The Gympie Field, Queensland.

Apart from Mt. Morgan (which, being a single mine only, can scarcely be classed as a gold-field) the two most important gold-fields of Queensland are Gympie and Charters Towers The Gympie rocks have been determined by Mr. J. Etheridge and others as Permo-Carboniferous. This field, which is only about two miles long and three-quarters of a mile wide, is remarkable for the evidence it affords of volcanic disturbance, contemporaneous with the formation of the sedimentary rocks This disturbance has also probably produced the extensive faults intersecting the country-rock in various directions, and bounding the auriferous area on the north, south and west.

Mr. W. H. Rands, in his "Report on the Gympie Gold Field" (Brisbane, 1889), says that the district presents a series of alternations of sedimentary strata with some intrusive igneous, and also some volcanic rocks. Taking the average clip at 22°, the aggregate thickness of the strata must be some what over 2000 feet, consisting chiefly of graywackes, altered sandstones, gray and black carbonaceous shales, grits, conglomerates, breccias and limestone. Interbedded with these are amygdaloid volcanic rocks and volcanic ash, and sheets of a much altered intrusive "greenstone." Most of the sedimentary rocks are more or less pyritous, some of the shales, espe page 15 cially, being full of small cubic crystals of pyrite. Most of them, especially the sandstones, graywackes and conglomerates, and some of the shales, are also calcareous. The faces of joints are almost invariably coated with calcite. The strata dip with great regularity on the average from 20° to 22°; the direction of dip being, as a rule, in the northern part of the field, a little N. of E.; in the southern part, E.S.E. There are altogether four beds of dark carbonaceous shale in the formation, locally known as the First, Second, Third and Fourth beds of slate; and it has been observed that the reefs are notably profitable only where they traverse these shales. I found in the latter, as a mean of four determinations, 1.07 per cent, of carbon; but I did not find the percentage of carbon increasing as a lode was approached—as was the case in the New Chum Railway mine of Bendigo, described above. They generally carry in very small cubes a considerable quantity of pyrite—the specimens of other sedimentary rocks of the district examined by me being much less pyritous. The intrusive "greenstone" interstratified with the shales, appears to have affected favorably the gold-bearing character of the rocks near it. Mr. Rands, while pointing out this favorable effect on the shales, says, however, that the reefs actually in contact with the "greenstone" have become impoverished.

The questions suggested by the foregoing conditions are:

Fig. 3.

These results seem to indicate that the gold found in the country-rock is derived from the lode, and that the origin of the gold in the lode is to be looked for neither in the kindly country-rock nor in the greenstone underlying it, but in some source lying deeper than any rocks now exposed in the workings.