356 Mr. W. J. Lewis on Forms of Glaucodote. 



The planes y are pitted and rough. Figs. 2 and 3 are repre- 

 sentations of simple crystals. 



The twins about (Oil) are a fresh illustration of the ten- 

 dency to twin about the face of a prism whose angle is near 

 60°. Fig. 4 is a representation of this twin, in which both mem- 

 bers are shown approximately in cequilibrio with the twin-axis 

 vertical. Fig. 5 represents somewhat closely the appearance of 

 a moderately large twin of this kind. The principal crystal, 

 denoted by Roman letters, is projected in the same way as in 

 fig. 1; and to it are attached two smaller crystals twinned about 

 (110), the one represented by Greek letters, the other by 

 barred Greek letters. The intersection of planes which corre- 

 spond is straight and definite, of planes which do not cor- 

 respond (as s and fi) is irregular and indefinite. The elements 

 obtained by measurements on this crystal differ slightly from 

 those given above, as is shown by the following Table: — 





Calculated. 



Found. 



D 



= 30° 14 J 





E 



50 2§ 





F 



55 10 





mm / 



69 40 



69° 38i 



s s / 



61 40 



61 341 



Is 



19 12§ 



19 14} 



m I 



64 2 



64 li 



ms 72 58J 72 53 



mit t 48 50£ 48 59 



One specimen in the British Museum consists of a triple twin, 

 resembling those of chrysoberyl, two of the members being 

 twinned on adjacent faces of the form {011} of the third. 

 Fig. 6 is an orthogonal projection on the plane (10 0) of this 

 twin, in which an attempt has been made to show the appear- 

 ance of the specimen and the relative magnitude of the mem- 

 bers. Fig. 6 a shows the simple crystal in the same projection. 

 Twins about the face of the prism {110} have been already 

 observed. These twins generally show the tendency to deve- 

 lop but slightly in the direction of the twin axis; and I was 

 fortunate enough to get a specimen showing this to a remark- 

 able degree. It is about the size of a penny-piece, and about 

 the thickness of the thick penny of George III. One member 

 is about half the width of the other, the remainder being 

 apparently untwinned without an increase of its thickness. 

 Close inspection, however, shows the existence of twin laminae 

 in this part. 



