42 THE MINERALS OF TASMANIA 



Eeferring to the same mineral from the Comet Mine, Dun- 

 das, " The crystals which occur on a matrix of galena and 

 powdery limonite are thin tabular on b and twinned on 

 m; the figured crystal is a trilling resembling the cerus- 

 site of the Magnet Mine. The two crystals twinned to 

 that in the conventional position are small in comparison, 

 and scarcely penetrate the larger." " At the Magnet 

 Mine, Tasmania, cerussite occurs in two different habits, 

 long prismatic or tabular on the b (010) pinacoid, and as 

 flat tables parallel to the basal plane. In both cases the 

 crystals are twinned on the faces 7?i (110) and m 111, 

 (110) resulting in trillings of pseudo-hexagonal form. A 

 specimen in the museum collection furnished crystals of 

 the first habit, while Mr. W. F. Petterd obligingly lent 

 some examples of the other. An interesting feature is 

 that the flat pseudo-hexagonal tables of the second habit 

 are invariably contaminated with chromate of lead, doubt- 

 less in the form of crocoisite, which imparts to them a 

 canary-yellow colour with occasional patches of red. 

 The two crystals measured were essentially similar, being 

 elongated along the vertical axis and tabular on the 

 b (010) pinacoid. The same forms are present in both, 

 namely c (001), a (100), b (010), m (110), r (130), t (021), 

 X (102), and p (HI)- In the figure the breadth along the 

 a axis is somewhat exaggerated, and the three individuals 

 are drawn in equi-poise, though really only one is well- 

 formed, the other two being quite subordinate. All the 

 forms except b are relatively narrow, and the prism zone 

 is much striated and interrupted. Of the three individ- 

 uals forming the trillings, I. is placed in the conventional 

 position, while II. and III. are twinned on the faces (110) 

 and (110) rsspectively of I. Thus the faces m and p are 

 coplanar with /n and j^, while m and p are coplanar with 

 m^ and p^, and similarly at the other end of the a 

 axis of I., but II. and III. have only one coplanar face, 

 namely the base c. The figure is similar to the well-known 

 drawing by Schrauf, but the Magnet mineral has three 

 more forms. The table of angles below gives the measured 

 and calculated values for I., and also the observed angles 

 belonging to forms on II. and III., as owing to the small 

 size and imperfect development of the crystals, on the 

 goniometer it was impossible to distinguish the reflections 

 belonging to the several individuals, and it was mainly 

 from the angular measurements that the twinning struc- 

 ture was deduced. Habit II. — The crystals with this 

 habit differ from the others mainly in having a large basal 



