AUSTRALIA Commonwealth of Australia, which includes Tasmania, has an area of 2,974,581 square miles, and is just over three-fourths the size of Europe. In its general form the island continent consists of a plateau, with an average elevation of about 1,000 feet, but a number of distinct physical regions may be recognised. The Eastern Highlands extend from Cape York southwards, and occupy the eastern parts of Queensland, New South Wales, and Victoria, and the whole of Tasmania. They do not form a true mountain range, but represent the dissected escarpment of a fractured pene plain, the eastern part of which has sunk below the level of the ocean. In the north of the range there is a great block of Archaean age ; in the centre Carboniferous rocks predominate, though others, ranging from Archaean to Triassic, are also found ; while in the south the strata belong to lower Palmozoic times with Archaean outcrops.
Between the Eastern Highlands and the sea, there stretches a series of coastal plains composed of materials washed down from the uplands and recently uplifted above sea-level. These plains vary in breadth, but seldom exceed fifty miles and are frequently much less.
The Eastern Highlands present their steep sides towards the east coast, and slope gently westwards towards the Great Plains, which form an area of depression stretching across Australia from the Gulf of Carpentaria to the west coast of Victoria. In the north and centre this region consists of Cretaceous rocks, but in the south it is covered with silt brought down by the Darling, Murrumbidgee, and other rivers during Tertiary times. Of the Cretaceous rocks the Rolling Downs formation, as will be shown later, is of great importance because of its influence upon the develop ment of irrigation. The northern part of the Great Plains drains to the Gulf of Carpentaria, the southern part to the Murray, and the western, which is an area of inland drainage, to Lake Eyre. West of the Great Plains, and of the South Australian Highlands which lie to the south of Lake Eyre, the Western Plateau covers the greater part of the remainder of the continent. It consists, in the main, of a block of Archaean age which does not appear to have been under water since early geological times. In the north east, however, considerable tracts are covered with lower Palaeozoic rocks, while, in the south, the Nullabor plains, which make a great indentation in the plateau, belong to the Tertiary period. Along
the west and north-west coasts there are narrow plains, also of Tertiary formation.
CLIMATE.--The climatic factor exercises a very important control upon the economic development of Australia. The continent lies between the tenth and the fortieth parallels, and from one-third to two-fifths of it falls within the tropics. During summer the heat of the continental interior is very great, and in some districts the maximum shade temperature may be above 100° F. for days, or even weeks, continuously. In January, the hottest month, the isotherms range from 64° F., in the south, to over 90° F., in the northern part of the Western Plateau. Variations of temperature are naturally greatest in the interior, and, south of the tropic, ground frosts occasionally occur at night. On the lowlands, and more especially around the coast, these are rare, and snow, though it occasionally falls in the south-east, never lies. In the highlands it is otherwise, and in the Australian Alps of New South Wales and Victoria the ground is white for several months each year. In July, the coldest month, the continent lies between the isotherms of 45° F. and 80° F.
The distribution of rainfall is determined by several factors. In summer, when the equatorial low pressure belt has moved southwards, and when the area of minimum pressure lies over the northern part of the Western Plateau, the trade winds of the northern hemisphere are pulled across the equator and blow as north and north-west monsoons. These bring much moisture, especially to the northern shores of the continent, though their influence is felt far to the south. At the same time, the east coast of Australia receives a considerable amount of rain from the south-east trade winds which blow upon it. On the west coast, on the other hand, the precipitation at this time of the year is very slight, as the winds, which blow towards it from the high-pressure area over the sea, are either turning round into the trade-wind system, and therefore away from the land ; or, as they have previously crossed the cold current flowing along the west coast of Australia, they are heated by their contact with the land, and therefore do not deposit the moisture they contain.