276 



G. H. Williams — Crystals of Pyroxene. 



group consists of six simple crystals of the usual tabular habit 

 and of two larger ones, which are at the same time twins and 

 hemimorphic. The 'largest of these crystals measures 3x8£ cm , 

 and is represented in figures 1 and 2. The upper portion 



belongs to a single individual, having the usual habit and 

 showing the usual forms: OP (c), — P (u), P (s), 2P (o), co P (m) r 

 ooP'oo (b) and ooPoc (a). Below, however, toward the front, 

 we have only the forms : 2P (o) and P oo (p), indicating that the 

 crystal is hemimorphic in the direction of the vertical axis, as 

 first noticed by Des Cloizeaux.* The lower back quarter of 

 the crystal is moreover exactly like the lower front quarter, 

 but in a reversed position, so that the lower half of the indi- 

 vidual is a twin as represented by vom Rath, while the upper 

 half is apparently simple and of the usual habit ! The bound- 

 ary of the quarter in twinning position is represented in the 

 figure by the light line. 



The second, slightly smaller crystal (2x2J cm ) is essentially 

 the same as the one just described, though it shows the twin- 

 ning structure even more plainly, as may be seen from figure 3. 



In all of these crystals the superficial alteration to hornblende 

 has progressed so far as to render the planes unfit for exact 

 measurements. The angles were, however, determined by a 

 contact goniometer with sufficient accuracy to identify the 

 symbols above given. The striation parallel to the basal pina- 

 coid, so common in all pyroxene crystals from the crystalline 

 limestones of New York, is here very distinctly marked. 



Petrographical Laboratory, Johns Hopkins University, 

 Baltimore, McL, April, 1887. 



* It is perhaps doubtful whether a crystal can properly be spoken of as hemi- 

 morphic in any direction except that of an axis of symmetry (cf. Groth), but this 

 case is so analogous that it is difficult to describe it by any other term. 



