DESCRIPTIONS OF CRITICAL AREAS 207 



On the east side of the north arm of Humboldt bay, about 2 miles from 

 the contact between Kewatin schists and the diabase, the remnant of the 

 trap sheet rests on Keweenawan sandstones ; these in turn rest on granite. 

 This area of sandstone is an isolated patch, with a minimum thickness of 

 50 feet. Since the diabase is found in actual contact, not only with the 

 sandstones but also with the underlying rocks in adjacent areas, the con- 

 tact is an unconformable one. 



POSSIBLE REMNANTS OF OLD SOILS IN SITU 



Within the Nipigon basin the greater number of contacts noted be- 

 tween the base of the remnants of the capping sheets of diabase and the 

 underlying rocks were in areas where the Keweenawan sediments had 

 largely been removed prior to the volcanic extrusion. In some four 

 places, two of which have been mentioned above, boulders of Archean 

 rocks have been found within the diabase close to the basal contact. Con- 

 tacts between Keweenawan sediments and the igneous rock are also fre- 

 quently found, but the best examples lie to the south of the Nipigon 

 basin, along the line of the Canadian Pacific railway, in the area that is 

 underlain chiefly by these sediments and the area in which the intrusive 

 sills are so abundant. 



Near the mouth of the Nipigon river, 3 miles southwest from Nipigon 

 station, the railway track skirts the foot of a diabase-capped bluff known 

 as Eed Eock (from the characteristic color of the underlying sediments). 

 At this locality (mile post 66, from Schreiber) the diabase sheet ascends 

 across the beds to a height of about 140 feet, and the upper part of the 

 sheet seems to rest nearly conformably on the upper beds. Where the 

 diabase crosses the truncated edges of the lower beds, close to the railway 

 track, between the base of the sheet and the undisturbed portion of the 

 beds is a mass of broken rock at least 10 feet in thickness at one point 

 and probably thicker elsewhere. This breccia consists of small fragments 

 of the sediments, many of them partly rounded and the whole now rece- 

 mented and bleached to a color lighter than that of the parent beds. The 

 recemented beds seem to have a slight downward dip toward the ascend- 

 ing trap sheet. 



At mile post 67, on the opposite side of the same ridge, are more ex- 

 posures of a similar breccia, not in immediate contact with the diabase 

 sheet. Nearly midway between these two points one of the railway cut- 

 tings passes through a dome in the sediments in which many of the thin 

 shaly beds are crumpled, while the heavier beds are folded or faulted 

 locally. While there are no exposures of diabase in the exposed portion 

 of the core of the dome, its form and structure strongly suggest that the 

 strata are arched up by a laccolitic mass below. 



