is also to be observed in 

 the chloritic schists. As a result of this metamorphism 

 the schists are crumpled, and epidote, magnetite and 

 hematite, which are readily noticed in the field, are formed. 



The most accessible of these dykes is that which was 

 mapped on Thompson island and Whitefish bay. It has 

 been traced almost without interruption from Darlington 

 bay to the east side of Whitefish bay, and is apparently 

 continuous with a dyke on Crow lake. In places, as on 



