118 



dicular on ш{10То} are far predominant over those parallel to 

 this surface. The most common twin-formation is after {ио}. 

 The crystals of type 4 are constructed about as the pre- 

 ceding ones; but the lamellae are rather unsharply bounded, 

 and are almost exclusively perpendicular on m{loTo}. 



Elpidite. 



This mineral is partly found in forms in no way differing 

 from those previously known ^), but partly also of an appearance 

 so peculiar, that it has only been possible, by more particular 

 examinations, to determine it as elpidite. 



The material seems to be considerably fresher than what 

 was known previously, and consists of yellowish white, needle- 

 shaped masses of a length of up to 2 cm and a breadth of 

 1 mm; some of the finer needles consist of a single crystal, 

 otherwise they consist of fibrous aggregates. Both kinds have 

 a strong silk-lustre , and cleave easily into quite exceedingly 

 fine needles; the most essential difference between the present 

 form and common elpidite is the fact that the needles, only 

 with the exception of the very finest ones, are quite stiff" and 

 inflexible, and easily penetrate into the skin. 



The single crystals are in the prismatic zone bounded by 

 a{lOO}, &{010}, m{lIO}, ?i[l20}, which are all highly striated 

 vertically. Terminal hmitation is wanting. A very strong cleavage 

 is found after ш{|10}. The specific gravity has been deter- 

 mined to 2-598. 



The optic axial angle is somewhat larger than that cal- 

 culated by Flink, being for yellow light 2F = 75° 12'. By 

 means of glass hemispheres with an index of refraction of 1'53 

 has been found, by several, very consistent measurements, on 



Described by Lindström: Geol. Foren. Förh. Stockholm, 16, 1894, 

 p. 330, by G. Nordenskiöld: ibid. p. 343, and by Flink: M. о. G. 

 24, 1901, p. 146. 



