216 A. W. G. WILSON TRAP SHEETS OE LAK^E NIPIGON BASIN 



rate of cooling, such as would normally take place only at some consider- 

 able distance below the surface. 



IX FAVOR OF SURFACE FLOWS 



1. The very widespread occurrence of unconformities between diabase 

 sheets and underlying formations. 



2. The occurrence of boulders of granite and gneiss and schist in dia- 

 base, the latter resting on similar rocks in situ in localities where there 

 is direct evidence that before the advent of the trap the underhung rocks 

 were buried beneath the sediments similar to those now present, near by. 

 under the same diabase sheet. 



3. The occurrence of old soils in situ at the bases and on the sides of 

 sedimentary ridges, the whole being covered in places with a diabase cap. 



4. The nicety of the adjustment by which the diabase sheets have 

 fitted themselves to the underlying topography. While the upper sur- 

 face 7 of the residuals of the capping sheets are everywhere fairly uniform 

 in height, the base of the sheet has adjusted itself to a topography where 

 the relief was at times as much as 300 feet. 



5. The mechanical problem which arises in explaining the numerous 

 unconformities, especially those on the embossed Archean surface, by the 

 theory of intrusion vanishes completely on the theory of surface erosion 

 prior to surface extrusion. 



6. The features characteristic of the upper surfaces of sills — the occur- 

 rence of overlying beds or fragments thereof, aphanitic structures, in- 

 cluded fragments in the upper part of the sheets — are not found. 



T. The medium to coarse texture, which characterizes the sheets, would 

 be found at the base of thick surface flows as well as in sills, being de- 

 pendent not on the nature and thickness of the cover so much as on the 

 rate of cooling. 



8. A glassy matrix, amygdaloidal or porous structure, basaltic texture, 

 flow structure, and associated voleanies would not be characteristic feat- 

 ures of the under parts of surface flows, and the upper parts of these 

 sheets are unquestionably removed, without a single exception. 



BALAXCIXG THE EVIDENCE 



It seems that we have no data relative to the actual character of the 

 upper surface of the trap sheets. Such negative evidence as is available 

 is equally applicable to both theories. With regard to the texture of the 

 residual basal portions of the sheets, there are no recorded differences 



7 It is recognized that this even upper surface is now an erosion surface and does not 

 necessarily owe its even character to an original uniform structural surface. 



