730 
to be noted that epidote generally secretes itself on the felspar frontier rather than the 
quartz ; that is to say, a hornblende hounded by quartz and felspar prefers to secrete 
the secondary mineral on the felspar boundary. 
The miens, black and white, do not commonly exhibit that piercing with apatite 
which is frequent in the micas of typical German granites. 
'VhQ felspars are, as a rule, exquisitely banded, and sometimes perfectly limpid. 
The fibrous structure in some of the felspars is to be noted, but not confounded with 
twinning. 
Some of the orthoclases are very highly kaolinized, as is usual. Microcline 
intergrows with orthoclase, particularly in the Etheridge samples. 
Undulose extinction is most marked in the Charters Towers granites ; and with 
regard to this matter I may quote TealTs “ British Petrography” (page 33) Zonal 
structure unquestionably indicates changes in the environment of the crystal during 
the period of growth. Che separation of crystals in a magma must necessarily produce 
a change in the composition of the part which remains liquid ; and this cause alone may 
explain such cases as those observed by Hopfner and Becke. Changes from green to 
brown and vice versa observed in certain hornblendes can probably be explained on the 
assumption that the growing crystals were alternately subjected to oxidising and 
reducing agencies. Until we are more fully informed as to the chemical and physical 
changes which occur during the period of crystal-building in igneous magmas, and the 
effects of such changes on the growing crystals, we cannot, however, hope to render 
a complete account of the various structures observed in the crystalline constituents of 
igneous rocks.” 
The accessory minerals are not so minutely described as is usual in this branch 
of science. I have not recognised any of the cubes, supposed to be salt cubes, in the 
quartz inclusions, neither have I noted any microscopic zircons, which are generally 
found in some samples of granite ; but, as the prepared sections arc to be handed over 
to the Geological Survey Department, it will be possible in the future for others to 
search more diligently, and with better instruments than are at my disposal. 
Apatite, as an accessory mineral, nearly always is found in the vicinity of 
magnetite. Particularly is this so in the Charters Towers samples of granite. 
The tourmaline granite of Cooktown is interesting from the fact that the Cornish 
tourmaline granite may be described in nearly the same language, and both are tin- 
bearing districts. 
Teall says : — “ The amount of tourmaline increases towards the margins of the 
granitic masses. The increase in the amount of tourmaline is accompanied by the 
disappearance of mica, and finally of felspar, the ultimate result being a rock composed 
of tourmaline and quartz (schorl rock).” But whether this holds in the Cooktown 
district is unknown to the Authoj*. 
The garnetiferous granite of the Cape Biver Gold Field bears out Zirkel’s 
observation that the American garnet-bearing granites do not carry the mineral in 
microscopic size, all the garnets being macroscopic.* 
RHYOLITES. 
These rocks exhibit fluxion-structure. Zirkel describes them as follows f : — “ In 
the glassy and ha'f-glassy rocks, it is a widely-spread phenomenon for the colourless 
green and black, needle-formed, microscopical elements to be grouped together into 
strings, bands, and flocks. There are bodies among them which have the appearance of 
« 11 jjlleroscopical Petrography.” 
t Loc. cit., p. I. 
I 
