158 THE MARQUETTE IKON BEARING DISTRICT. 



like the tuffs of modern volcanoes, which have been tilted from their 

 original position and have been rendered schistose by mashing, as have 

 also many of the dike masses that intrude them. They possess many of the 

 characteristics of dynamically metamorphosed tuffs, and others due to 

 weathering processes. 



Other varieties. — lu additiou to tlic apluuiitic schists and the banded schists 

 of this area, there are three other phases that should be mentioned. The 

 first phase strongly resembles schistose varieties of the dike rocks to which 

 the name "epidiorite" is often given. These rocks probably represent the 

 coarser lavas that were associated with tlie glassy and fine-grained lavas 

 and the tuffs that gave rise to the more common types of schist in the 



district. 



In the second phase the structure is plainly diabasic, but the quan- 

 tity of hornblende is so great that the rocks might well be called 

 amphibolites. In a rock from about 200 paces east of the NW. corner of 

 sec. 35, T. 48 N., R. 21 AV. (Atlas Sheet XXVII), for instance, the hornblende 

 is very abundant. It is a fibrous variety, consisting of long, almost color- 

 less needles or thin prisms scattered through a felt of greener fibers, the 

 mass forming pseudomorphs after diabasic augite. Feldspar is not al)un- 

 dant in the rock. That which is present is penetrated by needles of liorn- 

 bleude and spicules of chlorite to such an extent that its characteristic 

 features are often almo.st completely obscured. 



The third phase is more nearly like the true crystalline schists than 

 are any others of the greenstone-schists. This is the least common type 

 in the Mona schist area. It apj^ears to be confined to its northeastern 

 portion. In the hand specimen the rocks of this type are dark-green in 

 color, verv fine in grain, and extremely schistose. Under the microscope 

 they appear to be much fresher than the other green schists. They are 

 composed almost wIkiUv of long, narrow prisms and needles of light-green 

 hornblende, lying in a mass of tiny, clear grains of plagioclase, which 

 interlock in the manner of the grains of a crystalline schist. Intermingled 

 with these clear feldspars are a few larger grains of reddish, altered ones, 

 clouded by inclusions of epidote, kaolin, and sericite. On their edges 

 some of these seem to l)e passing into the clearer, fresher-looking feldspar, 



