CampBeLL.—On a Pseudomorphous Form of Gold. 457 
which sometimes show under 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 Rutley’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 
trystalline 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 
oa as petrographic research has been making rapid strides in the 
years. 
Arr. LXXIV.—Notes on a Pseudomorphous Form of Gold. 
By W. D. Campzett, F.G.S. | 
[Read before the Auckland Institute, 5th September, 1881.] 
. Plate XXXIX. 
To Occurrence of gold in pseudomorphous erystals has not been hhitharto 
observed, I believe, so that this specimen, which was obtained from a 
a ay ton from a lender i a drive of the igh Pe SS 
