216 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



cleavage. As this alteration i^rogresses there is a Hghteuiug- of the color of 

 the biotite, and, as a consequence of this the whole cause of the metallic 

 luster and the partial cause of the color of the biotite is disclosed. In the 

 lighter biotite one by careful examination can see innumerable small plates 

 of a brown or smoky color. At first sight they remind one strongly of the 

 inclusions so common in many hypersthenes. Closer examination only 

 emphasizes this resemblance, and they are believed to be micaceous ilmenite 

 plates. These inclusions were studied by means of an oil immersion 

 objective givmg a magnification of about 1,250, and were found to have 

 mainly a roundish or hexagonal outline. In addition to these, some plates 

 of long, irregular form were observed. These are all isotropic and non- 

 pleochroic. These minute plates lie parallel to the biotite lamellae. The 

 consequence of this is that in sections parallel to c one sees, for the most 

 part, only short black streaks — the edges of the plates — whereas in the basal 

 sections of the biotite one can determine the irregular or rounded contours 

 of the plates. The plates are too small to allow the metallic luster to be 

 seen on an isolated one. En masse they produce a very decided blue 

 metallic slnmmer, as seen in some of the biotite fragments. 



Numerous apatite crystals occur. They are usuallj^ clear white, but 

 one crystal was seen showing a diclu*oisixi from faintest brownish for rays 

 perpendicular to crystallographic c to light smoky brown for rays jDarallel 

 thereto. This crystal contains a core of brown glass. 



Some of the iron oxide is in roughly rectangular masses, and appears 

 to be magnetite. This is associated with an iron oxide, which occurs in 

 opaque, ragged masses formed of long, irregular, and knotty stringers. 

 These at places are parallel to one another and at other places cut one 

 another at various angles, and at still other places meet at a common cen- 

 ter, forming an opaque mass of varying dimensions, but usually small. 

 Now and then one of the large magnetite masses constitutes a center from 

 which extend the knotty, irregular stringers. The general appearaiice of 

 these ragged masses is that described by Grerman petrographers as serhackt. 



When these stringers pierce the section at an oblique angle, the ends 

 are translucent, with a brown color, becoming more opaque as the section 

 gets thicker. Such masses have all the appearance of ilmenite, and are 

 believed to be that mineral. Similar ilmenite stringers are included in the 

 chlorite, which i-esults from the alteration of the biotite 



