67 



Type III. (Crystals of short prismatic habit.) 



The epididymite crystals from Narsarsuk that belong to 

 the third type are. Ике those of the first type, developed prism- 

 atically parallel to the zone of the a-axis, but they differ from 

 the latter by their considerable thickness. Their thickness is 

 not infrequently as great as their length ; but rarely they 

 exceed И" in either dimension. 



The predominant zone, that of the «-axis, is on these 

 crystals less striated and less rich in vicinal planes than it is 

 on the epididymite crystals described above. The prism d (Oll) 

 is generally dominating (Fig. 8, Plate III); it is, as a rule, accom- 

 panied only by one prismatic form with a higher, and one with 

 a lower value of the c-axis. These forms usually are e (023/ 

 and f {02l}, but also h (038} and i (013J have been observed. 

 The second pinacoid, 6, is generally present; sometimes it is 

 tolerably broad. The third pinacoid (the base), on the contrary, 

 has hardly ever been found on simple crystals except as a 

 cleavage plane. 



The other well marked zone on these crystals is that of the 

 c-axis. The individuals generally have crystalline terminations 

 at both ends , almost always consisting only of the vertical 

 faces. Of these the prism и is nearly always predominant. On 

 simple crystals the fundamental prism, ш, is sometimes largely 

 developed. The first pinacoid, a, is always represented by 

 narrow faces; such is also the case with the prism I {^Ф^, j2ö 

 which is rather rare. The pyramid j> (31|} is, likewise, only 

 seldom observed on crystals belonging here. 



Fig. 8, Plate III shows a simple crystal of type III. Such 

 crystals are, however, of rare occurrence, twinning being much 

 more common among crystals of this type than among those 

 of the two foregoing types. The twinning follows the usual 

 law; real penetration twins are not, however, likely to be met 

 with. The normal simple crystal is divided into two halves by a 



5* 



