Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/843



USTRALIA is the only continent entirely in the southern hemisphere. A mostly unbroken coast-line faces the sea in majestic and stately curves, giving the whole land a character of compactness, which likens it in some way to portions of the African continent. Quite one-half of its area is drained towards the interior. There are, however, no rivers like the Congo or Nile, and no snow-capped mountains to act as feeders. The average height of the land is not more than 300 feet, the highest point, Mount Kosciusko, being only 7328 feet above sea-level, and well below the limits of perpetual snow. Australia lies between 10° 39’ and 39° 11½′ south lat., and between meridians of 113° 5’ and 153° 16’ east long. Its greatest length is 2400 miles from east to west, and the greatest breadth 1971 miles from north to south. The area is, approximately, 2,946,691 square miles, with a coast line measuring about 8850. This is equal to one mile to each 333 square miles of land, the smallest proportion of coast shown by any of the continents. The salient features of the Australian continent are its compact outline, the absence of navigable rivers communicating with the interior, the absence of active volcanoes or snow-capped mountains, its isolation from other lands, and its antiquity. Some of the most profound changes that have taken place on this globe in lifting mountains such as the Alps and the Himalayas, have taken place in Mesozoic times. Rocks high up on the shoulders of these great ranges were formed beneath the sea at a time when a great portion of the Australian continent was already dry land. Vast tracts of Europe and Asia have been submerged in Tertiary times. Australia has been for the most part above the sea since the period anterior to the great earth movements named. In this sense Australia has been rightly referred to as one of the oldest existing land surfaces. It has been described as at once the largest island and the smallest continent on the globe. While in one sense a large island, it conforms in general to the continental model. The general contours exemplify the law of geographers, as to having a high border around a depressed interior, and as to having the highest mountains on the side of the greatest ocean. The main dividing range of Eastern Australia looks out upon the greatest and deepest water mass on the globe, the Tasman Sea and the South Pacific.

Australia stands up from the ocean depths in three fairly well marked shelves. The basal plain of these terraces is the bed of the Pacific, having an average depth of 15,000 feet. From this profound foundation rise Australia, New Guinea, and Melanesia, in varying slopes. The first ledge rising from the ocean floor has a depth averaging 8000 feet below sea-level. The outer edge of this basal ledge is roughly parallel to the coast of West Australia, and more than 150 miles from the land. Round the Australian Bight it continues parallel to the coast, until south of Spencer Gulf (the basal ledge still averaging 8000 feet in depth) it sweeps southwards to lat. 55°, and forms a submarine promontory 1000 miles long.

General character

The edge of the abysmal area comes close to the eastern coasts of Tasmania and New South Wales, approaching to within 60 miles of Cape Howe. The terrace closest to the land, known as the continental shelf, has an average depth of 600 feet, and connects Australia, New Guinea, and Tasmania in one unbroken sweep. Compared with other continents, the Australian continental shelf is extremely narrow. There are points on the eastern coast where the land plunges down to oceanic depths with an abruptness rarely paralleled. Off the Queensland coast the shelf broadens, its outer edge being lined by the seaward face of the Great Barrier Reef. From Torres Strait to Dampier Land the shelf spreads out, and connects Australia with New Guinea and the Malay Archipelago. An elongation of the shelf to the south joins Tasmania with the mainland. The vertical relief of the land above the ocean is one of the chief factors in determining the climate as well as the distribution of the fauna and flora of a continent. The land mass of Australia rises to a lesser mean height than that of any other continent. The chief mountain systems are parallel to, and not far from, the coast-line. Thus, taking the continent as a whole, it may be described as plateau fringed by a low-lying well-watered coast, with a depressed, and for the most part arid, interior. A great central plain covers quite 500,000 square miles of the continent. Although termed the Central Plain, it is situated a little to the east of a meridional line bisecting the continent. The vast bulk of this plain is situated to the south of the 22nd degree, but portions of it reach very close to the low-lying flats south of the Gulf of Carpentaria. The contour of the continent in the latitude of the Richmond River is as follows:—A short strip of coastal plain; then a sharp incline rising to a mountain range 4000 feet above sea-level, at a distance of 40 miles from the coast. From this a gently-sloping plateau keeps deepening until nearly down again to the sea-level, in a line due north of Spencer Gulf. Then there is a gentle rise to the Low Steppes, much of which country is spinifex desert, 500 to 1000 feet above sea-level. A further gentle rise in the High Steppes leads to the mountains of the West Australian coast, and another strip of low-lying coastal land to the sea. The Great Central Plain is certainly Australia’s most notable inland feature. Comparatively little of the rainfall over its vast extent ever reaches the sea by any known river system. Indeed the river systems as shown on maps leave a false impression as to the actual condition of things. The absence of rivers connecting the coast-line with the interior has already been noticed. In keeping with this is the solid outline of the coast generally. On the north and north-west some notable indentations lead to the Fitzroy, the Victoria, the Daley, and the Roper rivers.

The network of streams forming the tributaries of the Darling and Murray system give an idea of a well-watered country. The so-called rivers have running water only after heavy rains, and very few of these tributaries ever reach the main drainage line. Flood waters disappear often within a distance of a few miles, being absorbed by porous soil, stretches of sand, and sometimes by the underlying bed-rocks. The waters even of the Macquarie do not usually reach the Darling, but break up into innumerable gutters, and spread out over vast flats. In flood times only, the river overflows its banks, and flooding the flat country for miles around, reaches the Darling. Oxley went down the Lachlan, 1817, during one of these periods of flood, and the great plains appeared to him to be the fringe of a vast inland sea. As a matter of fact, they are an alluvial deposit spread out by the same flood waters. The great rivers of Australia, draining inland for the most part, carve out valleys, dissolve limestone, and spread out their deposit over the plains when the waters become too sluggish to bear their burden farther. From a geological standpoint, the Great Australian Plain and the fertile valley of the Nile have had a similar origin. Taking the Lachlan as one type of Australian river, we find it takes its rise amongst the precipitous and almost unexplored valleys of the Main Dividing Range. With the help of its tributaries it acts as a denuding agent for 14,000 square miles of country, and carries its burden of sediment westwards. A point is reached about 200 miles from the Dividing Range, where the river ceases to act as a denuding agent, and the area of deposition begins. But the river is still about 1000 miles from the sea. The Darling is reckoned amongst the longest rivers in the world, for it is navigable, part of the year, from Walgett to its confluence with the Murray, 1758 miles, and then to the sea, a further distance of 587 miles,—making in all 2345 miles of navigable water. But this gives no correct idea of the true character of the Darling, for it can hardly be said to drain its own watershed. From the sources of its various tributaries to the town of Bourke, the river may be described as draining a watershed. But from Bourke to the sea, 550 miles in a direct line, the river gives rather than receives water from the country it flows through.

The annual rainfall and the catchment area afford no data whatever as to the size of a river in the interior of Australia. In Europe it has been estimated that from 20 per cent. to 50 per cent. of the rainfall flows away in rivers. The discharge of the Darling River at Bourke does not amount to more than 10 per cent. of the rainfall of its catchment area. It was this startling fact which first led to the idea that, as the rainfall could not be accounted for either by evaporation or by the river discharge, much of the unaccounted-for 90 per cent. must in part sink into the ground, and in part be absorbed by some underlying bed-rock. All Australian rivers depend entirely and directly on the rainfall. They are flooded after rain, and in seasons of drought become dry water-courses. Springs which equalize the discharge of rivers by continuing to pour water into their beds, long after a rainy season,