DISTRIBUTION OF THE DIABASE 221 



present lake Nipigon is underlain by diabase, and beyond the lake as far 

 as the Hudson Bay divide, mesas occur. Northeast of the lake the prin- 

 cipal remnants of the main sheet forms the north and south peninsulas 

 of Ombabika, and another large area, possibly a part of this sheet, occurs 

 12 miles farther northeast, at Grass lake. Since the oldland gradually 

 increases in elevation northeastward, it is doubtful if the northeast edge 

 of the sheet extended many miles beyond Grass lake. 



At the south end of Pijitawabikong bay the remnant of the sheet is at 

 least 600 feet thick. Near the middle of the crescentiform main mass 

 of diabase, at Tchiatang bluff, on Black Sturgeon bay, it is at least 650 

 feet from base to summit. The remnant at Inner Barn island, in Wabi- 

 nosh bay, is 600 feet thick. In all these localities and in numerous other 

 places not here indicated specifically a careful examination was made of 

 the sheet from base to summit, and, apart from the fact that at some 

 points erosion has developed great benches, no noticeable change in the 

 noncrystalline texture of the rock was observed and no indication was 

 found that would lead one to think that the sheet consisted of successive 

 flows. 



At the northeast angle of lake Nipigon, on the peninsula of Ombabika, 

 and at Livingstone point the thickness of the remnant of the sheet is 

 about 300 feet, thinning to about 250 feet where masses of sediments 

 underlie it. At various points around the margin of the oldland, rem- 

 nants on the adjacent uplands vary in thickness from 12 feet in one noted 

 instance to 150 feet. With regard to the masses of trap capping blocks 

 of sediments on the cuesta to the southwest of the lowland, unless there 

 is specific evidence to the contrary, it can not always be decided whether 

 they are remnants of an original capping sheet or are parts of a sheet 

 that had been intruded between sedimentary beds, the overlying beds 

 and the upper parts of the sheet having since been removed by erosion. 



The relations of the thinner capping sheets around the lake basin on 

 all sides of the lowland to the main mass in the lowland can be well com- 

 prehended by inferring that the molten diabase probably filled the low- 

 land basin to overflowing and then spread out on the adjacent higher 

 ground on all sides. Undoubtedly some of the diabase came to the sur- 

 face through fissures on the margin, but not within the basin, but the 

 tendency would be for it to move toward the lower ground. Eventually 

 the diabase from its several sources probably filled the whole basin and 

 spread far out on the adjacent uplands on all sides. 



As to the maximum depth of the molten diabase over the lowest parts 

 of the flooded lowland, no accurate figures can be given. An approxima- 

 tion based on the thickness of the remnants in the basin and of some out- 



XIX — Bull. Geol. Soc. Am., Vol. 20, 1908 



