148 TRAVIS— PYRITE FROM CORNWALL. [June 22 



hedron zone was developed on one crystal, affording (221) and 

 (774), each of which was in a zone with (211) and an observed 

 diploid. Thus, — 



211 211 



212 747 



213 8.5.11 



This is not true of (552), which would agree with the diploid (741). 

 No diploids were observed between (321) and (210), (321) being 

 the last of the series. 



The development of the zone between the pyritohedron and a 

 non-adjacent (321) face, e. g., (210) (132) on crystal 24 is prob- 

 ably accidental, due to the irregular development of the crystal. 

 This zone affords (552) and (474). 



The writer wishes to call attention to the method described 

 above, used in determining the small faces (221) and (774). He 

 has not heard of its previous use as a practical method of meas- 

 urement. The angle p can be measured to within 5' or 10' with a 

 microscope, and this is accurate enough for all purposes if </> is not 

 large. The easiest way of holding the crystal on the stage is to set 

 it on a cone of wax on a glass slip ; the face chosen as a reference 

 face can be made approximately horizontal by hand. Having meas- 

 ured p, the crystal can be set up on an ordinary one-circle goniometer, 

 and, using the Websky signal, 4> i^i^y be measured by observing the 

 face directly and finding the point of maximum brilliancy of re- 

 flection. 



In concluding, the writer wishes to express his indebtedness to 

 Professor Amos P. Brown, not only for the specimens which fur- 

 nished the material for this paper, but also for valuable assistance 

 in its preparation. Thanks are also due to Mr. Kenneth Williams 

 for crystals of Type H used in the above series. 



