Maclaben. — On Gold at Coromandel. 493 



goniometer at my dieposal. There is no appearance of any 

 twinning on an octahedral plane. 



The reverse side (fig. 3) presents the same dodecahedral 

 outline. Here, however, two dodecahedral planes form the 

 basis, surmounted by octahedral planes (1, 1), and these 

 again by a final dodecahedral plane (i). On this side, and 

 with an edge parallel to the edge of an octahedron, is a cube 

 corner (H, fig. 8). This cube has the apex bevelled inwards, 

 forming a depression, as shown in fig. 6. In none of the 

 cubes or octahedra that I have examined have salient edges 

 been discovered. The cube faces have plane and interfacial 

 angles of about 90°. 



An apparently similar form has been described by a pre- 

 vious observer''' as a pseudomorph after botryogen or red iron 

 vitriol, but, as I have been unable to detect botryogen in the 

 reef, and as pyrites is also far from common, it is hardly likely 

 that the gold, in this particular case at any rate, has crystal- 

 lized as a pseudomorph, more especially as the angles (fig. 4) 

 approximate much more closely to the 120° of the rhombic 

 dodecahedron than to the 117° 24' of botryogen, and therefore 

 the most logical conclusion seems to me that it has crystal- 

 lized in its natural system. However, as I have mentioned, 

 unequal development of the faces will produce a strongly 

 marked monoclinic appearance. 



The specific gravity of the " gold" is about 17'6, and the 

 fineness is therefore 0-8842, assuming the other constituent to 

 be silver, which from the colour is probably the case. 



Gold in Calcite. 



Several instances of this rare occurrence have lately been 

 brought under my notice. The following appears to be a 

 typical mode : The calcite appears as an incrustation on the 

 walls of a quartz reef, and the deposition of gold and of calcite 

 appears to have been contemporaneous. The calcite is in the 

 form of Iceland spar or double-refracting spar occurring in 

 clear rhombohedra. The gold is scattered in thin scales and 

 plates throughout the Iceland spar, with, apparently, no re- 

 lation to the cleavage planes of the spar, for the scales cross 

 these planes at all angles. The gold presents no crystalline 

 faces, and from the colour appears to be much poorer in 

 quality than that from the adjoining quartz reefs. 



The very limited leisure time at my disposal has hitherto 

 prevented me from devoting my attention to the mode of de- 

 position of the gold in the calcite, and I must plead the same 

 excuse for the general brevity of these notes. 



* Trans. N.Z. Inst., vol. xiv., pp. 457, 458. 



