STRUCTURAL FEATURES OF THE PRE-CAMBRIAN 187 



north limb is found on Windy Lake, and by the crest of the next 

 anticline ; or else it must be assumed that another strike fault again 

 upthrows the lower beds on the south. 



The determination of the attitude of the beds on the south side 

 of Kaopatina Lake, the correctness of which is a matter of great 

 importance, was made with the greatest care in three different 

 places, in each case by tracing a flow from its coarse-grained base 

 up to its fine-grained top, where it was found in contact with the 

 coarse-grained base of another flow. Lying above these basalts, 

 and conformable on them, is a thick series of thin-bedded tuffs, 

 interbedded with which, and rather high up in the series, is a thin 

 flow of rhyolite porphyry. This flow is of small areal extent and 

 appears only in the west end of the lake. 



On the Opawika River above Kaopatina Lake the same relations 

 were observed. Rather thick flows of basalt with a strike of N. 80° 

 E. and a dip of 80° south 'are here conformably overlain by thin- 

 bedded, rather basic tuffs. The plane of contact is occupied by a 

 dyke of granite, which appears to be an offshoot of a larger mass 

 to the east, as the dyke was not observed along the contact farther 

 to the west, and where observed widens rapidly toward the east. 

 The strike and dip of the tuffs is identical with that of the under- 

 lying basalt. About thirty feet above the contact a basalt flow 

 about four feet in thickness is interbedded with the tuffs. 



Passing higher into the tuffs toward the south, it was observed 

 that the beds of tuff begin to be mingled with beds of metamor- 

 phosed clastic sediment. The latter become more numerous and 

 finally replace the tuffaceous beds completely. At the same time 

 the thickness of the separate beds increases until on the south side 

 of Lake Shishib the beds are so heavy and massive that a bedding 

 plane can rarely be observed. 



The thick-bedded rocks on Lake Shishib are garnetiferous mica 

 gneisses, probably recrystallized impure sandstones, or sandy shales. 

 They continue to outcrop at intervals up the Opawika River to 

 Dinachagama Lake, where they begin to be intruded by granite 

 masses. They were traced across the Height of Land into Lake 

 Nemenjish, on the north shore of which the sediments are well 

 exposed and comparatively undisturbed by granite intrusion. Here 



