Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/358

 Metamorphic.

The mineral districts already opened in Queensland not situated on areas occupied by Palaeozoic rocks of the Devonian epoch are : —

1. Cape River. 6. Kilkivan.

2. Portion of Gilbert. 7. Gooroomjam.

3. Peak Downs. 8. Broughton.

4. Cloncurry. 9. Etheridge.

5. Black Snake. 10. Ravenswood.

In each of these localities gold has been found in alluvial deposits, resting on rocks of a metamorphic character, derived in part from auriferous lodes, which traverse these strata. At the seven first- mentioned diggings mica and hornblende schists are the commonest forms of metamorphism developed ; while at the three latter the mining-area is entirely confined to syenitic granite.

An analysis of this rock from Ravenswood gave the following results : —

Silica 60.066 Alumina 21.180 Ferrous oxide 5.533 Lime 4.833 Magnesia 2.073 Potash 2.120 Soda 2.233 Water, constitution 1.305 Water, hygroscopic 0.650 99.993

Its contained minerals were triclinic felspar, quartz, brown mica, and a little hornblende, though some of the syenitic granites from the same locality have a much larger percentage of hornblende. This was a typical specimen of the hornblendic granites, largely developed in various localities in Queensland, which evidently have been formed by chemical and mechanical changes from stratified sedimentary rocks, as evidenced by the occasional bands of mica- and other schists observed in them at intervals.

As far as my individual observations are to be trusted, all the granites of the colony that contain hornblende in addition to mica, and the greater proportion of whose felspar is triclinic, may be classed among the metamorphic series.

The decomposition of such granites forms fair soils, yielding good grasses ; and their physical aspect is that of an undulating country, with gently sloping ridges ; and -as a whole they are favourable to the production of other metallic minerals besides gold.

What has been asserted as to the value of certain trappean rocks in influencing the production of mineral lodes in the Devonian system, is also borne out in several of the localities enumerated above, included within metamorphic areas.

Whilst, however, the mineralizing traps of Devonian areas are