SOURCE AND NATURE OF THE FLOWS 217 



which would indicate that it belonged to a flow and not to a sheet. On 

 the other hand, numerous unconformities exist and the diabases are 

 known to rest successively upon Laurentian, Kewatin, Huronian (pos- 

 sibly Middle and Lower, certainly Animikie), and Keweenawan, and 

 these unconformities are very widely distributed. Owing to the mechan- 

 ical difficulties involved by any other interpretation, it seems to the 

 writer that the balance of evidence available is distinctly in favor of con- 

 sidering these capping sheets as the basal residuals of a once very exten- 

 sive flow or series of flows of a very fluid diabase over the well dissected 

 topography of a previous cycle. 



Source and Nature of the Flows 



Information about the source of the diabase is scant. No specific 

 centers of eruption are known. Some of the sills in the sediments are 

 found associated with dikes of similar rock. Instances where the actual 

 connections between the two have been observed are rare, and the writer 

 has never had the opportunity of studying one in detail. Two localities 

 have been found where the writer thinks he has located a connection be- 

 tween a dike from below and the overlying diabase cap. In both in- 

 stances only one side of the supposed dike in contact with the adjacent 

 rock was found, though the connection with the -sheet is distinct, and it 

 can not be regarded as certain that the intrusive is a dike from below. 



One of these lies east of the south end of Black Sturgeon lake, about 3 

 miles from the lake, near the head of a dry canyon cut in Kewatin 

 schists. Near the head of this gorge, on the north side, the schists are 

 found to overlie the diabase, an actual contact having been found. The 

 contact occurs in the face of the cliff which forms the north wall of the 

 canyon, and rises upward at an angle of about 20 degrees from tjie hori- 

 zontal, the exposed portion being about 125 feet in length. This diabase, 

 here lying below Kewatin schists, is directly connected with the main 

 sheet of diabase that covers all that section of country south of lake 

 Nipigon and extends eastward beyond the Nipigon river. If this is a 

 feeding dike, the other contact lies farther east, beneath the diabase 

 sheet, and would not now be accessible. 



The second locality is near the head of Pine portage, on the Nipigon 

 river, at the north end of the area of gneiss exposed in the gorge. Oppo- 

 site the head of the portage are high bluffs of diabase, rising over 300 

 feet above the river. Near the summit we found a large mass of gneiss 

 overlying the diabase, which forms the whole front of the cliff facing the 

 portage. No actual contact could be found, though the two rocks were 

 traced to within about 10 feet of each other. A small intervening hoi- 



