202 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



Biotite occurs in larg-e irreg-ular masses wliicli are considered to be 

 primary, as well as in the scales which occur within secondary products of 

 the rock and are considered to be secondary. It is scattered throughout the 

 rocks in irregular pieces, usually associated with iron oxide. Where fairly 

 fresh, it is brown and shows its ordinary character. By weathering it 

 becomes green, having still a high double refraction. By further weath- 

 ering it passes into a nearly colorless mass that has the faintest tinge of 

 green and scarcely polarizes light. Such masses ai'e crossed by lines of 

 hair-like crystals, some of which intersect one another at angles of 60 

 degrees, extinguish parallel to their long directions, and show high single and 

 double refraction. These are taken to be rutile. Other crystals, somewhat 

 coarser, also lie irregularly in the biotite masses. They show the same inter- 

 sections as the rutile. They are very faintly greenish, have a high single and 

 double refraction, are positive in the long direction, and have a maximum 

 extinction angle of 46 degrees. These characters were not sufficient to 

 determine the mineral by, and no other characteristics could be observed. 



Ilmenite and titano-magnetite are in irregular grains. These minerals 

 are more or less altered to leucoxene or to sphene. Very frequently the 

 alteration product incloses bands of the iron oxide, which intersect one 

 another at angles of 60 degrees, pointing toward the hexagonal character 

 of the ore. In one case a beautiful example of the alteration of such an 

 ilmenite to rutile was observed. It is exactly similar to that described by 

 Williams in the case of the greenstones of the Menominee district.^ By 

 low power the mass has a semimetallic kister, and, as it seems to be almost 

 solid, has very much the general appearance of an ore, but by higher power 

 it is resolved into a mass of small golden-brown crystals. These frequently 

 intersect one another at angles approximating 60 degrees (120°), similar to 

 the fine needles in sagenite. 



The hornblende is mainly in large xenomorphic plates inclosing the 

 automorphic feldspars. This is the variety of hornblende known as uralite 

 and is all presumed to be of a secondary nature. In no case does it pos- 

 sess the compact nature of original hornblende, but is always more or less 

 fibrous, its fibrous nature being best seen along the edges, and less clearly 

 shown, though still observable, where the sections are thicker. It varies 



' A letter to Neues Jahrbueh,Vol. II, 1887, p. 263. The greenstone-schist areas of the Menominee- 

 and Marqnette resions of Michigan. Bull. U. S. Geol. Survey, No. 62, 1890, p. 99. 



