DISCUSSION OF EVIDENCE 211 



scribes the rock as usually a compact bluish black basalt with frequently 

 a well defined columnar structure, Cf but also at times it is vesicular and 

 scoriaceous, especially on the surface of sheets." 5 



If, in the Lake Nipigon area, the lavas were very fluid, and if the flows 

 followed one another closely, so that the earlier flows had not had time to 

 solidify before the later followed upon them, or if there were only one 

 large flow, the fluid lava would tend to accumulate in the basins and hol- 

 lows of the invaded district, would heat the underlying rocks, possibly 

 even re-fusing parts of the lava that had cooled on the first contact with 

 the cold underlying beds on which it rested, and would remain fluid for 

 a very long time, giving ample opportunity for the development of the 

 crystalline texture so characteristic of the basal portion of the remnants 

 of the sheets. 



In this event the characteristic upper surface would be confined to the 

 highest members only, and below a certain limiting depth the cooling of 

 the mass would proceed as if it were a laccolitic mass below a sedimentary 

 cover. For this reason also no argument as to the nature of the upper 

 surface and as to the character of the intrusion can be based on variations 

 in the texture of the crystalline rock. While the marked uniformity of 

 the grain of the rock, except that close to the base of the sheets, suggests 

 slow cooling, nearly simultaneous solidification throughout the greater 

 portion of the mass of the sheet, and the existence of a cover, it offers no 

 evidence as to the character of that cover. 



UNCONFORMITIES OF THREE TYPES 



General characteristics. — The occurrence of numerous unconformities 

 is an established fact. Broadly, they are of three types : 



The diabase sheet appears to truncate the edges of the nearly hori- 

 zontal beds of sedimentary rocks in situ on Archean rocks. 



The diabase rests on an erosion and uneven surface which truncates 

 the structures of metamorphic Archean rocks. 



The diabase rests on an uneven surface which truncates both Archean 

 metamorphics and overlying later sediments — rocks of two or more for- 

 mations. 



Examples of each type. — Contacts of the first type are particularly 

 numerous along the line of the Canadian Pacific railway. Reference has 

 been made to the unconformity at Red Eock. A number of other exam- 

 ples are described by Lawson, more particularly, however, with respect to 

 contacts between the diabase and underlying Animikie rocks. One of 



5 1. C. Russell : A geological reconnaissance in central Washington. U. S. Geological 

 Survey, Bulletin 108, p. 21. 



