On Crystallographical Forms of Glaucodote. 355 



prism, calculation gives the angle (1 1 0, 1 6 0) =27° 56^, 

 which agrees very well with that obtained by measurement, 

 27° 54J'. 



The elements and angles given below have been taken from 

 measurements obtained on two good twin-crystals, each about 

 the size of a cherry. 



System prismatic : 



D = (010, 011) = 30° 12£'; E = (001, 101) = 50°1'; 



F = (10 0, 110) = 55° 14', 



a:5:c=l-4406: 1:1-71784. 



Forms observed: 



MO 1 0}, ro{l 1 0}, y(20 1}, 1{1 1}, s{l 2}, n{0 1 1}, 

 o{lll},w{122},p{160}. 





Calculated. 



Found. 



mm, 







= 69 



32 



69 32 



11, 



100 



2 



100 1 



Is 



19 



12| 



19 



h 



17 



14 



17 13 



yy< 



134 



30 



134 31J 



h/ 



62 



44 



62 43| 



my 



58 



16i 



58 15 



ml 



64 



H 



64 4J 



nw 



16 



42 



16 43J 



wiv, 



33 



24 



33 27 



010 



14 



15f 



14 7 



mn 



44 



46 





n I 



71 



H 





mfx, 



89 



32 



89 36 



m\ 



26 



221 



26 32 



The observed angles agree fairly well with those obtained by 

 calculation, with the exception of / s and m X. The observa- 

 tion of the latter was one of no great weight ; but the former 

 was repeatedly measured with great care. As moreover an 

 increase of W in the angle E involves an increase in Is of J' 

 only, I have been obliged to regard the discrepancy as due to 

 a distortion of the face 5 in the crystal on which the angle was 

 measured. This face s, though large, is generally one of the 

 worst on the crystal. The plane n is but poorly developed, 

 and therefore does not serve for a direct determination of the 

 element D. The plane b is deeply striated parallel to its inter- 

 section with m ; the planes I and s are striated parallel to their 

 intersections with one another, s being much the rougher. 



2A2 



