DISCUSSION OF EVIDENCE . 215 



It seems very improbable that the several masses of diabase in which 

 these boulders now lie were intruded between the basal beds similar to 

 those in adjacent masses of sediments and the Archean rock on which 

 the trap containing these boulders now rests. 



COLUMNAR STRUCTURE 



Columnar structure is a feature characteristic of all the sheets, whether 

 distinctly intrusive or possibly surface flows. In nearly every instance it 

 extends from the lower surface to the upper surface, as it does from wall 

 to wall in dikes. Structures of this type, however, are not confined 

 strictly to sills. We find that the flows of both the Snake river and the 

 Columbia river lava beds exhibit the same type of structure. It must 

 also be pointed out that with respect to the capping sheets we know only 

 the base of the sheet and possibly its middle parts, the upper part having 

 been removed. There are also many localities where horizontal jointing 

 is as well developed as the vertical jointing by which the columnar struc- 

 ture is produced. In very many instances (three localities are : east side of 

 Pijitawabikong bay ; near Speke point on the east side of lake Mpigon ; 

 and the west side of the portage into Waweig lake from the south) this 

 double system of jointing is well developed, and a moderate amount of 

 erosion along the fissures has slightly rounded the exposed corners of the 

 joint blocks, so that the face of the cliff, sometimes for many hundreds of 

 square feet, closely resembles an ancient wall. The resemblance to a 

 wall is very close and remarkable, because the individual blocks are 

 strikingly uniform in size and are usually oblong in section, the hori- 

 zontal joints being usually closer to one another than the vertical ones. 



On many of the thicker sheets so well is horizontal jointing developed 

 that the vertical columnar structure which the cliff faces of these sheets 

 present is largely due to the accident that gravity — which causes some 

 of the joint blocks to fall down the fronts of the cliffs while others re- 

 main standing as columns — happens to act in a vertical direction only. 



Summary of the Evidence available 

 in favor of intrusive sills 



The arguments in favor of regarding the "caps" as intrusive sills may 

 be briefly summarized as follows : 



1. Entire absence of any -of those features that are usually associated 

 with the upper parts of a surface flow — glassy matrix; amygdaloidal, 

 porous, or basaltic texture; flow structure; associated volcanics, either 

 lava breccias or pyroclastics. 



2. A medium to coarse crystalline texture, usually indicative of a slow 



