ON THE NORTHWEST SHORE OF LAKE SUPERIOR. 405 



portion of them do not, I think it most probable that the dike, as it now exists, is 

 due to two eruptions, the fissures having been reopened after the first filling had 

 become partially or wholly cooled and consolidated. I could discover, however, no 

 difference in the composition of the rock on different sides of the seam. 



ENCLOSED DIKE OF COLUMNAR BASALT, LAKE SUPERIOR. 



21. Pigeon River. — Between the mouth of Pigeon River and the first fails, the 

 only rocks exposed are alternations of siliceous and argillaceous slates, which are 

 seen in the banks of the river, about half a mile above the old Mission. The falls 

 are about a mile and a half above the mouth, following the windings of the stream. 

 The river, which is about twenty-five yards wide above the fall, descends perpendi- 

 cularly sixty feet. Immediately below is a deep gorge, which soon contracts to 

 fifteen or twenty feet in width, with mural walls, between which the stream chafes 

 its way for the distance of two hundred yards. The sketch on the next page, by 

 Major Owen, taken from a point a short distance below the fall, exhibits the slate- 

 beds at the upper end of the gorge, with the overlying trap. 



The rock over which the water falls is a compact greenstone (No. 1), and is a, 

 continuation of the dike which passes back of Grand Portage Bay. On the lower 

 side of the dike, the slate through which it cuts is altered for some distance. 

 About thirty-six feet from the trap, it is but slightly changed, except where 

 it is intercalated with a bed of basaltic rock, when it assumes something of a trap- 

 pous character (No. 2). Twenty-five feet from the dike, it is still more altered 

 (No. 4), and in still nearer proximity, some of the beds are converted into a 

 schistose quartz-rock. The width of the dike is two hundred and twelve feet ,: 

 and its height above the fall, in the direction of Grand Portage Bay, is one hun- 

 dred and ninety-four feet. It bears north 45° east. The slates, which, with 



