ARID PORTIONS OF SOUTH AUSTRALIA. 53 



stones, it may well be travertine or desert limestone. The nature of 

 the sandhills has already been mentioned and need not be taken up 

 further. As to the clay-pans of the sandhill neighborhood, it need only 

 be said that in them, as elsewhere where the drainage is poor or lacking, 

 there is an excessive accumulation of salts. In the clay-pans the flood- 

 ing water is at first fresh, but as it evaporates it becomes highly charged, 

 and upon drying leaves the salts in the fine silt or on its surface. In 

 other situations during dry seasons salts are frequently exposed in 

 considerable quantity, especially under conditions of poor surface- 

 drainage. It seems probable, however, that where the salts are not 

 present in the soil solution in sufficient quantity to form a visible crust 

 upon the surface of the ground when the water has evaporated, as on 

 the lower plain especially, yet they are relatively abundant. This 

 characteristic of the soil of all arid regions is probably nowhere more 

 marked than in the vicinity of Oodnadatta. 



So far as regards the water relations of the different physiographical 

 areas mentioned, it would appear from inspection and from the 

 character of the soil that they may be said to be as various as the 

 areas themselves. At the one extreme should be placed the flood-plain 

 of Neales River, which receives not only water directly from the rains, 

 but also such as comes to it from the drainage of the river system of 

 which it is a part. The gradient of the flood-plain is apparently slight, 

 the soil is deep and relatively coarse-grained, and there is no continuous 

 and well-defined channel at the place visited. All of these features 

 make for the maximum reception, storage, and retention of water. At 

 the other extreme should probably be placed the flat-topped hills, 

 representing the upper plain. Here the water is merely lost and not 

 received by run-off, and the soil is moistened directly by the rains and 

 in no other way. Percolation of the rains into the soil is indeed 

 forwarded by the presence of stones of various sizes, but is relatively 

 slow because of the fineness of the soil. Only in the slight depressions 

 which are the centers of diminutive drainage systems is there appre- 

 ciable accumulation of water after rains ; otherwise, there are no proper 

 soil reservoirs for water retention. However, owing to the closely 

 fitted mosaic, the "gibbers," the water is conserved very much better 

 than would otherwise be the case. 



The sandhills and the lower plain occupy an intermediate position 

 with respect to the water relation, although their relative position is 

 not very clearly defined. So far as regards the quality of the soil 

 solution and the retention and storage of the soil moisture, the sand- 

 hills are to be considered less arid than the lower plain, but where 

 the latter joins higher ground, as the upper plain, it may receive 

 seepage water, and in this regard only the water relation of the lower 

 plain can be said to be more favorable than that of the sandhills. Other 

 than this the water relations of the lower plain are about the same as 



