324 Hidden and Pratt — Twinned, Crystals of Zircon. 



this form with the unit pyramid. Rarely the crystals have the 

 ditetragonal pyramid x, 311, in combination with the unit 

 pyramid. Fig. 1 represents one of a rarer type of crystals 

 which is a combination of the prisms of both orders, m, 110 

 and a, 100, terminated by the unit pyramid, p, 111 and the 

 more acute pyramid u, 331. 



U^h 



The only type of twinning heretofore known for zircon* is 

 that with e, 101 as twinning plane, forming geniculations like 

 those of rutile and cassiterite. The same law of twinning 

 was observed on the Henderson County zircons, but the crys- 

 tals are cruciform as shown in fig. 2. 



*This Journal, xxi, 507, 1881, Hidden; Phil. Mag., xii, 26, 1881, Fletcher ,- 

 and Dana's Mineralogy, sixth edition, 1892, p. 486. 



