394 THE VERMILION IRON-BEARING DISTRICT. 



Point, Minnesota, certain gabbroic intrusives are known to have had a very 

 far-reaching contact effect on these sediments." Along the southern and 

 southeastern shores of Loon Lake were collected several specimens of 

 sediments which were near, although not in actual contact with, the sills. 

 One of these specimens shows a spotted character and is a spilosite such 

 as is fairly common in sediments near the contact with the great mass of 

 gabbro and occurs also in other districts near great dolerite dikes. This 

 spilosite contains a large amount of chlorite in clumps embedded in a 

 matrix of quartz and presumably some feldspar, and forms the microscopic 

 spots. In the Mesabi range some of the slates near the gabbro contact 

 show clearly recognizable cordierite, forming the white spots, and the slates 

 have been metamorphosed to a cordierite-hornstone.'' In general the slate 

 adjacent to thege sills in the Vermilion district shows its normal characters 

 with at most a little metanaorphism due to cementation. 



A contact of the gabbro with the Rove formation at a point at the 

 southwest end of Loon Lake was examined. This contact is of the gabbro 

 on the "graywacke slate member" of Grrant. The sediments at the top 

 of the section were within about 3 feet of the gabbro. This is as near as 

 we found the sediments to the gabbro. Here the rocks are interbanded 

 slates and graywackes which were quite crystalline and hard. Microscopic 

 examination of them shows that the gabbro had effected a partial recrys- 

 tallization of the sediments and discloses in the sediments a large amount of 

 secondary biotite and muscovite. Both of these occur in relatively large 

 porphyritic plates inclosing grains of the other materials constituting the 

 slate, recognizable quartz, and ferruginous material. As the rocks are 

 studied, as we go down the slope, they are seen to be less indui-ated, until 

 near the bottom of the section at the water's edge, about 50 feet below the 

 gabbro, the sediments do not appear essentially different from the ordinary 

 rocks of this character and of this age. It is clear from this that the effect 

 of the gabbro has not been felt at a very great distance from the actual 

 plane of contact with the sediments. 



■ «0n some peculiarly spotted rocks from Pigeon Point, Minnesota, by W. S. Bay ley: Am. Jour. 

 Sci., 3d series. Vol. XXXV, 1888, pp. 388-393. Abstract, Nature, Vol. XXXVII, 1888, p. 91 (5 lines). 

 Rocks on Pigeon Point, Minnesota, and their contact phenomena: Bull. U. S. Geol. Survey No. 109, 

 1893, pp. 121. 



6 The Mesabi iron-bearing district of Minnesota, by C. K. Leith: Mon. U. S. Geol. Survey Vol. 

 XLIII, 1903, pp. 171-172. 



