713 
No. 11. Mackat (Qtjeenslafd). 
A. Gr. Maitland’s Collection. 
This rock is opaque, with a dirty reddish-yellow colour, and might belong to 
either the obsidian or tachylite group. 
Section. — Only shows a few completely altered felspars and a dense opaque 
yellow glassy base, which is isotropic on the feather-edge. Mr. P. P. Sellheim, Under 
Secretary for Mines, kindly prepared this section for me. 
No. 196. Sheepield (Tasmania). 
11. L. Jack’s Collection. 
Colour, black. Vitreous. Glass with clear little white crystals. This is by far 
the most beautiful exain|)le of pitchstonel have yet seen. The crystals are olivine, and 
they are mostly preserved in exquisitely regular forms in the glass ; the faces o F { 001) 
are wanting. The section being a little thick and the glass very transparent, together 
with their high angle of refraction, enables one to recognise in these crystals many of 
the characteristic planes of the typical olivine crystal. In this rock, students of micro- 
scopical crystallography have an opportunity of study'ing olivine in a perfect crystal 
form. Under the i-inch objective the olivines are seen to include glass with 
fixed glass bubbles. The glassy base carries nothing else but opaque dusty matter in 
spots, which is probably a darker glass nucleus fringed with dusty matter, in whose 
neighbourhood gas bubbles are commonly found. The glass has bubbles at tolerably 
regular intervals, and in one or two cases, bubbles ai’e strung out in shapes or outlines. 
The microlites are very small, and play no great part in the constitution of the 
rock. They are tabular felspars, not unlike the New Guinea felspars in obsidian 
described in the No. 1 of the Rhyolites. {For Drawing see Plate GI,/y. 2.) 
No. 58. Mackat (Queensland) : Selection No. 164-2. 
A. G. Maitland’s Collection. 
Colour, light-brown. Vitreous. Fracture conchoidal, with strongly marked fissure 
planes, not unlike slate in this respect, rendering it difficult to mount when the section 
is removed from the grinding-slip. Under a lens, the rock is seen to carry numbex’less 
whitish-yellow granules, which .appear as yellowish patches on the plate. 
Section. — Isotropic glassy base filled with most delicately matted microlites. In 
' the plate they appear too- white ; the real colour is a faint yellow. By reflected light 
they have just the colour and sheen of the hair of a fair young child. They mark the 
fluxion, which would otherwise be invisible. I cannot suggest a name for the yellow 
patches, which are amorphous and well distributed, often enveloping the smaller 
f ©1 S pSifS 
The twm ovate bodies in the sanidine crystal are composed of the same substance, 
and are between the two faces of the sanidine section, as verified by the micrometer- 
gcrtjw, , , 
The felspars are fairly fresh sanidines. The extinction-angle of the sanidine in 
Plate 62, fig. 2, with reference to the crystal edge, is 24°, so that the section must be cut 
slightly obrique to the clinopinakoid (010), since, were it exactly parallel, it would be 
about 21°.* The rock is not quartzless, but only three small rounded quartzes occur m 
a section '75 inch square. 
* Kosenbusoh. Physiography of the Kock-making Minerals, p. 281. 
