STRUCTURAL GEOLOGY. 



17 



Upper Keewatin.] 



formation is so much like the Saganaga granite (and is really a "recomposed 

 granite '') that it has been mistaken in the field for granite by nearly all who have 

 seen it. In other places it appears that a very fine debris from the quartz-porphyries 

 or from the greenstones of the Lower Keewatin has accumulated in the same quiet 

 manner at the base of the Upper Keewatin, the coarser and even conglomeratic 

 composition coming in at a higher horizon. This fact makes it very difficult to 

 establish the precise base of the Upper Keewatin, except where this early fine 

 debris was carried away and the conglomerate lies immediately on the older rock. 

 The Upper Keewatin is cut by granite in the form of dikes and bosses. As it holds 

 granite boulders there must have been an older granite. About Snowbank lake, and 

 also about Moose and Disappointment lakes, it is changed to a mica schist by a 

 regional metamorphism, but only at Snowbank lake is it seen to be penetrated 

 extensively by intrusive dikes of granite. The two most notable masses of the Upper 

 Keewatin are the Stuntz and the Ogishke conglomerates. These are believed to be 

 substantially of the same age, although they are separated by an upward swell in 

 the bottom of the great trough by reason of which they are in isolated areas. 



The conditions of rock formation at the time of accumulation of the Upper 

 Keewatin were the same as in Lower Keewatin time, but the detrital forces were 

 much more powerful. It is evident that after the folding of the Lower Keewatin 

 the igneous forces suffered a great abatement, though they did not cease. Isolated 

 volcanic craters continued to throw out volcanic ash, and to charge the atmosphere 

 with acid gases, and the ocean with the same chemical precipitates. This is 

 evinced not only by the occurrence of much of the same volcanic greenstone ash in 

 the rocks, but also by the existence of extensive deposits of jaspilyte in the later 

 schists and conglomerates, as at Moose lake. 



The Upper and Lower Keewatin have been subjected to another folding which 

 had a general parallelism with the earlier folding. This pressure was so powerful 

 and the folding so close that the Upper Keewatin lies in narrow synclines in the 

 Lower Keewatin, and in nearly all places its attitude is nearly or quite vertical. 

 With this general statement must be inserted some exceptions, viz., the strike of the 

 bedding suffers sudden local changes. It runs north and south instead of northeast 

 and southwest. Such an exception has been noted on the south shore of Vermilion 

 lake, another on the east side of Disappointment lake (sec. 34, T. 64-8 W.), and still 

 another southeast from Knife lake (sees. 7 and 8, T. 65-6 W). The strike sometimes 

 is even 10 or more to the west of north. Such irregularities have, so far as observed, 

 no systematic occurrence, but still, perhaps, indicate a pressure and a folding 

 tendency oblique to the general direction. They may also be explained by the 

 occurrence of local obstructions such as isolated bosses of the older formation 



