Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/354

 Calliope, and Gympie,— the best reefs lying at or near the junction of the diorite and the Devonian " country."

In fact it would be only a repetition to describe the mode of occurrence of the gold at Lucky Valley, Talgai, Rosewood, Mount Wyatt, the Broken River, and the western portion of the Gilbert diggings, so nearly identical is it with those above mentioned.

The ground-plan of a portion of bare rock exposed in the bed of

Fig. 14. — Ground-plan showing Quartz-reef at the intersection of the intrusive Greenstone and Palaeozoic sediments, Gilbert River, 1-1/2 mile below the junction of the Robertson and Gilbert Rivers.

A, C, D. Diorite dykes. B. Thin-bedded slates ; strike E. and W., dip 75°.

E. Slate, e. E faulted down, apparently by intrusion of C.

F, G, H. Alternating beds of slate and sandstone.

f, g, h. Beds corresponding to F, G, H, faulted down by C and D.

I. Auriferous quartz-vein on line of fault.

J. Auriferous quartz-vein on line of contact of dyke with slate.

the Gilbert river (Fig. 14) gives a good illustration of the relation of intrusive trap-rocks to the auriferous vein-stones.

A view of the section (Fig. 15) shows also the faulting action of the intrusive rocks at this point.

In nearly all veins due to this apparent cause cale-spar and pyrites are largely represented, some of them being composed entirely of pyrites.

All vein-stones of this class are disintegrated by acid, and are rarely composed of quartz alone. The associated gold is always alloyed with a large percentage of silver, and assumes also much of the character of the latter metal in its native state, being filiform and dendritic. It often takes a peculiar form, combining a thread-like structure with a semicrystalline surface, which is technically known among Queensland diggers as " spider -leg " gold.

The analysis of the " spider-leg " metal, from the " Upper-Cap