402 THE VERMILION IRON-BEARING DISTRICT. 



by Bayley." The gabbro, as a rule, was found to be a medium- to coarse- 

 grained rock, with essentially the same granular texture throughout. 

 However, near the contact of the gabbro with the other rocks to the 

 north of it, it is found to grow much finer grained. This gradation 

 is rapid. Thus the gradation from a medium fine-grained to a normal 

 coarse-grained rock was completed within a distance of about 10 paces. 

 Occasionally large areas of the fine-grained phase of rock which has been 

 called granulitic gabbro occur in the midst of the main gabbro mass. This 

 fine-grained material is found to have very sharp contacts'' with the coarser- 

 grained gabbro, and small areas of this fine-grained material are also 

 included in rounded as well as irregular masses within a coarse-grained 

 gabbro, possibly indicating that there is a slight difference in age between 

 the two. This fine-grained gabbro at the point referred to has a remarkable 

 horizontal jointing, as the result of which it looks at a short distance like a 

 bedded rock in layers of from 2 to 6 inches thick. It also has a sheeted 

 structure striking in a general way east and west and dipping to the south. 

 This structure is brought out by the differential weathering. In some 

 cases also the gabbro has a very distinctly banded structure, as has been 

 described by Grant." He describes an exposure near south end of Bald 

 Eagle Lake as having a gneissic structure which is practically vertical and 

 runs N. 15° W., making the rock break more readily in this direction than 

 in any other. "In some places the gabbro lies in horizontal beds from 

 2 to 4 inches thick. The rock seems to be almost entirely composed of a 

 feldspar (probably labradorite) and a mineral which is probably olivine; 

 this, when not decayed, is of a yellowish green color." Microscopic studies 

 show this mineral to be olivine. This occurrence is very similar to the 

 banded faces of the gabbro which may be seen upon the St. Paul and 

 Duluth Railroad between Short Line Park and Smithville, and also upon 

 the shore of Lake Superior between Split Rock Bay and Beaver Bay. 



A number of varieties of the gabbro were seen upon Little Saganaga 

 Lake. The gabbro varies from the very coarse-grained varieties to forms 



"The basic massive rocks of Lake Superior, by W. S. Bayley: Geol. and Nat. Hist. Survey of 

 Minnesota, Final Kept., Vol. I, 1884, pp. 688-716; Vol. II, 1888, pp. 819-825; Vol. Ill, 1895, pp. 1-20. 



6 Grant, TJ. S. Geol. and Nat. Hist. Survey of Minnesota, Final Rept., Vol. IV, 1899, p. 447, 

 PI. MM, figs. 5 and 6, and p. 489, fig. 89. 



«Geol. and Nat. Hist. Survey of Minnesota, Seventeenth Ann. Rept., 1888, p. 164. 



