In the winter months, conditions are greatly changed. There is no longer a heavy rainfall in the north, and, while the trade winds still bring a certain amount of moisture to the east coast, they do not penetrate far inland. But, as the whole wind system has moved northwards, the south-west of Western Australia, the southern part of South Australia, and the west and south of Victoria now lie within the belt of westerly winds, and receive their rainfall from the various cyclonic disturbances which move' across the continent from west to east.
As a result of these conditions, the north and east coasts of Australia receive an average annual rainfall of at least 40 inches. This amount rapidly decreases inland, both towards the south and the west ; and over part of the Great Plains and most of the Western Plateau the precipitation does not exceed 10 inches per year. In the region of winter rainfall, the south-west corner of Western Australia and part of Victoria have over 20 inches ; but elsewhere, less than that amount falls.
IRRIGATION.—On considering these facts it is obvious that a large part of the continent is destitute of sufficient supplies of moisture to permit of its settlement ; and that other parts, which receive in years of average rainfall the minimum amount necessary for this purpose, are in years of low rainfall subject to great disaster. Efforts have therefore been made to augment, by irrigation, the water supply of the marginal districts. The rivers offer no adequate means of so doing. On the north and east coasts they are perennial, but in the interior the only one which does not fail in times of drought is the Murray-Darling, which is snow-fed, and, although its waters are being utilised, it can never provide for more than a mere fraction of the waterless area. Another source of supply from which much has been hoped lies in the artesian basin of the Great Plains. In the Cretaceous system of that region an important series of rocks is composed of marine clays, and is known as the Rolling Downs formation. In the underlying sandstones, great supplies of water at high pressure are prevented from reaching the surface by the impermeable nature of the Rolling Downs, but when these are bored through, as has been done in Queensland, New South Wales, and South Australia, the water rushes upwards with great force. Concerning the origin of this water, there still is much dispute. Professor Gregory maintains " that much of this water is not rain-water which has worked its way downwards ; but it is plutonic water, which has risen from the deeper layers of the earth's crust ; and that the water rushes up the wells owing to the tension of its included gases and the pressure of the overlying sheets of rocks." This view is vigorously opposed by most Australian geologists, who hold that much of the rain which falls on the Queensland hills finds its way to the south coast of Australia by great subterranean channels, and that it is these channels which have been tapped by the artesian bores. The practical importance of the dispute is,
that, if the first theory be true, the supply of water is not necessarily inexhaustible, while, if the second be true, there would seem to be no reason why steps should be taken to conserve the outflow. Un fortunately, owing to its mineralised character, the water obtained from many of the wells is unsuitable for the irrigation of crops, and its chief use is for the watering of stock, the keeping open of stock routes across the interior, wool-scouring, domestic purposes, and in some cases the cultivation of lucerne. On the Western Plateau, many shallow wells which collect rain-water have proved of great value to the development of the pastoral industry.
VEGETATION.—The distribution of plant life follows that of rain fall in a marked degree. On the exposed slopes of Victoria and New South Wales grows the temperate rain forest, among the most important trees of which are numerous species of the genus Eucalyp tus, including iron-bark, black-butt, and various gums. Over the remainder of the north and east coasts of Australia and for a varying distance inland, the prevailing type of vegetation is savanna forest, distinguished alike by the gigantic eucalypti of which it is composed, and by the rich growth of grass between the trunks of the open wood. In the north there are, in addition to the eucalypti, various palms, bamboos, and other trees belonging to a more tropical climate. Further inland, including, and extending beyond, the area with a rainfall of ten to twenty inches, lies a great grassland region. Here the trees generally occur only as an edaphic formation, while the grass is xerophilous in character. In some places the grass gives place to scrub, such as mallee and wattle, and in others to salt-bush, a herb on which sheep manage to thrive. Over a great part of the remainder of Australia, desert or semi-desert conditions prevail. The Western Plateau is covered in places with mulga scrub, but the characteristic vegetation over wide areas is spinifex : " spinifex in low straggling tussocks, or in high round compact stools . . everywhere in loathsome profusion grows that most useless and unattractive plant." The region of winter rains in south-west Australia has an evergreen sderophyllous forest in which jarrah is an important tree, but from there eastwards to the similar forest in Victoria the south coast is bordered by scrubland except on the Nullabor plains where savanna prevails.