CXViII. IRRIGATION GEOLOGICALLY CONSIDERED. 
the authors,’ and the following extracts will convey an 
idea as to their general character:—‘‘ The sandhills which. 
vary from small mounds to hills 50 feet in height are 
formed of blown sand. . . . There can be no doubt 
that the sand of which these hills are formed is due to 
the disintegration in Post Tertiary times of the Upper 
Cretaceous or Desert Sandstones. The claypans, which 
are invariably met with in proximity to the sandhills, are 
shallow, flat bottomed depressions; they vary in depth 
from a few inches to three feet, and their floors consist of 
a thin bed of fine clay, upon which the water lies for a 
considerable time after rain. They are often quite circular 
in shape, while, in other instances they form long channels 
of regular width. . . . It seems probable that they 
have been formed by the whirlwinds (the Burramugga 
of the aborigines) which are of very common occurrence in 
this country. Some of these whirlwinds remain stationary 
for a considerable time (forming columns of whirling sand, 
sometimes a quarter of a mile high), which suggests the 
formation of the circular depressions—while a travelling 
whirlwind, such asis frequently met with, might be expected 
to sweep up the sand in such a manner as to form one of 
the long narrow channels. The depressions having thus 
been formed, subsequent rains would carry into them, in a 
state of suspension, fine clay washed out of the surrounding — 
sandy soil. When the water was afterwards evaporated 
by the heat of the sun, or had sunk into the floor of the . 
depression, a coating of clay would be left, and frequent | 
repetitions of this process would leave a fairly thick bed q 
of impervious clay.” | 
Mound Springs or Mud Springs.—These have been 
described by one of the authors in the work already referred 
1 <The Mineral Resources of New South Wales,” by E. F. Pittman, 
1901, pp. 465 — 466. 
