118 



G. H. Williams — Hemihedrism in 



(133); s, +P (in); o, + 2P (221); X, +3P (331); y, +|P (553); 

 c, OP(OOl); e, Pbo (Oil); s, 2P^o (021); o-\¥ (112); u, 

 — P(lll); and v, — 2P (221). At the opposite extremity occur 

 only the three forms u, 0, and L The source of this remarka- 

 ble crystal is not definitely given. It agrees in all respects 

 with the well-known diopsides from Mussa Alp, except in the 

 matter of attachment. 



Inquiry has shown that inclined-faced hemihedrism is not 

 altogether rare among the yellowish white pyroxene crystals 

 from near Warwick, in Orange Co., N. Y. These are usually 

 tabular, parallel to the base, and occur either singly or in 

 groups, imbedded in crystalline limestone. A figure of one 

 of these crystals given by DesCloizeaux,* shows above the 



forms c, s, 0, e and /i, — 2P2 (211). At the lower end there 

 are present c, u and a, +fPs (31 2). 



e Fig. 6, taken from my above-men- 



tioned ".Note," shows 'a crystal with 

 the usual combination above 0, s, and 

 -w, while below are the two halves of a 

 crystal in twinning position showing 

 only the two forms o and p. The crys- 

 tal was carefully tested for pyro-electric- 

 ity, but no trace of this property could 

 be discovered even with a delicate Thom- 

 son electrometer. 

 For permission to use the following figure, 7, I am in- 

 7 indebted to the U. S. Geological Survey, and also 



to Professor S. L. Penfield, of New Haven, by 

 whom it was drawn. It represents a perfectly 

 formed pyroxene crystal, 3£ inches in length, 

 from Grassy Lake, near Rossie, St. Lawrence Co., 

 1ST. Y., upon which the hemihedral distribution 

 of planes is very apparent. At the upper end 

 occur the planes u, very large, s and 0. At the 

 lower end are the forms p, c, u, small ; s and 0. 

 In the prismatic zone are the pinacoids a. b and 

 the prism m. This crystal was probably formed 

 in calcite, as it is doubly terminated. It was 

 collected by Professor Penfield during the summer of 1888, 

 and is now deposited in the U. S. National Museum at Wash- 

 ington. 



Prof. E. S. Dana has called my attention to the fact that 

 certain of the tabular white pyroxene crystals occurring in the 

 dolomite at Canaan, Conn., also show different forms at oppo- 

 site extremities of the vertical axis. Indications of such a de- 



* Manuel de Mineralogie, vol. i, p. 54. Atlas, fig. 58, 1862. 



