Apart, therefore, from the 5.552 per cent. of chloride of sodium, the deposit consists of sequi carbonate of soda or native “Trona,” and as such is used by the settlers for culinary purposes, etc.
PALÆOZOIC.
“Carboniferous.”—Whilst the affinities of the southern coalfield of Queensland are mesozoic, a northern field, of even larger extent, has a distinct fauna more resembling the Palæozoic Carboniferous areas of Europe.
The Dawson, Comet, Mackenzie, Isaacs, and Bowen Rivers drain this carboniferous area; and numerous outcrops of coal have been observed on these streams. No commercial use, however, has yet been made of any of these deposits, as the measures generally are too far inland to be made available until the railway system of the country is extended in that direction.
“Devonian.”—From the southern boundary of Queensland up to latitude 18 deg. S., a series of slates, sandstones, coral limestones, and conglomerates extend to a distance 200 miles inland; these are sometimes overlain by coal measures, sometimes by volcanic rocks, and consequently do not crop out on the surface over such districts. North of latitude 18 deg. S., however, over the Cape York Peninsula, this series (so far as we have any evidence), is absent, granites and porphyries capped by “Desert Sandstone” forming the ranges on the eastern, and their abraded ingredients the sandy ti-tree flats, those on the western side of that inhospitable tract of country, a never-ending flat of poor desert-looking sandy ti-tree country, stretching away to the shores of the Gulf of Carpentaria.
In the limestone bands, which form the lower portion of the series, corals are very numerous; in fact, the limestones, where little alteration has taken place, are a mass of aggregated corals; and as this class of rock has resisted aerial destruction better than the associated slates and sandstones, the barriers thus formed mark the trend of the rock system to which they belong, in a very picturesque and decided manner; their bold, massive, and varied outline chiselled into the most delicate fretwork by Nature’s hand, is relieved by a wealth of richly-tinted foliage, unknown in the surrounding bush; and the eye jaded with the monotony of the eternal gum tree turns with delight to the changing tints and varied scenery presented by these barrier-like records of the past. This class of country is very much in evidence at Chillagoe. On the track from the Broken River to the Gilbert diggings, Devonian rocks several thousand feet thick may be observed, as they are continuous in dip, without being repeated, for at least five miles across the strike, with an average inclination of 60 deg.
Although on the Broken River and its tributaries a breadth of thirty miles with a length of sixty miles, is occupied by a persistent outcrop of Devonian strata, gold has only been discovered in remunerative quantities in a small gully, where a trapdyke has penetrated the Palæozoic rocks of the district.
The following districts, however, where Devonian rocks prevail, have been the centres of gold mining enterprise:—Lucky Valley, Talgai, Gympie, Calliope, Boyne, Morinish, Rosewood, Mount Wyatt, Broken River, portion of Gilbert.
In every case here cited, the country is traversed by trap rocks of a peculiar character, either diorite, diabase, or porphyrite; and tufaceous representatives of these are also found interstratified in the upper portion of the same formation, and occasionally throughout the other beds.
At Gympie, the auriferous area is confined to veins traversing a crystalline diorite, or within a certain limit of its boundary, marked by the presence of fossiliferous diabase tufas.