ARID PORTIONS OF SOUTH AUSTRALIA. 21 



associated with poor surface drainage. These areas are so numerous 

 and may be so extensive that they constitute a very important portion 

 of the habitats of the interior. The non-sahne habitats are to be dis- 

 tinguished from one another in part by physical and chemical com- 

 position and in part by their phj^siographical relations. The leading 

 differences between them, at least from a biological point of view, rest 

 mainly on differences in their water-content and (probably also asso- 

 ciated with this) on their relative temperatures. Thus there are 

 stream-ways and flood-plains and often terraces, plains of several 

 levels. The first two are subject to occasional flooding, but the latter 

 may or may not receive water by seepage from still higher ground. 

 Other plains may occupy the highest elevations and thus may have 

 water relations quite different from those of the plains last mentioned. 

 The plains may, or may not, be protected against wind erosion by a 

 covering of coarse stones, "gibbers." There are also hills and low 

 mountains and the slopes of these. Of the hills, the moving or sta- 

 tionary dunes constitute important features of the physiography of the 

 dry interior. Between the sandy ridges there are often fiat ' ' clay-pans" 

 which have interesting features of their own. Ina,smuch as the species 

 to be found in these habitats are often, possibly largely, characteristic 

 of them, the subaerial environment constitutes a very important factor 

 in the environment as a whole. 



The subaerial environment of plants thus has interesting connection 

 with surface geology and its history would be that of physiography. 

 Without entering into a discussion of this phase of the matter, however, 

 it will be instructive to note certain characteristics of the dynamics of 

 the general subject. Thus, Howchin and Gregory (1909 : 103) point out : 



"An inland basin, like that of Lake Eyre, can not get rid of its worn-down 

 material, such as occurs when the drainage of the country flows into the sea, 

 whilst from a deficiency of moisture vegetation is scarce and the soil is but 

 loosely held together. From this cause the soil and sand are constantly on 

 the move, and with the ever accumulating products of waste, the highest 

 hills are gradually covered by drift, and the country is ultimately buried under 

 its own ruins." 



But the region of the sandhills is not confined to such a depressed 

 area as the great central basin. Thus D. W. Carnegie (1898:178) 

 describes in a very vivid way a sand plain-sandhill region in central 

 Western Australia, nearly 200 miles across in a straight line. Here the 

 general level of the country is considerably above sea-level, but the 

 drainage is inland, or at any rate not directly to the sea, and it can 

 possibly be described as being undeveloped. Without going into the 

 subject much further, another region can be mentioned having an 

 accumulation of detritus (and sand is here especially in mind), where 

 the drainage is not well defined. This is the Ooldea sandhill region. 

 Here are sand ridges of prominence which alternate with narrow flats 

 over a region about 50 miles wide. Apparently the rains are absorbed 



