42 THE CHICAGO ACADEMY OF SCIENCES. 



green which are predominant colors, are due to polarization 

 phenomena dependent upon the lamellar structure. The pearl 

 grey yellow and red colors are are due to clouds of the finest 

 microlites of ilmenite, hematite, pyroxene, etc. the color vary- 

 ing not only as the species of the microlites varies, but also as 

 their positions vary in regard to the vertical axis of the crystal. 

 (See Hintze, 1. c., Vol. II, p. 1,512.) The containing rock is a 

 gabbro or diorite. Labradorite splinters melt more easily than 

 those of oligoclase. Powdered labradorite decomposes in warm 

 hydrochloric acid. Labradorite easily metamorphoses to epi- 

 dote. In a specimen examined, the change has begun to color 

 a large crystal with a greenish hue, and has given the specimen 

 an oily lustre, so that it was mistaken for a specimen of neph- 

 elite. 



The PYROXENE and AMPHIBOLE Groups are represented by 

 both orthorhombic and monoclinic species, but as in the case of 

 the feldspars, all the material at hand has been collected from 

 the igneous drift rocks of the region, and the study and deter- 

 mination of the species is carried on chiefly by microscopical 

 and chemical means, rather than by macroscopical and phys- 

 ical. 



Hypersthene. (FeMg)SiO 3 Orthorhombic. 



The same gabbro in which labradorite is found contains 

 small brown crystals of hypersthene which show perfect cleav- 

 age parallel to the brachypinacoid (oio), and distinct cleavage 

 parallel to the prisms (no) and the macropinacoid (100). The 

 fracture is uneven, the tenacity brittle, the hardness 5 to 6, 

 specific gravity 3.4 to 3.5. The cleavage surface shows a 

 pearly or metallic luster. The color is a dark brownish green 

 or greenish black. The streak greyish to brown. Translu- 

 cent to opaque. Under the microscope and in polarized light 

 which vibrates parallel to the a axis the mineral appears 

 brownish red, while if the light vibrates parallel to the b axis, 

 the mineral appears reddish yellow and if parallel to the c axis 

 the mineral is green. The metal-like luster of the mineral is 

 caused by minute tabular scales of metallic enclosures, such as 

 gothite, hematite, brookite, arranged parallel to the basal 

 plane. Before the blow pipe, fuses, yielding a magnetic mass. 

 It is partly decomposed by hydrochloric acid. 



