During the gradual recession of the palæozoic ocean towards the south and south-east, the waves of that shallow ocean had a longer time in which to obliterate the unevennesses of the archæan rocks situated in the southern and south-eastern portion of the gold region, than they had in the northern.
Comparatively low and rugged ranges and hills, consisting chiefly of altered schists and of massive and schistose greenstones follow the more or less irregular courses of divisional fractures in the archæan lythosphere.
Within depressions, following also chiefly divisional breaks, occur numerous saline flats and so-called salt lakes, some of which occupy hundreds of square miles.
As this region has hardly any drainage towards the sea, those saline depressions receive the bulk of the meteoric water, which descend within their respective water-sheds. The collected water forms generally extensive shallow sheets, and during dry seasons the largest part, if not the whole of it, disappears through evaporation and absorption.
The strata forming the bottom of such salt lakes are usually permeable for water, and the water level recedes, in some cases to a considerable depth underground, leaving behind crusts of salt efflorescences which cover the bottom of the lake basins.
The collected water reaches those lakes partly in flood channels, and partly in subterranean conduits, and by percolation. In some of the smaller lacustral basins a new supply of rainwater remains potable for some time.
The saltness of the lacustrine depressions, and also the saltness of the bulk of the subterranean waters in this region, is attributable to some extent to oceanic leavings, but chiefly to the salt formed by the decomposition of the rocks, and especially the salt liberated by the disintegration of rock-forming minerals. In nearly all the quartz, feldspar, hornblende and augite-crystals of the igneous rocks, visicular cavities, containing salt solutions, or even common salt in cubic crystal form, occur.
The lacustrine depressions are usually surrounded by argillaceous reddish-brown coloured löess flats. Their soil is of æolian origin, and they occupy a very large portion of the interior gold region.
The archæan elevations, as well as greenstone hills and ranges, rise above those löess flats, and have supplied, and still supply, the material of which the löess strata are built.
The elevations, consisting of solid rocks, are usually surrounded and often covered by strata of accumulative decomposition, the material of which strata consists of the unremoved detritus derived from the decomposition of those rocks.
Most of the surface formations are neogene, of subærial, fluviatile, lacustrine, and æolian origin.
In depressions, neogene features probably overlay, comformably, lacustral and fluviatile formations of the mesozoic era.
As there exists hardly any drainage towards the sea, the meteoric water-supply is fully balanced by evaporation, and also by absorption in the continuous process of mineral alteration.
The annual rainfall is not a large one (about nine inches), and the surface strata are very porous. There are no running rivers and rivulets. Creeks and gullies, eroded in declines of ranges and elevations, are short, and all traces of them are soon lost in sandy flats. Their beds are mostly dry, but in some cases water may be found for some considerable time after a rainfall in intervening deepenings. Such water reservoirs occur more frequently in the northern parts.
Massive granite outcrops contain water collecting and retaining basins and rock holes, which have gradually weathered out in places where the granite had less resistibility against decomposition. Such rock holes are locally called "namma holes."
"Native wells and native soaks" are usually situated at the base of outcropping granite rocks, and roughly deepened into rock fissures. Water collected in such fissures, and in higher situated detritus standing in communication with them, will drain into these wells and soaks until the subterranean supply down to the level of the well-bottom becomes exhausted.
Fresh-water pools into which the rain-water of an adjacent area is collected, and which in consequence of the imperviousness of the clayish beds is retained for some time, are locally termed "clay pans."
Elevated sand plains, extensive löess flats, saline depressions, and salt lakes—the latter often bordered by sand-dunes—succeed one another with tiresome monotony throughout this region; a monotony which is only scantily relieved by low and rugged greenstone ranges and by a few larger granite outcrops.
Eucalyptus forests and mulga bush cover the largest portion of the gold region. The former predominate south, the latter north, of the 30th degree S. Lat. Elevated sand plains and sand drifts bear usually a stunted scrub vegetation, and are always covered with prickly spinifex.
The occurrence of nutrious grasses in the southern portion is confined chiefly to the soil which surrounds the outcropping granite rocks (which are also the areas of rock holes and soaks), and also to the bases of greenstone hills and ranges.
The vegetation of saline depressions and salt-lake borders consists chiefly of dense pale-green and reddish-green salt bush scrubs, which seldom reach a height of more than three feet. Several species of the dreaded Western Australian poison plants (gastrolobium and oxylobium) occur on the high sand plains and in the surroundings of granite outcrops.
III.—ROCKS OF THE INTERIOR GOLD REGION.
Archæn Gneissic-Granites, although mostly covered by subaërial formations, are the most predominant rocks within the interior gold region. Their outcrops appear sometimes as genuine gneiss, with a more or less developed parallel arrangement of their mica-scales, and also as huge, often dome-shaped, massive granite rocks. In some places, especially where fractures and dislocations have occurred, the surface decomposition has produced the well-known wool-bag-shaped blocks. The mica of these rocks is usually biotite (magnesia mica) of a dark green or black colour. Sienitic features, in which the mica, or portion of it, is replaced by hornblende, occasionally occur. Various gradations between micaschist and typical gneiss, and between the latter and massive granite, are frequently met with.
The gneisso-granitic rocks of the interior gold region belong to the oldest strata of the geologically known earth-crust.
Palæozoic Greenstones.—These rocks are highly crystalline, partially of schistose and partially of massive texture. They form mountains and hill ranges.
Amongst the schistose greenstones the feldspar-amphibolites, or diorite schists, are of most general occurrence. Occasionally they overlay the gneissic-granite directly; sometimes beds and seams of quartzite of sienitic gneiss, or of hornblendic schist, appear interpolated. The diorite schists are found both coarse and fine grained, sometimes almost perfectly stratified, in other cases breaking up in cubes, thin or massive slabs, and even in
