PRE-CAMBRIAN NORTH AMERICAN LITERA TURK 747 



into rouniih' rhoniboidal 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 difference in the elasticity of the 

 two rocks. The slate inclusions readily yielded and finally took a per- 

 manent set under the deforming forces, while the harder and more 

 rigid jasper, in fragments of limited size and diverse orientation, 

 behaved like the enclosing 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 porphyrv inclusions. 



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

 south of this area the jasper succeeds the slates. The southern jasper 

 continues in a complex 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 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, (i) they occur for the most part in pitching 

 troughs with impervious basements. Usually this impervious basement 

 is one or more of the different varieties of the eruptive rocks. (2) 

 They are secondary concentrations produced by downward percolat- 

 ing waters, the silica being leached out and the iron ore deposited. 



Smyth, (H. L.),' describes a quartzite tongue in the jasper at Repub- 

 lic. This tongue branches from the main mass of quartzite, and after 

 continuing nearly parallel with it for a long distance, tapers to a point 

 toward the north in a mass of specular jasper. The quartzite tongue 

 includes between itself and the main quartzite a similar jasper tongue, 

 which starts in the north from the jasper, and tapers to a point toward 

 the south in the quartzite, the two tongues interlocking. These 



' The Quartzite Tongue at Republic, Michigan, by H. L. Smyth. Journal 

 OF Geology, Vol. II, 1894, pp. 680-691. 



