Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/361

 Silica 61.433 Alumina 20.293 Ferric oxide 1.220 Ferrous oxide 3.260 Lime 3.233 Magnesia 1.566 Potash 1.686 Soda 6.173 Water, constitution 0.725 Water, hygroscopic 0.300 Specific gravity 2.712. 99.889

Mr. Allport's description of this, from a microscopic examination, was : — " That it was an altered porphyrite, with pseudomorphs after felspar, in a granular felsitic base, and contained minute grains of magnetite, and also a little chlorite." The specimen analyzed did not contain pyrites, but was slightly tinged with carbonate of copper in the mass, a portion not so stained being selected for the determination of the ingredients.

Rocks of this type, associated with auriferous veins, are developed in the Black Snake " reefing districts," on the flanks of the Berserker Range (Pl. XI. fig. 3), in New-Zealand Gully, and in other mining- areas near Rockhampton, also within the auriferous radius around Mount Wyatt.

Group 2.

Pyritous Diorite and Diabase.

In the chapter on the Palaeozoic rocks, correlated with the Devonian of Europe, the analysis of a type specimen of the Gympic crystalline Diorite is given. That of a diabase tufa, containing organic remains, which forms an interstratified portion of the Devonian series, the one probably representing the core, the other the ejecta, of a Devonian volcano is also given.

As this group, however, is widely distributed along the eastern portions of Queensland, and seems so important an indicator of auriferous wealth, the analysis of another specimen (Pl. X. fig. 3) taken from near the junction of Granite and Twenty-mile Creek, in the Gilbert district, is here given.

Ultimate Analysis.

Silica 47.465 Alumina 19.485 Ferric oxide 1.570 Ferrous oxide 11.735 Lime 7.400 Magnesia 5.670 Sulphur 0.335 Potash 0.280 Soda 2.725 Carbonic acid 1.461 Water, constitution 1.150 Water, hygroscopic, 0.350 99-626