Page:History of West Australia.djvu/393



Not far inland from the western seashore of Australia, the great West Australian tableland rises, more or less abruptly, to elevations of over 1,000 feet. In the south-western portion of the colony this tableland commences with the Darling Ranges. The latter run almost parallel with the western coast line, from the Southern Ocean to Geraldton, and the plateau itself extends from north to south throughout the continent.

Proceeding from the coast eastwards, in the gneissic-granite of the Darling Ranges, archæan strata are encountered for the first time. The eastern slopes of those ranges sink into a number of erosive valleys, where, favoured by fertile soil and a copious rainfall, the principal agricultural settlements of the colony are thriving. Beyond these valleys the country is undulating. The eastern extent of the plateau approaches in the southern latitudes the 123rd meridian; it passes this meridian in the middle, and goes considerably beyond it; then it again recedes westwards in the northern latitudes.

The basis and nucleus of this extensive elevation consists of archæan gneissic granite.

From this nucleus coastwards, and more or less parallel with the coast-lines, belts of palæozoic, mesozoic, and tertiary sedimentary strata have found a greater or lesser development.

The recent coast features are chiefly estuary deposits and sand-dunes.

The eastern declines of the tableland sink gradually into desolate sandy depressions, in which mesozoic strata rise to the surface.

The level of those depressions is sometimes only 700 feet above sea.

The interior gold region of Western Australia comprises the central and eastern portions of this tableland.

The presence and great development of palaeozoic sedimentary rocks in the northern and north-western portion of this plateau, and the insignificant occurrence of such rocks in the interior gold region, form conditions for a natural and traceable boundary of that region north and north-westwards.

That portion of the plateau which is situated to the west and south-west of the interior gold region—although to a great extent of physical and geological similarity to it—shows sufficient distinction for the demarcation of the boundaries between the two.

In the western and southern portions of the plateau the archæan strata have been subjected to a general fracturing and folding process, in a similar manner as within the gold region; but the palæozoic greenstones and igneous rocks in general are of limited occurrence.

Contrary to this, the archæan strata within the gold region are traversed by gigantic divisional fractures, running more or less parallel in a north-westerly direction. Greenstone ranges and hills extend along these fractures, and the occurrence of gold also follows their course.

Eastwards, the previously indicated termination of the tableland coincides with the eastern boundary of the interior gold region, which, accordingly, would occupy an approximate area of 120,000 to 140,000 English square miles.

The physiographical designations adopted in Western Australia do not correspond with the usual nomenclature; thus the term mountains is very often applied to hill ranges of no great extent and elevation. Most of the so-called rivers are a succession of waterholes, contained in flood channels of great length, and the so-called lakes are saline marshy depressions, with an occasional shallow sheet of water. During dry seasons they usually appear as arid flats, covered by glittering sheets of salt efflorescences.

Elevations topped by extensive undulating sand plains, granite hills covered by wool-bag-shaped boulders, steep gneissic and granitic cliffs, and huge dome-shaped and rounded granite rocks, are features of the archæan strata.

Within the gneissic-granitic areas, erosion has often imparted a precipitous character to ravines, intervening between alternating plain levels. Most of those ravines are old lacustrine strands, and in the northern portions of the gold region their height often reaches 50 feet, and sometimes more. Their gradual decrease into hardly perceptible rises of the country is frequent.

The table tops, as locally named, are remnants of denuded portions of high plains; they are flat-topped elevations, bordered all round by more or less precipitous descents. Such ravines and granite outcrops often follow fissures and faults.

Proceeding from the north, in a southerly direction, the borders of archæn elevations assume successively more and more the forms of gentle slopes, which occasionally are interrupted by massive granite outcrops, these being generally the more resistive portions of the archæan strata along spices of monoclinal folds.