Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/359

 generally confined to pyritous diorite or diabase, whose percentage of silica rarely exceeds one half of their total constituents, another typical intrusive rock appears to perform similar functions in the more siliceous rocks of metamorphic districts, such as those of the Cape River.

Sometimes this partakes of the character of felspar porphyry, and sometimes of a porphyrite.

It is at the intersection of these acid-felspathic dykes with the mica- and hornblende-schists, and in areas within the influence of the said intrusions, that several of the auriferous veins round Mount Remarkable, Mount Davenport, and Mount Elvan, in the Cape- River Mining- district, are found.

What the age of these metamorphic rocks relatively to the Devonian system may be is uncertain, though it is probable that they may represent the Lower Silurian series of Victoria, or the still older metamorphic system of that colony.

The richest copper lodes yet opened in Queensland, viz. " The Peak Downs " and " Mount Perry," lie within such areas. Neither of these seems to be in any way in contact with igneous dykes, but to be true lodes. The former of them had, up to the 30th June, 1870, smelted 29,168 tons of ore for a yield of 5839 tons refined copper, and had been proved, at the time of our visit at the end of that year, to extend at the 40-fathom level 1500 feet in an east and west strike, with an average width of 2 feet. The Mount Perry, lately opened, with very favourable prospects, is bounded by granite of metamorphic origin, very similar in character to that of Ravenswood.

In this last- mentioned gold-mining district nearly 200 distinct reefs are now being actively worked, the yield, from 2120 tons of the quartz first crushed from various claims, having been 5682 oz. of smelted gold, or at the rate of 2 oz. 14 dwts. per ton*.

There is no evidence of trappean action influencing the production of the veins at Ravenswood ; or if there be, it is deep-seated ; and there is, therefore, this practical difference to be borne in mind when considering the mode of occurrence of metallic minerals in Queensland, viz. that in the fossiliferous palaeozoic equivalent of the Devonian no case has yet been observed free from trappean disturbance where paying quantities of metallic ore or metal have been found, whilst in the metamorphic areas this has not been shown to be an absolute necessity. That the stanniferous granites of the Severn river, which are now yielding such marvellous quantities of tin ore, are of metamorphic origin, seems clear from the description contained in a private letter just received by me from Mr. Aplin, lately the Government Geologist for Southern Queensland, who says : —

" The rock is a loosely aggregated, coarse-grained, highly micaceous granite, abounding in thin threads and veins of quartz. It seemed to me to be metamorphic, and was not in large bosses and broad sheets, but in numerously jointed beds.

of gold.
 * The total yield for 1871 from this gold-field, just received, gives 60,444 oz