82 PLANT HABITS AND HABITATS IN THE 



the west. It is 427 miles from Port Augusta and about 80 miles, in a 

 direct line, from the Great Australian Bight. The main features of 

 physiographical interest are the sandhills to the east and the Nullarbor 

 Plain on the west. It lies thus at the division between the two. 



The Nullarbor Plain is a formation of very great interest and unique- 

 ness. Its east-west extent is about 450 miles and its north-south 

 extent about 200 miles. The plain is a sea-floor, of the Mesozoic 

 age, underlain by Paleozoic rocks (Jutson, 1914:71). Rocks of the 

 same age outcrop to the west of the plain in Western Australia, and 

 passing beneath the plain appear again about 100 miles to the east 

 and again at intervals farther toward Port Augusta, as at Tarcoola. 

 The Nullarbor Plain is composed of limestone of a thickness approxi- 

 mating 500 feet. The surface of the plain is nearly level, as would 

 appear from the fact that the transcontinental railway runs for 330 

 miles, nearly across it, without a curve. The topography, however, 

 is not absolutely flat, but is very gently rolhng and dips (less than 

 1 foot to the mile) to the east. The Umestone is covered by about a 

 foot of red soil, although fragments of the underlying rock are scattered 

 about on the surface. 



The most striking feature of the plain, however, aside from its 

 level character and great extent, is the presence here and there of 

 slight depressions, known as "dongas," which vary in size from a few 

 to hundreds of acres. There are, in addition, as would be expected in 

 the limestone country, various fissures or openings of various shapes 

 and sizes which communicate with subterranean cavities. Such open- 

 ings are locally known as "blow holes" from the air-movements asso- 

 ciated with them. The "blow holes" often appear to be situated in 

 depressions of small extent. From the structure of the plain it will 

 be seen that such water as falls on it must quickly disappear from the 

 surface. The soil is so thin that it constitutes a very inadequate 

 water-reservoir. In the dongas, however, there is a greater accumula- 

 tion of soil and hence greater possibiUties in the way of water-storage,' 

 and in such situations the vegetation is relatively abundant and 

 wholly unlike that of the surrounding plain. 



The sandhills region east of the Nullarbor Plain has an east-west 

 extent of 100 miles, more or less, and a larger extent in the north- 

 south direction. As seen from Ooldea and from the railways trav- 

 ersing it, the region has a certain monotony. The hills are of nearly 

 equal size and run from north to south, or approximately so. A 

 hardpan resembling travertine, or desert limestone, is often present, 

 lying about 1 meter beneath the sandy surface. In the fairly deep 

 hollows between the ridges, however, the sand appears to be deeper 

 than on the ridges themselves. A short distance to the north of 

 Ooldea, where an impervious substratum underlies thq hollows, water 

 collects. This may be either brackish or "sweet." Such a place is 



