560 WATSON AND WHERRY: PYROLUSITE FROM VIRGINIA 



the other hand the wedge-shaped ones are thicker because of 

 greater development of prism d or others near it. The height 

 also varies somewhat, the thinner crystals tending to be longer 

 in the c direction. The curvature of the faces which is almost 

 universally present is indicated; it is actually the more promi- 

 nent on the thicker, wedge-shaped crystals. Forms h, m, and 

 k are narrow or lacking on some crystals, and each of the three 

 pyramids has been observed but once, but the other forms, es- 

 pecially those at the right or positive end, are practically constant. 

 The domes fall excellently into harmonic series Ns, although 

 the two vicinal to the base, 018 and 014, are extra. 



Form 001 018 014 013 012 — Oil 032 021 — 010 



^ i i i I - 1 f 2 - inf. 



Ns — — I h f 1 t 2 3 inf. 



A list of the forms and their angles is presented in table 2, 

 new ones being marked by an asterisk. The formulas used for 

 calculating <f> and p are as follows: 



k k 



Prisms: cot (/)=-•«; domes: tan p =-• c; 



h I 



Pyramids: cot </> = -•«; tan p = — • 



k I cos 4> 



