VIEWS AS TO ORIGIN 201 



"1. They are strictly analogous to the great dikes of the region: (a) In their 

 general relations to the adjacent rocks and in their field aspect. (&) In that 

 both the upper and lower sides of the sheets have the facies of a dense aphan- 

 itic rock, which grades toward the middle into a coarsely crystalline rock. 



"2. They have practically a uniform thickness over large areas. 



"3. The columnar structure extends from the lower surface to the upper 

 surface, as it does from wall to wall in dikes. 



"4. They intersected the strata above and below them after the latter had 

 been hard and brittle. 



"5. They may be observed in direct continuity with dikes. 



"6. They pass from one horizon to another. 



"7. The bottom of the sedimentary strata above them, wherever it is ob- 

 servable, is a freshly ruptured surface. 



"8. Apophyses of the trap pass from the main sheet into the cracks of the 

 slate above and below. 



"9. The trap sheets, particularly at the upper contact, hold included frag- 

 ments of the overlying slates. 



"10. They locally alter the slates above and below them." 



For purposes of discussion the diabase sheets may be divided into two 

 great groups : That group of sheets both the upper and lower surfaces of 

 which are known or can be readily inferred, and a group consisting of 

 all those sheets whose under surface only is known — the group of sheets 

 which forms the various topographic "caps" throughout the whole region. 



With reference to the first group, the observations of the writer, con- 

 ducted over a wider area, emphatically confirm Lawson's conclusions as 

 outlined above. With reference to the capping sheets, the writer's obser- 

 vations are in accord with Lawson so far as the areas examined by both 

 are concerned, but data obtained largely in the basin of lake Nipigon lead 

 him to make a somewhat different interpretation of facts noted both by 

 Lawson and himself, and to different conclusions. 



Scope of the present Discussion 



The writer wishes to confine the present discussion wholly to the con- 

 sideration of the nature of the relations which now exist, and possibly 

 formerly existed, between that group of sheets of diabase which now 

 form the "caps" and the underlying rocks. So far as all other masses of 

 diabase within the area are concerned, his observations confirm in every 

 respect those of Lawson. Hence that there are a group of diabase sheets 

 in this region which are not volcanic flows but which are intrusive sills 

 of a laccolitic type is not a subject of discussion in this paper. 



With reference to the "caps," it must be recognized at the outset that 

 the only contact surfaces we can study are those at the base. The char- 

 acter of the upper surface and the nature of the contacts, if any, can be 



