Il6 THE JOURNAL OF GEOLOGY. 



tances. Whether this denudation extended everywhere deep 

 enough to remove all surface volcanic material, and to leave only 

 deep seated igneous material, is undetermined. At the beginning 

 of the Lower Huronian time, the Basement Complex was, in the 

 Lake Superior region, a universal system. 



The Lower Huro7iian. — After the forces of erosion had nearly 

 exhausted themselves, there was the first advance of the sea over 

 the Lake Superior region of which we have any evidence, as a 

 result of which the Lower Huronian was deposited. 



The well-known characteristic rocks of the Lower Huronian, 

 are (i) conglomerates, quartzites, quartz -schists and mica- 

 schists, (2) limestones, (3) various ferruginous schists, (4) basic 

 and acid eruptives, which occur both as deep seated and as effu- 

 sive rocks. The order given, with the exception of the erup- 

 tives, is the order of age from the base upward. 



The inferior formation is usually a quartzite or a feldspathic 

 quartzite. Where metamorphism has been severe it passes into a 

 quartz -schist, mica -schist or gneiss. The lowest horizon of 

 the formation is in places a coarse conglomerate, and this when 

 metamorphosed may become a conglomerate -schist. This con- 

 glomerate is of two types, depending upon the character of the 

 underlying formation, which is here granitic and there schistic. 

 The limestone formation, when at its maximum, is of very con- 

 siderable thickness. The limestone is magnesian and so very 

 crystalline as to make the name marble appropriate. It fre- 

 quently contains a considerable amount of chert. In places it 

 may be divided into two horizons, one of which is nearly pure 

 marble, and the other nearly pure chert. At other times the 

 limestone becomes very siliceous by a mingling of fragmental 

 quartz, while zones of wholly fragmental material may occur. 

 These impure phases are often at the lower part of the limestones, 

 where they may be considered as a transition from the under- 

 lying formation. The formation overlying the limestone is 

 usually known as the iron -bearing member, since it contains all 

 the ore bodies of the Lower Huronian. It has varied aspects, but 

 the different varieties grade into one another both vertically and 



