Page:History of West Australia.djvu/397

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The depressions within the hydrographic basins of the interior gold region are generally occupied by marshy salt flats or so-called salt lakes. The latter are areas of surface collection and evaporation. The water influx into those lacustrine basins is chiefly subterranean, although during heavy thunderstorms a few flood channels may also contribute considerably. The salt accumulations within these areas are due to an excessive evaporation. During the dry seasons, almost all water within the salt lakes disappears, and the damp and muddy lacustrine beds become covered with hard, sandy salt crusts and salt efflorescences. In the vicinity of flat shores, such sandy salt crusts usually cover a permeable pulverulent stratum of lacustrine sediment. Here the subterranean water influx can be observed after a heavy rainfall. This influx causes a pressure on the sandy, salt crust from below, and forces it into calotte-shaped elevations, sometimes above an already accumulated shallow sheet of water. Finally, the tops of those elevations burst, and water flows out of the crevices. Part of the salt within the lacustral basins pursues a circular current. In dry seasons salt dust is lifted from the 'dried-up lake bottom by the winds and deposited on higher levels. Then, during wet periods, it is re-dissolved and returned to the lake by rain. The extension of a saline vegetation, far beyond the borders of a lake, and above its level, is due to the salt being transported and deposited in the above-described manner.

The annual average rainfall for this region has not yet been sufficiently ascertained, but it scarcely will exceed nine inches. Of this, a large portion falls in brief showers, which hardly soak a few inches of the loose, sandy surface soil. The result is that it becomes partly absorbed by the vegetation, and partly re-evaporated during the next few days. Only heavy thunderstorms contribute towards subterranean storage.

The subterranean waters within this region are either percolating or resting. In both those conditions they are found either potable or mineralised and salt.

Water under artesian pressure is here non-existent, the reasons for this being as follow:—

(a) There are no interchanging impermeable and permeable strata, of which one of the latter is confined by two of the former, and all of which are regularly dipping under a low, flat country.

(b) There is no elevated water-collecting area within the interior gold region which could sufficiently supply a subterranean stream, even if the strata necessary for its reception and conduct would be in existence, which they are not.

(c) The numerous and extensive outcrops of the archæan gneissic granites exclude absolutely every supposition that regular stratified rocks could reach this region from outside.

The only spaces where subterranean water, under some slight hydrostatic pressure, could occur, are the monoclinal folds within the archæan rocks. Some of those folds may contain stratified beds, which, if they exist, would be like the mesozoic strata near the south coast, almost horizontal. Water is certain to have collected within the permeable rocks which fill the spaces of V-shaped cross-section, in the archæan monoclinal folds; but as the hydrographic basins enclosing such folds (like the rest of this region), have hardly any drainage towards the sea, such water, if tapped, will be more or less salt.

There are neither running creeks nor springs within this region and even water accumulations in flood channels during occasional thunderstorms generally disappear underground as soon as they enter certain sandy flats.

A certain part of the rain precipitations, after soaking through the porous, sandy surface soils, enters the pores, joints, and fissures of the underlaying rocks.

The rain-water, passing through the atmosphere, becomes charged with carbonic acid, and being provided with that powerful agent, is enabled to perform far-reaching alterations within the percolated rocks.

The upper strata of the gneissic granites have generally suffered such alteration; they are decomposed. This decomposition reaches various depths, and often consists in a complete kaolinisation of the feldspars, and in the more or less advanced change of the mica (biotite) into a pale, talc-like hydrous mineral, the quartz grains being hardly affected.

The greenstones are subjected to a similar alteration, in which the feldspars become also kaolinised, and the other component minerals, hornblende or augite, transformed into varieties of hydrous minerals.

In very large portions of the decomposed greenstones within this region, hornblende and augite are altered into chlorite and delossite; therefore, the original rocks into chloritic rocks.

With the kaolinisation of the feldspars in the upper strata of the archæan rocks, and the alteration of the feldspar amphibolites (diorite schists) into chloritic schists, easily soluble mineral constituents of the country were replaced by more resistive ones, and the rocks, after alteration or decomposition, became far more permeable for water than they were in their primary state.

During the process of decomposition or alteration, large quantities of mineral matter were removed, and, as this process continues, are still continuously being removed towards and into the water-collecting depressions of the various hydrographic basins. As the rocks have retained their original texture, their storage capacity for water has necessarily largely increased. Decomposition reaches the greatest depths along fissures and lodes. In portions of country not traversed by fissures and lodes, decomposition does not reach great depths, but the latter generally increases in approaching the next lode or fissure.

The subterranean physiography of the surface of the un-decomposed and still solid country rock prescribes the movements of the percolating waters until they reach a level of complete saturation, and from thence the outfall towards the collecting depression of the hydrographic basin decides their further movements.

The rain-water entering such decomposed porous rocks, soaks centripetally downwards till the underlaying, solid, impermeable rock, or a plane of complete saturation within the former, is reached.

Various degrees of resistibility against decomposition, as well as number and size of joints and fissures, chiefly influence the subterranean physiography of the still solid rocks.

Supposing the decomposed rocks were removed, the so exposed surfaces of the still solid impermeable rocks would present elevations and depressions (ridges, flat-topped elevations, channels, and basins). The percolation towards the collecting basins will necessarily follow such depressions. Conduits may spread over a wide area of the subterannean [sic] solid rock, or they may be confined to narrow channels. Basin-form reservoirs are also certain to occur.

In the schistose greenstones the decomposition reaches generally greater depths than in the gneissic granites. The