112 THE VERMILION IRON-BEARING DISTRICT. 



both basic and acid, are frequently included fragments from both the 

 slate and iron formations, from those of small size to masses more than 

 100 feet long. 



The conglomerate breccias are of djaiamic origin. The first step in 

 the development of the breccias was the formation of two intersecting sets 

 of planes of .fracture, dividing the originally massive rocks into roughly 

 rhomboidal blocks. Their further development depended on continued 

 movement between these blocks under pressure, which resulted in enlarging 

 the shearing zones at the surfaces of contact, and rounding the angles. 

 The slate and jasper inclusions originally plucked off from the rocks which 

 the porphyries and greenstones invaded shared, of course, the subsequent 

 history of their captors. The fact that the jasper inclusions are frequently 

 rounded, while those of slate are not, is explained by the diflFerence in the 

 elasticity of the two rocks. The slate inclusions readily yielded and finally 

 took a permanent set under the deforming forces, while the harder and 

 more rigid jasp,er, in fragments of limited size and diverse orientation, 

 behaved like the inclosing porphyry. The boundaries of the inclusions 

 were generally the surfaces along which rupture took place, although, as 

 has already been said, jasper in a few instances is found partly held in 

 porphyry inclusions. 



As to structure, the main slate area is anticlinal; both north and south 

 of this area the jasper succeeds the slates. The southern ja^sper continues in 

 a comj)lex syncline, and south of this is found the northern limb of another 

 anticline of slates, the southern limb not being exposed. Still farther south 

 is the jasper of Lee and Tower hills, which appears to form the southern 

 and western edges of a complex syncline. All of these folds pitch toward 

 the east. 



The ore deposits, a number of figures of which are given, as well as 

 many details concerning them, are found to conform in occurrence to the 

 laws worked out by Van Hise in reference to other districts of the Lake 

 Superior region; that is, (1) they occur for the most part in pitching 

 troughs having impervious basements, the basement being usually one or 

 more of the different varieties of the eruptive rocks ; (2) they are secondary 

 concentrations produced by downward-percolating waters, the silica being 

 leached out and the iron deposited. 



