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THK CHICAGO ACADEMY OF SCIENCES. 



PI. VII, fig. I. Neither the crypto-crystalline nor the phanero- 

 crystalline variety is peculiar to any one source, each being 

 found both in the drift and in the native limestone. The larg- 

 est crystals found by the writer on the lake shore are in a piece 

 of amygdoloidal melaphyre, characteristic of the Michigan 

 copper region. They are half an inch high and three-eighths 

 of an inch in diameter, bounded by the common striated 

 prisms, capped by positive and negative rhombohedral planes. 



The alternate rhombohedrons are tarnished with a brown 

 coating, probably of an iron oxide. The specimens in chert 

 nodules are also terminated with the positive .and negative 

 rhombohedrons and in many of these crystals under examina- 

 tion the rhombohedrons are so evenly developed as to appear 

 to be hexagonal pyramids. This is illustrated in the specimen 

 at the right hand upper corner of the photograph PL VII, fig. 

 2 and shown in the drawing Fig. 10. Prisms with double ter- 

 minations occur more commonly in the limestone than in the 

 drift material, see Fig. u. 



Since the crystals line the wall of cavities and their long 

 axes are perpendicular to the base on which the crystal rests, 

 the c axes form all possible angles with each other, and the 



Fig. 10. 



Fig. n. 



Fig. 12. 



resulting groups are a confusion of interfering crystals. The 

 direction of the c axis can be distinguished by the predomi 

 nant striations transverse to this direction. Fig. 12 represents 

 the striations on a prism face with striations showing the di- 

 rections of rhombohedrons which may be distinguished in as 

 much as the latter forms reentering angles with each other. The 

 small quartz crystals lining the geode in PI. VII, fig. 2 are clear, 

 have sharp edges and well formed planes, and show the polar- 

 ization colors due to the refraction of light. The cryptocrys- 

 talline specimens are most commonly white to grey in color 

 and translucent to opaque. They are represented by several 



