THE KEWEENAWAN. 401 



tially parallel to the bedding of the sediments, although occasionally they 

 are found cutting aci'oss the beds at low angles. Erosion has been most 

 active in this portion of the district in a direction parallel to the strike of 

 the beds, and consequently most of the large valleys trend in agreement 

 with these, approximately east and west. The resistant sills now form the 

 caps of the ridges, the slates having been removed down to the sills. The 

 massive rock forming the sills breaks off along the joint planes, and this 

 breaking results in forming perpendicular cliffs, below the foot of which 

 talus from the sills and from the easily weathering Rove slates give a 

 gentle slope. These sills are sometimes very nearly concealed by the 

 accumulated talus derived from them. 



The effects of erosion have produced a series of hills with very nearly 

 vertical north escarpments, and a gentle slope from the crest of the hills to 

 the south. This slope corresponds very closely to the dips of the Rove 

 slates and the upper surface of the dolerite sills. Fig. 22 shows a some- 

 what idealized section through the ridge on the south side of Gunflint 

 Lake, taken from W. M. Chauvenet's manuscript notes. 



PETROGRAPHIC CHARACTERS OF THE GABBRO. 



Macrosco])ic characters. — It is not the purpose of this report to consider 

 in detail other rocks than those of pre-Keweenawan age which make up 

 the Vermilion district in its sti'ict sense. In order, however, to give a 

 complete description of the area shown on the maps in the accompanying 

 atlas, it is essential to consider at least briefly tlie Duluth gabbro. Speci- 

 mens have been taken here and there along its margin, and several trips 

 have been made well down into the gabbro, during which specimens were 

 collected of the varieties seen and observations made on their relations. 

 Tlie following brief descrijotion of the gabbro is the result chiefly of the study 

 of these specimens. No attempt has been made to obtain specimens from all 

 parts of the gabbro, and consequently numerous facies which would be seen 

 only after very detailed studies of the gabbro have, of course, not been 

 found. For more detailed descriptions of this gabbro the reader is referred 

 to the reports of the Minnesota survey, especially to the articles by 

 Elftman", Grant, ** and WinchelV and to the petrographic study of the gabbro 



«Am. Geologist, Vol. XXII, 1898, pp. 131-149. 



6Geol. and Nat. Hist. Survey of Minnesota, Final Kept., Vol. IV, 1899, p. 476 et seq. ; Twenty- 

 third Ann. Kept., 1894, pp. 224-230. 



(■Am. Geologist, Vol. XXVI, 1900, pp. 1.51-188, 197-245, 261-306, 348-388. 



MON XLV— 03 26 



