Campbell. — On a Pseudomorx)hotis Form of Gold. 457 



which sometimes show mider the microscope a base of homogeneous glass 

 without a trace of double refraction ; in some cases the glassy structure is 

 destroyed by crystallization, and a micro-crystalline base formed, possessing 

 the double refraction characteristic of felstones, so that what was once 

 glassy lava is now a felsite with a crystalline structure. (See also Eutley's 

 Petrology, p. 169.) 



It may be mentioned as illustrative of the changes which glassy forms 

 undergo that water extracts potash and soda from glass, together with por- 

 tions of silica, the decomposition taking place with greater ease in propor- 

 tion as the glass is richer in these alkalies and more minutely divided, and 

 the temperature of the water higher. The pearly stratum with which speci- 

 mens of antique glass found buried in the earth are covered, consists almost 

 wholly of silica. 



In thus briefly reviewing the general relations of the various kinds of 

 crystalline rocks, the chief leading characters of the more important groups 

 have been referred to only, the numerous varieties diverging from these 

 groups forming intermediate forms of more or less subordinate interest. 

 The consideration of the changes that rocks undergo, leads us to a certain 

 extent into speculative ground, where different interpretations of facts are 

 tenable. The chief differences of opinion occur with reference to the 

 relations and formation of granite. Though these uncertainties may encircle 

 the subject at the present time, we may expect before long to have a clearer 

 knowledge, as petrographic research has been making rapid strides in the 

 last few years. 



Akt. LXXIV. — Notes on a Pseudomorjplious Form of Gold. 



By W. D. Campbell, F.G.S. 



[Bead before the AucMand Institute, 5th September, 1881.] 



Plate XXXIX. 



The occurrence of gold in pseudomorphous crystals has not been hitherto 

 observed, I believe, so that this specimen, which was obtained from a 

 white clay, taken from a leader in a drive of the Evening Star Claim, at the 

 Waiotahi Creek, Thames, is of great interest. It is reniform-shaped, about 

 ^ inch in length, and covered with minute crystals and filaments studded 

 sparsely with minute irregular- shaped grains of a yellowish-brown mineral, 

 insoluble in acids, which are probably zircon. It is, a^Dparently, the same 

 mineral alluded to by Captain Hutton, in his second report, page 29, on the 

 50 



