50 PROCEEDINGS OF THE ROYAL SOCIETY OF QUEENSLAND. 
(111) DESCRIPTION OF ROCK. 
Megascopic-—In the hand specimen the rock appears to 
be made up entirely of black tourmaline and creamy pink 
felspar. The tourmaline, which consists of a matted aggregate 
of minute prismatic crystals, is the more abundant mineral, 
and forms a black background against which the light-coloured 
felspar stands out in high relief as a series of crumpled, con- 
torted bands of varying thickness. The individual bands are 
sometimes persistent, but are often pinched out and discon- 
tinuous. They vary in width from 2 mm. to barely discernible 
threads. 
Microscopic.—Microscopic inspection confirms the general 
impressions gained in an examination of the hand specimens. 
The rock is seen to consist almost entirely of tourmaline and 
felspar. 
The tourmaline forms a matted aggregate of prismatic 
crystals averaging about 0-15 mm. in length and 0-03 mm. in 
breadth, and of fairly uniform size. 
In transmitted light the colour is bluish green. Absorp- 
tion is strong and characteristic (O>E) and the dichroism is 
marked, O being dark bluish green, and E light brown with 
a greenish tint. All of the tourmaline crystals appear to be 
of the one type and no colour zoning is apparent in individuals. 
The felspar is albite, with typical double refraction and 
low refractive index. Polysynthetic twinning is commonly 
present both on the Albite and Pericline law, both types being 
sometimes displayed on the one crystal. The twinning is 
somewhat irregularly developed and sometimes gives rise to a 
structure which may be compared with the ‘ Chequer” 
structure of albite described by Flett,t Hughes,® and Tilley.‘ 
The felspar crystals contain numerous minute inclusions. 
The junction between the tourmaline and felspar is well 
defined. A few crystals project into the felspar, and occasional 
isolated crystals are seen enclosed in the felspar itself, but the 
general nature of the contact militates against the view that 
the tourmaline has been formed at the expense of the albite. 
The general structure suggests that the albite is a later mineral 
4“°The Geology of Newton Abbot,” 1913, p. 60. 
5“ The Geology of Part of Carnarvonshire,’’ Geol. Mag., 1917, p. 18. 
6 Proc. Roy. Soc. S. Aus., 1919, r. 328 also Pl. xxxi, fig. 2. 
