214 A. W. G. WILSON TRAP SHEETS OF LAKE NIPIGON BASIN 



numerous instances in which the diabase occupies not only major de- 

 pressions in the surface, but has even insinuated itself into minor irregu- 

 larities. It has done this not in a few localities, but in many places 

 widely distributed. Had it insinuated itself between an overlying cover 

 and the rock on which it now rests, normally one would expect to find 

 numerous small remnants of the sediments in the bottoms of the major 

 hollows, at least, rather than to find so complete a stripping as seems to 

 have taken place. In the second place, the edges of the sheets, with the 

 sediments underlying, have been followed and examined for many miles 

 and by many observers, and no single instance has been recorded where 

 an intruded diabase sheet has followed the sinuosities of the contact sur- 

 face between the sediments and the underlying Archean. In other words, 

 while in the whole area numerous sheets have been intruded into the 

 sediments in addition to the sheets that now form the "caps," yet in no 

 instance has one of them been found to have intruded itself along the 

 supposed plane of weakness at the contact between the sediments and the 

 underlying rocks. 



In the case of unconformities of the third type, under the intrusive 

 theory the diabase must have occasionally been unable to insinuate itself 

 along the intricate surface of the Archean and must have broken across 

 the beds of the sediments to a higher horizon, leaving a remnant firmly 

 attached to the original basement, and again descending after the ob- 

 structing mass was passed — an improbability not only because of the 

 physical difficulties involved, but also because at the contacts, so far as 

 recorded or examined, no evidence of violent disruption or intrusion has 

 been found. 



Again, numerous contacts between the sediments and the Archean show 

 that usually the basal beds are arkoses or conglomerates, and contain at 

 least a few pebbles, cobbles, or boulders, derived from the underlying 

 rock. In four widely separated localities, yet each in the vicinity of un- 

 conformable contacts between the diabase and two other formations, 

 boulders of the underlying Archean have been found in the diabase at or 

 close to the contact. The diabase in actual contact with these boulders 

 is fine textured, but it is not glassy, and the zone of alteration, even in 

 the immediate vicinity of one boulder about 5 feet across, is very narrow, 

 from which one can infer that the boulder lay in the fluid or semifluid 

 diabase long enough to become heated through, and that it had but little 

 effect on the cooling of the diabase magma in its immediate vicinity. 



No fragments of any sandstone or other cement material similar to 

 that found in the basal conglomerate was found either in the diabase or 

 clinging to the boulders, so far as seen. 



