ARID PORTIONS OF SOUTH AUSTRALIA. 



PHYSICAL ENVIRONMENT OF THE VEGETATION OF 

 AUSTRALIA. 



The vegetational environment of Australia, including the more 

 arid portions, has a complex geographical background. The island 

 continent is separated biologically, as well as physically, from other 

 continents and has been so separated for an immense period of time. 

 Long geological ages also have passed since a large portion of the 

 surface was covered by the sea. The physiography is relatively 

 monotonous, as might be expected from the fact that possibly the area 

 may be regarded as a vast peneplain. The latitudinal situation also is 

 of importance in influencing, really in shaping, the leading characteristic 

 of its climate. Projecting as it does far into the interior, the state of 

 South Australia shares in the general physical characteristics of the 

 continent, but it also holds in certain regards a peculiar relation to the 

 sister states. Its southern shores are washed by the cool seas, while the 

 northern boundaries are parched and baked under a tropic sun. It has 

 the most typically continental climate of all the states (Howchin and 

 Gregory, 1909:17). Such are some of the features which have con- 

 stituted and which now constitute the broad characters of the physical 

 environment of the vegetation of Australia, including that of the 

 central state, and under which by physiological reactions there has 

 slowly developed the vegetation familiar to us at the present day. 



GENERAL PHYSIOGRAPHICAL CONDITIONS. 

 The general physiographical conditions of Australia have many 

 points of interest in connection with the present paper. Gregory 

 (1916:25-27) states that: 



"Australia as a whole is a great plateau land. It is a fragment of a large 

 continent, the rest of which has been snapped off along great fractures. 

 . . . The mountain system is not determined by any dominant lines of 

 folding of the earth's crust, like those which have formed the Alps and the 

 Himalayas. Australia was folded at an early period in the earth's history; 

 and all its ancient fold-mountains have long since been worn down. The chief 

 existing features in the relief of Australia are due to vertical earth-movements, 

 by which some parts of the area have been raised to high plateaus and others 

 have sagged downward into deep basins. The margins of the plateaus have 

 been carved into valleys. . . . The eastern margin of the old plateau 

 has been dissected by powerful streams into deep valleys, which are separated 

 by steep-sided and flat-topped ridges ; and in some districts river erosion has 

 been so active that very little of the original surface has been left. . . . 

 Western Australia, on the other hand, owing to its smaller rainfall and feebler 

 rivers, retains more of the old plateau surface. . . . The inner part of 

 the plateau is a vast gently undulating country, with low rounded hills, except 

 where some hard wind-etched boss of rock rises abruptly from the plains. 

 Wide, shallow depressions run together like the converging branches of a river; 

 and these valleys are divided by the irregularities of their floors into basins, 

 which in wet seasons may contain lakes of little depth; but usually they are 



