210 S. L. Pen field — Chalcopyrite from Chester Co., Pa. 



prisms which have been very symmetrically tapered by oscilla- 

 tions with the positive unit sphenoid. Measuring in the zone 

 between p, 111 and p' 111, starting from j9 there followed an 

 unbroken band of signals which continued for about 35°, that 

 is, from j?, 111 to a pyramid of the second order e, 101, after 

 which no reflection was obtained till p' was reached. From 

 this it may be assumed that there is probably no definite 

 sphenoid in this zone and that ^ which we have assumed as 

 122, 1-2, results from the oscillations of e, 101 with p 111. 

 Some of the accurate measurements between p, p' and c are as 

 follows : 







Times 

 measured. 



Limiting 

 measurements. 



Average. 



Calculated. 



J)aC 



111.. 001 



5 



54° 4'-54° 27' 



54° ]9' 



54° 20' 



p*p' 



111*111 



5 



70° 2-70° 22' 



70° 10' 



70° 8' 



Twin crystals are not rare, the twinning plane being always 

 a unit sphenoid. Two crystals with the habit shown in fig. 3, 

 if symmetrically twinned about 111, would appear as is fig. 6, 

 with one projecting through the other, while in reality all 

 which have been observed show a slight modification and 

 adaptation in that the shorter pole edge of one individual is a 

 continuation of the longer pole edge of the twinned crystal, 

 fig. 7. The striations and lettering are the same as in figs. 3 

 and 1. A still more interesting twin is represented in fig. 8, 

 where the principal crystal is a combination of a unit prism, m, 

 and a pyramid of the second order, e, equally developed. In 

 the upper, front, right hand octant three faces of a twinned in- 

 dividual occur, let into the principal crystal, as shown in the 

 figure, while on both individuals the basal planes are present. 

 To represent the ci*ystal 111 has been taken as the twinning 



plane. On the reverse side the crystals show no penetration, 

 and only three faces occur, one of which is a prism and two 

 pyramids of the second order, these are equally developed, are 

 striated parallel to their intersection with the unit sphenoid, 

 and have their angles slightly modified owing to this oscillatory 

 combination. These twins do not seem to be very rare and are 



