Mr. TV. J. Lewis's CrystattograpMc Notes. 121 



course of the long vacation to be able to settle definitely the 

 crystallography of the mineral. 



Idocrase. — The Brooke collection contains a small crystal of 

 idocrase, apparently from Zermatt, which has minute planes 

 adjoining the base which do not seem to have been hitherto 

 noticed. They are striated, and the measurements obtained 

 are not good. Using p and^, to denote two of the planes in 

 a zone with c. and p t , one of the others, the measurements 

 obtained were cp- 6° 3' (best), cp y =6 19', an&pp„=9° 26'. 

 The form (117) is that which agrees best with these measure- 

 ments; the angles required by it being (0 01. 1 1 7) = 6° lOi 7 , 

 and(117,lT7)=8°42i / . 



Zoisite. — In the Cambridge collection is a small specimen of 

 zoisite in small bright green crystals imbedded in calcite. 

 One of them had terminal planes ; and by careful extraction 

 from the calcite. I obtained a crystal showing four terminal 

 pyramidal faces. The faces were rough, and deeply striated 

 parallel to the edge lying in the braehydiagonal plane, so that 

 the measurements obtained were not good. They agree, how- 

 ever, sufficiently well with those given by Brogger (Groth's 

 Zeitschrift, iii. p. 471); and they have led me to the conclu- 

 sion that Brooke must have been mistaken in giving the angle 

 w k =56° 30', an angle which has been assumed in all the 

 mineralogical works to give the elements of the crystal. 

 DesCloizeaux observed a poor dome-plane, of which I seemed 

 to have doubtful indications, and has given a table of angles 

 calculated from Brooke's data. The positions of the faces are 

 shown in the stereographic projection (fig. 3). The angles 

 given by M. DesCloizeaux are compared in the following table 

 with those of Brogger and with those observed and calculated 

 bv me. 



r me. 



Calculated. 



Means of 

 observed 

 angles. 





Angles 



adopted bv 



Brogger. 



Angles cal- 

 evdated bv 

 Dx. * 



bs 

 s s. 



*58 8 

 14 36 

 63 44 



58 9i 

 14 36 

 63 35 





*58 17 



63 26 



*58 8 

 63 44 



bw *73 9 



~sic 56 42 

 jo 10 j j 66 36 



73 10i 



73 19 (Miller) 



56 52i 

 66 14" 



73 9 



56 321 

 66 55 



72 28* 



55 13i 

 69 33" 



~aw 



62 121 

 ; 55 35 



62 54 

 55 5 





55 57 



61 1 

 57 58 



s y w 

 w h 



75 56 



57 541 



58 19 



75 35 (Miller) 

 60° 'near (Dx.) 



75 43 



75 23 



*56 30 



61 3 



