Hydrography. In its hydrography, Australia is peculiar, in the fact that two-thirds of its area — indeed all of it, excepting strips of varying breadth on the east, north, and west coasts — has no drainage to the sea. In all this vast region of nearly 2,000,000 square miles, comprising the whole interior of the continent, and most of the southern coast strip, the scanty rain that falls is absorbed by the thirsty soil. In this region are rivers of great length, which, in rainy seasons, may be of considerable volume, but the desert sands absorb their entire flow. On the east or seaward slope of the Great Divide are many short rivers, but none of them are of importance for navigation, as they run very low at times. Among them are the Brisbane. Burnet, Titnoy, Burdekin, Herbert, Normanby, and Kennedy. The only river rising in the west side of this mountain system which reaches the sea is the Murray, with its great branch, the Darling; this is far the largest stream of the continent, having a drainage area of 300,000 square miles; with the Darling, it is navigable at high water for 1700 miles. The Murrumbidgee, another large branch, is navigable for 500 miles at high water. The south coast for nearly 1000 miles has no stream flowing to the sea. On the west coast the streams are short, and are dry, except in the rainy season. On the north coast the principal streams flow into the Gulf of Carpentaria: these are the Leichman, Flinders, Roper. Gilbert, and Mitchell. Into Queen Channel flows the Victoria, while farther north are the Adelaide and Daly rivers. All these streams are long, but are torrential in character, and of no value for navigation.
The rivers of the interior, though of no service as means of communication, may be put to use for irrigation purposes as settlement progresses; among them are the Cooper and Macumber, flowing into Lake Eyre, the first of which is not less than 1200 miles in length, the Herbert, and Diamantina. There are many salt lakes in the interior, which serve as sinks for the streams which flow into them. The largest of these is Lake Eyre, in South Australia, which receives the waters of many streams from a large area of surrounding country. Its surface is said to be a few feet below sea level. South of it are lakes Torrens and Gairdner, and far to the northwest is Lake Amadeus.
Geology. The Great Dividing Range is composed mainly of Silurian beds. with, however, large areas of Carboniferous formation, while in the neighborhood of Sydney and Ipswich, New South Wales, are large areas of what are probably Jurassic beds. Throughout the range intrusions of granite are abundant. The Darling Range, on the west coast, is composed of more recent beds, probably Jurassic. East of this range, stretching far into the interior, the country is overlaid irregularly with granite, and the same rock appears in many, if not most, of the mountain ranges of the interior, breaking through the Tertiary and Cretaceous beds which floor the plains and plateaus of this region. The distribution of these last two formations in the interior may be broadly characterized by the statement that the interior portions of Queensland and New South Wales are underlain mainly by Cretaceous beds, while the interior of Western and South Australia is mainly covered with Tertiary formations. There are, however, considerable tracts in the interior which are covered by older rocks. Thus, in the southern part, north of Spencer Gulf and the Gulf of Saint Vincent, is a large area of Paleozoic beds extending inland as far as Lake Eyre, while from the head of the Gulf of Carpentaria an area of the same age extends southward. Tasmania is composed mainly of Silurian strata; Carboniferous and Jurassic beds and older crystalline rocks appear, however, both in the interior and on the east coast. Gold-bearing rocks and other valuable mineral deposits are widely disseminated throughout Australia.
There are no active volcanoes on the continent, but there are several areas of extinct volcanoes. They arc very numerous in the State of Victoria in the southeast, where they have had much to do with shaping the present conformation of the land. There are but few in New South Wales, although outflows of basalt through low vents were numerous and extensive in that colony during Tertiary times. In Queensland the volcanic area is extensive, especially in the interior. In South Australia is the well-known volcanic group which has Mount Gambier for its central figure.
Climate. Australia lies half within the tropics, and half within the South Temperate Zone; while it has a wide range of temperature and rainfall, this is not attributable to the range in latitude, but to the topography of the continent. The rainfall is heaviest upon the north and east coasts, while upon the south and west coasts, except upon the southwest corner of the continent, and the eastern part of the southern coast, it is light, and throughout the interior it is almost wanting. Thus only a small part of the whole area has sufficient rain for agriculture. The northern part of the east coast enjoys a tropical rainfall amounting to 80 inches or more annually; southward it diminishes, being 50 inches at Sydney, and at Melbourne 25 inches. The rainfall diminishes inland along the whole coast. The east coast of the Gulf of Carpentaria has about 40 inches, while the west coast has 50 inches or more. Most of the south and west coasts have less than 20 inches, while most of the interior has less than ten inches. The rainfall is exceedingly irregular in all parts of the island, even upon the east coast, and the streams are correspondingly torrential in character. Most of the moisture brought to the island comes with the trade winds from the southeast, and is of course deposited on the east and north coasts, whose mountain ranges strain nearly all the moisture from the air currents, leaving them to flow as dry winds over the plains of the interior. The moisture in the southwest and in the State of Victoria on the south coast, on the other hand, is brought by the prevailing westerly winds, whose influence extends up the west coast only as far north as Perth.
Australia presents also extremes of temperature. The east coast has a fairly uniform temperature, the mean annual temperature ranging from 80° in the north to 55° in the south, and with a range between the monthly means of July and January of only 15° to 20°. On the other hand, in the interior, while the mean annual temperature ranges with the latitude from 60° to 80°, the contrast between summer and winter is great, reaching 40°, while the mean
