252 THE VERMILION IRON-BEARING DISTRICT. 



ones by systems of still smaller joints and parting planes having the same 

 trend as the larger fractures. After the formation of the joints movement 

 took place along the fracture planes, and as a result of this movement the 

 rhombs have been rubbed against one another and their angles have been 

 more or less completely rounded, so that the rhombs have acquired now a 

 more or less perfect oval outline. The long axes of the ovals agree, of 

 course, since the ovals were all produced by the same processes. The 

 rock is now strikingly like a conglomerate, and forms a fine example 

 of a pseudo-conglomerate. This pseudo-conglomerate may, nevertheless, 

 readily be distinguished from true conglomerates, such as occur in abundant 

 typical development at Vermilion Lake, and especially on the shores of 

 Stuntz Bay, by the fact that all of the pebbles of the pseudo-conglomerate 

 are of exactly the same kind of porphyry, and that the matrix between 

 the pebbles is merely a sheared form of the same porphyry. Moreover, 

 no indication of bedding whatsoever is found in this pseudo-conglomerate. 

 The true conglomerates contain pebbles of various porphyries, as well as 

 of the older greenstone and the iron-bearing formation, and gradations can 

 be followed from these conglomerates into the graywackes, and through 

 these into the overlying slates. The pseudo-conglomerate is most typically 

 developed at the location given above. It is seen in less typical develop- 

 ment on the point south of Mud Creek Bay and at some other localities on 

 Vermilion Lake. It is not so common, however, as one might be led to 

 suppose from previous descriptions of the Vermilion district." Far more 

 numerous are the exposures of the poi'phyry on which the fracturing is not 

 very distinct, and on which movement has not been sufficient to produce 

 the rounding of the rhombs and the pseudo-conglomeratic structure. These 

 pseudo-conglomerates were described by Smyth and Finlay* under the 

 name "conglomerate breccias," and the manner in which they were formed 

 was correctly interpreted. However, these authors unfortunately classed 

 in their conglomerate breccias the enormously and typically developed 

 sedimentary conglomerates that occur on the islands and shores at the east 

 and southeast side of Vermilion Lake, and in particular on Stuntz Bay of 

 that lake, where they are interbedded with and grade into the normal 

 finer-grained graywackes and slates. 



« The geological structure of the western part of the Vermilion Range, Minnesota, by H. L. Smyth, 

 and J. Ralph Finlay: Trans. Am. Inst. Min. Eng., Vol. XXV, 1895, pp. 610-613. 

 6 Op. cit., pp. 629-633. 



