268 RECORDS OF THE AUSTRALIAN MUSEUM. 



law, or the segment on the left of the figure with faces labelled 

 b, c, 0, X, may be regarded as twinned to the segment in the 

 normal position on the Manebacli law ; it is not possible 

 without very exact measurement to decide between these alter- 

 natives. This specimen, like the last, is much corroded in 

 approximately parallel lines and it shows here and there small 

 scales of a yellowish micaceous mineral. It measures about 9 x 

 3 "5 cm. 



Another isolated Baveno doublet (PL xlix., fig. 1) exhibits an 

 irregular junction of the two segments, that on the left partially 

 enveloping the other. This crystal, which is drawn with the 

 edge \b, c] perpendicular to the plane of the paper, measures 3-5 

 X l'^5 cm. 



An interesting crystal of which the exact locality is not known 

 is similarly drawn in PL xlix., fig. 2. It was acquired in a 

 collection of Australian and New Caledonian minerals from 

 Mr. A. H. F. Stephens, who gave the locality as New South 

 Wales. It bears a close resemblance to the Oban twins, and, 

 like them, is accompanied by yellowish mica scales ; hence we 

 may fairly assume that it was derived from the granite of Oban 

 or the neighbourhood. Like the specimen described above (PL 

 xlviii., fig. 3), it may be regarded either as a Baveno triplet or as 

 a combined Baveno and Manebach group. The junctions are 

 remarkably regular and the crystal as a whole is well balanced ; 

 it is, though not the largest, perhaps the finest example of a 

 Baveno twin in the collection of New South Wales orthoclase. 

 It measures 4*5 X 1 cm. 



While the Baveno twins are the finest, crystals also twinned 

 according to the other well established laws, the Carlsbad and 

 Manebach, are forthcoming from Oban. Of the former the 

 crystal represented in PL xlviii., fig. 4, may be taken as typical. 

 It shows th_e forms c(001),6(010),a(100),m (110), ~(130),;i-(r01), 

 y (201), o (111); of these the pinacoid a is of infrequent occurence 

 on orthoclase. The 7n faces are comparatively bright (it seems as 

 if these resist corrosion with greater success than do the other 

 faces). The terminal faces are marked by irregular branching 

 lines with a general direction parallel to the edge [c, a] ; these 

 markings are of very usual occurence on orthoclase crystals, and, 

 as they are accentuated on worn crystals, are probably due to 

 corrosion. The b pinacoid is finely striated parallel to the inter- 

 secting edges of the prismatic zone. The faces c and .r are 

 represented in the figure as coplanar ; strictly speaking c 

 (p ~ 26* 3') is somewhat steeper than x (p = 24° 13') ; 



