GEOLOGIC HISTORY. 441 



down the great thickness of mechanical sediments marked by the Knife 

 Lake slates. These were deposited as mnds, grits, and fine gravels. The 

 sea, therefore, must have been one of moderate depth; land areas must 

 have been adjacent, as the formation is so thick that a continual subsidence 

 must have occurred in order that the great mass of material could be piled 

 up. A part of the material was clearly derived from the adjacent land 

 areas; some portion was contributed by contemporaneous volcanoes, for 

 within the Knife Lake slates at various localities are considerable propor- 

 tions of volcanic ash, showing that volcanic ash was spread far and wide 

 from volcanic centers, and was thus important for upbuilding the Knife 

 Lake slates. The source of this ash has not been discovered. The 

 Ogishke conglomerate, Agawa formation and the Knife Lake slates together 

 constitute the sediments of the Lower Huronian series. 



Following the upbuilding of the sediments was another great period 

 of igneous intrusion marked by the Giants Range granite, the Snowbank 

 granite, and the Cacaquabic granite. While the acid intrusives mentioned 

 were the dominant ones, there are certain more or less schistose basic and 

 intermediate intrusives occurring in isolated dikes in the various rocks thus 

 far described which belong to this same general period of igneous activity. 

 The Snowbank granite and granites of equivalent age, etc., are placed with 

 the Lower Huronian series, although they more probably belong with the 

 great geologic revolution following the comparatively quiescent conditions 

 of Lower Huronian sedimentation, and contemporaneous with lavas j^robably 

 formed at the surface. If such lavas were laid down they were apparently 

 wholly removed by the deep-seated erosion mentioned below. 



This revolution was caused by the orogenic movements which now 

 followed. These were of the most severe kind, and continued long. As a 

 result the Lower Huronian series were thrown into a set of close east-west 

 folds and steeper cross folds. This folding must have produced great 

 mountain masses. No sooner did the orogenic movements raise the land 

 above the sea than the epigene forces again began their work of hewing it 

 down. The period of erosion was very long, so long in fact that in various 

 places the entire Lower Huronian series was cut through, laying bare the 

 Archean. Contemporaneous with and largely caused by the folding and 

 partly by the intrusion of the igneous rocks and by the erosion was a 

 second great period of metamorphism. The great mass of muds of the 



