36. NOTES on the BINGERA DIAMOND-FIELD, with NOTES on the MUDGEE DIAMOND-FIELD. By ARCHIBALD LIVERSIDGE, Esq., F.G.S., Reader in Geology and Mineralogy, University of Sydney. (Read June 24, 1874.)

[Abridged *.]

THE first announcement of the occurrence of the diamond in Australia was made by the Rev. W. B. Clarke, F.G.S., in 1860. Specicimens were found, in 1852, at Calula Creek, and, in 1859, near Sutton's Bar, in the Macquarie river, at Burrendong, and at Pyramul Creek.

In 1867 diamonds were discovered by gold-diggers in the Mudgee district on the Cudgegong river, which flows into the Macquarie. In 1869 they were worked pretty extensively at this spot; and in 1870 Mr. Norman Taylor and Dr. Thomson described the Mudgee diamond-field in a paper read before the Royal Society of New South Wales. From this it appears that the diamond-bearing spots lie along the river in outliers of an old river-drift, at varying distances from the river and at an elevation of 40 feet or more above it. The outliers are capped by deposits of hard and compact, sometimes columnar basalt, regarded by Mr. Taylor as of older Pliocene age. These outliers, with their basaltic capping, may be traced for about seventeen miles up the river; in some of them the diamantiferous drift is still 70 feet in thickness.

The outliers which have been worked are enumerated by Messrs. Taylor and Thomson as follows:-Jordan's Hill, 40 acres ; Two Mile Flat, 70 acres; Rocky Ridge, 40 acres; Horse-shoe Bend, 20 acres ; and Hassall's Hill, 340 acres: in all, 510 acres. The drift has invariably been met, with in these localities on tunnelling under or sinking through the basalt. In one patch a peculiar deposit of crystalline cinnabar was met with.

No diamonds have been found in the river-bed, except where the diggers have discharged their tailings into the river, or where gold has been washed.

The basalt, when not resting upon the drift, frequently lies upon metamorphic shales, slates, sandstones, or greenstone. The general formation of the country is regarded as Upper Silurian, with overlying outliers of Carboniferous rocks. The rocks in the vicinity are nearly vertical, with a general N.N.W. strike, and consist of red and yellow coarse- and fine-grained indurated sandstones, thin white argillaceous shales, pink and brown fine-grained sandstones with purple stripes, slates and hard metamorphic schists, hard brecciated conglomerate with limestone nodules, flint and red felspar in a

[As this paper is to a great extent identical with one communicated by the author to the Royal Society of New South Wales in October 1873, and pub lished in their Proceedings,' it is here given only in abstract.-ED. Q. J. G. S.]

greenish siliceous base, and dykes and ejections of intrusive greenstone. The rocks are generally devoid of mica.

The older Pliocene diamantiferous drift is generally coarse and loose, but in parts cemented together by a white siliceous cement, sometimes green by admixture with silicate of iron; sometimes oxides of iron and manganese are the agglutinating agents. Diamonds were found in this solid portion.

The drift consists chiefly of boulders and pebbles of quartz, jasper, agate, quartzite, flint, slate, shale, and sandstone, with coarse sand and clay. The quartz-pebbles are often incrusted with oxide of iron or manganese. Many of the boulders and pebbles show a peculiar brilliant polish or glaze, not due to friction, since the cavities are equally well polished. Silicified wood is common; and coal has been found in the river higher up-also Carboniferous fossils, such as Favosites gotlandica, and Orthis.

The minerals associated with the diamond are:-1, black vesicular pleonast; 2, topaz; 3, quartz; 4, corundum; 5, zircon ; 6, tourmaline; 7, black titaniferous iron-sand; 8, black magnetic iron-sand; 9, brookite; 10, wood-tin; 11, garnet; 12, iron, from tools; 13, gold.

The diamonds are distributed sparingly and irregularly through the older Pliocene drift. The average weight of the diamonds was 0.23 carat or nearly 1 carat grain. The largest discovered was a colourless octahedron of 55 carats. Average sp. gr. 3.44.

The newer Pliocene drift also furnishes a few diamonds; and, being chiefly derived from the older drift, it contains the same minerals; but a few grains of osmiridium are also found in it.

A few diamonds have also been found in Victoria.

The Bingera Diamond-field.

The workings are about seven or eight miles south of Bingera, and are situated in a basin-shaped valley among the mountains of the Drummond range. This valley is about four miles long, and three miles broad, and opens to the south. The surrounding district is of Carboniferous or Devonian age. The valley seems to have been covered by diamantiferous drift; but much of this has been removed by denudation, leaving patches. Running into the basin are spurs of basalt, which also probably overlies the drift.

The bed-rock under the drift is an argillaceous shale. Blocks of this are scattered through the lower part of the drift; and small outcrops of it are seen in one or two places. In some parts there are outcrops of a siliceous conglomerate composed of subangular pebbles with ferruginous cement, sometimes replaced by manganese; and the junction of the conglomerate with the shale is shown by a small gully. The rocks are much contorted and almost vertical. Diamonds occur in the surface soil over the conglomerate here, but not over the shale.

All the workings for diamonds are confined to the surface, the drift not being removed to a greater depth than 2 or 3 feet. The

author describes the process adopted, which he thinks must cause the loss of many diamouds. He also mentions the average number of diamonds per load of drift obtained in certain workings.

The Drift.-The characters of the diamantiferous drift of the Bingera diamond-field are as follows:-On the surface there are boulders and large pebbles of various kinds, principally siliceous, and elongated pebbles of a very fissile argillaceous sandstone, the smaller of which are termed "finger-stones" by the miners, and regarded as a favourable sign. Blocks of very compact conglomerate occur among the other rolled stones; the smaller pebbles in this are generally subangular.

The drift itself is largely composed of clay and sand. The clay shows a brecciated structure, as if recently derived from a comminuted shale; and parts of it, when freshly dug out, are of a bright green colour, probably due to ferrous silicate, as it becomes pale brown on exposure. Large blocks of the bed-rock are found in the drift. Minute crystals of selenite occur in the clay.

The larger pebbles include rolled white quartz (apparently veinquartz), red, green, brown, black, yellow, and veined jasper, a little cacholong, black flinty slate, concretions of limonite, generally spherical (occasionally imbedded in rolled masses of magnetite, within which the nodules seem to have formed by segregation of iron oxide), botryoidal carbonate of iron, rolled masses of very soft shale, and a little petrified wood. The pebbles are not glazed as in the Cudgegong deposits.

The residue left after the puddling process consists of sand and small pebbles, and is known as "gem sand." It is from this that the diamonds are picked out. Associated with the diamonds in it are the following minerals :

:

1. Topaz, generally rolled and water-worn, colourless and transparent, sometimes greenish. 2. Corundum, usually in small angular blue and green fragments. 3. Quartz, as rolled pebbles and worn crystals, also jasper and small black pebbles of flinty slate, the presence of which is regarded as a good sign by the miners. Small jaspery pebbles, usually measuring about in. by in., showing light mottled tints of drab, yellow, brown, pink, &c., and pretty highly polished, are known as "morlops" by the miners, and regarded as furnishing favourable indications. 4. Tourmaline, in rolled black crystals about inch long. 5. Spinel, of a dull red or pinkish colour, often much broken. 6. Zircon, in small rolled fragments and crystals, colourless, red or brown. 7. Wood-tin, rare, in small rolled pieces. 8. Ilmenite, not common. ite, rare, in flat, rolled, translucent plates. 10. Magnetic iron-sand, in minute brown grains, sometimes showing octahedral form under the microscope, strongly magnetic. 11. Garnet, in small, ill-formed reddish-brown crystals, not very common. 12. Gold, rather scarce, usually in very thin spangles, generally attached to the magnetite. 13. Osmiridium, rare, in small, flat, heavy plates.

9. Brook

The diamonds are generally small, and the crystals not particucularly well developed; the faces are usually much rounded. They

vary from colourless to pale yellow and green. Their specific gravity is 3.42, that of the Mudgee diamonds being 3.44.

In all parts of the world where the diamond is found it seems to occur in sandstone regions, associated with basaltic rocks, and usually in a drift or conglomerate. This statement applies to India, Russia, Borneo, Brazil, and Africa. The true source of the diamond has evidently not yet been ascertained; for in all the cases mentioned the sources may be regarded as secondary.