I 



354 THE PENOKEE IRON BEARING SERIES. 



consolidation of the rock has occurred. This growth is much better iUus- 

 trated by the diabases of tlie eastern area and its description is deferred to 

 chapter viii. 



The series of aUerations has not ended in the chang-e from pyroxene 

 to amphibole. The secondary amphibole has to quite an extent ahered to 

 biotite. The biotite is as clearly secondary to amphibole as the amphibole 

 is to augite. In certain rocks in the Northwest^ the alteration of horn- 

 blende to biotite occurs, definite crystallographic relations obtaining 

 between the two; but in the Penokee rocks the alteration of each of the 

 amphibole individuals has resulted in the formation, not of a single biotite, 

 but in many small folia. All of the biotite is, however, not derived from 

 the amphibole, for a considerable quantity of that mineral is found sur- 

 rounding the magnetite areas, and, when the latter occurs in grate-like 

 forms, tilling the spaces between the bars. This biotite is also plainly 

 secondary and is in the nature of a reaction mineral, the magnetite furnish- 

 ing the oxide of iron necessary for its formation, while the surrounding 

 minerals have furnished the remaining constituents. Magnetite afeas 

 inclosed wholly within feldspar are in some cases surrounded by guch 

 secondary biotite, and it would then seem that the plagioclase has largely 

 furnished the material for the formation of the biotite. 



All the minerals of the diabases have now been considered, except the 

 rare rhombic })yroxene. This has its usual line fibrous rectangular cleav- 

 age, and cliaracteristically includes very numerous minute black jmrticles 

 along its parting. It is somewhat feebly pleochroic and is probably 

 bronzite or hypersthene. It has a tendency to occur in idiomorphic 

 six-sided forms, showing apparently two macropinacoids and the four 

 planes of the domes. It is quite frequently suri-ounded by reaction- 

 ary or intergrowth rings, which consist in part of minute, brilliantly polariz- 

 ing particles. The rhombic pyroxene is in strong contrast with the more 

 abundant colorless or pale brown augite. The augite has no black inclu- 

 sions, does not ordinarily show rectangular extinction, and has a coarsely 

 prismatic, nearly rectangular cleavage, instead of a iinely fibrous I'ectan- 

 gular one. 



