Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/353

 examined in polarized light, are of course triclinic, whilst those of one colour are probably orthoclase. There are some twins showing two colours sharply divided by the line of junction of the two halves ; and these are certainly orthoclase.

" The secondary products are a little chlorite, and two or three blebs of quartz, filling spaces between the crystals.

" The rock is not much altered."

Of the fossiliferous tufa he says : —

" It is a fragmentary rock, and has undergone so much alteration that the constituents cannot be well identified. There are, however, some grains of augite and magnetite. The fragments are imbedded in a chloritic base, which is itself a product of alteration."

Of another specimen of the same character from the West-coast Reef, Gympie, he remarks : —

" This is a characteristic trappean ash or breccia ; it consists of numerous fragments of rather fine-grained trap-rocks, broken crystals of felspar and augite, many black grains of magnetite, and fragments of other rocks, the whole forming a compact mass in which there is now much chlorite."

The analysis of the crystalline rock indicates no very determinate results, but suggests the presence of two felspars and two varieties of hornblende, with a little chlorite, carbonate of lime, and pyrites.

The insoluble portion may be a mixture of perthite and an insoluble aluminous hornblende, whilst the soluble portion indicates a mixture of oligoclase, an iron and magnesian hornblende, like pargasite, and a smaller proportion of a chloritic mineral, as indicated by the water of constitution.

The absence of augite in its composition would, notwithstanding the presence of the products of decomposition, chlorite and carbonate of lime, place it among the diorites. The so-called fossiliferous greenstone, however, would perhaps be most correctly called a "diabase tufa," as shown both by analysis and microscopic examination.

Whatever may be the proper technical term for the class of rock of which the Gympie crystalline diorite is a typical example, the fact remains that throughout the whole extent of the great Devonian area of Queensland no portion has yet been found to include auriferous veinstones which will pay to work, where trappean disturbance of this character is not present.

At the Boyne the Devonian slates are cut at various angles by dykes of hard crystalline diorite ; and for the most part the auriferous quartz-veins are found at the absolute intersection of the two. The very intelligent manager of the Boyne Company's leased ground had found this fact verified in all the mining operations conducted there.

At Calliope intersecting dykes of serpentine, or diorite, are the surest guides to the richest vein- stones, the Devonian slates and limestones being the rocks intersected.

At Crocodile, Blackfellows, and Morinish diggings, in the Rockhampton district, the conditions are analogous to those of the Boyne,