32 



Pleistocene. — The Pleistocene has not been carefully 

 studied, but consists not only of till, but also of assorted 

 sands and gravels which frequently show marked cross- 

 bedding. This latter feature is to be seen at many of the 

 gravel pits along the railways where excavation has been 

 recently done. 



Diabase sill intrusive into slate; Current River Park, Port Arthur. 



Keweenawan. — The Keweenawan in this region con- 

 sists largely, if not entirely, of great masses of diabase 

 which were intruded into the Animikie slates and iron for- 

 mation in the form of extensive sills, known as the Logan 

 sills. These sills have a marked effect upon the topography 

 of the region, giving rise to flat topped hills, 

 the upper parts of which are very precipitous, though 

 the lower parts have gentle slopes due in part to the forma- 

 tion of talus. It was formerly supposed that these sills 

 were surface flows as they were found capping the hills, 

 but Dr. A. C. Lawson (7) showed that they were intrusions. 



