188 REV. J. MILNE CURRAN. 



mass of the rock, are, in reality, negative crystals.* Under 

 •crossed nicols, instead of lighting up, as the slice is rotated, they 

 remain dark in strong contrast to the devitrified base that shows 

 •colour somewhat after the manner of a felsite. This structure 

 can only be accounted for by supposing that the silicates were 

 profoundly altered, or removed in solution, and pseudomorphs 

 deposited in their places. The opaline silica now filling the 

 cavities, was probably derived in this way from the rock. 



In the granites that are described further on, we have repre- 

 sented many interesting types of structure. Along the Queens- 

 land coast above Bowen, and about the Burdekin there is a vast 

 •area of granites varying in structure from fine-grained to porphy- 

 ritic. Hornblende is a constituent, but quite subordinate to 

 biotite in the localities I examined. The biotites show a peculiarly 

 bent and contorted structure. A grey granite (biotite granite) 

 iorming the hills at the back of Cowra is interesting for the large 

 number of macroscopic Carlsbad twins that the felspars show. 



There is a hornblende-biotite granite around Bafchurst that 

 alters in character as it approaches the surrounding slate country. 

 The hornblendes get less common and finally disappear as one 

 travels on towards the granitic boundaries. The whole structure 

 of the rock alters along its edges. This points to the absorption 

 of a certain amount of the sedimentary rocks in the intrusion of 

 the granite. 



The structure of the Moruya granite, on the south coast of New 

 South Wales may be taken as typical of our best granitic rocks, f 

 The rock is a holo-crystalline compound of orthoclase, quartz, 

 biotite, and hornblende. The hornblende is not too plentiful. 

 The quartz fills up the spaces between the other minerals and 

 ■binds the rock into a most enduring building material. 



* Professor J. W. Judd describes negative crystals formed by the action 

 of mutual solvents in larger crystals, and the filling up of the spaces 

 with decomposition products. See Judd on the Tertiary and Older Peri- 

 dotites of Scotland — Quart. Journ. Geol. Soc, Vol. xli., p. 385. 



f An analysis of the rock and note on its minerals by Prof. Liversidge, 

 f.k.s., will be found in the Journ. Eoy. Soc, N. S. Wales, Vol. xvi., p. 42. 



