INTERESTING FORM OF SUBSURFACE DRAINAGE. 827 



nel along great lengths of the course, though no trace of outwash 

 material at lower levels could be found at any point. At 

 Greenruount, in one instance, a small tree on the course had been 

 completely undermined and uprooted (PL xlv., Fig. 1). This 

 would indicate a considerable strength of flow. 



The formation of lines of sub-surface drainage in the Darling 

 Range is to be attributed not to any peculiarity of the soil, 

 which is derived from granites and laterite (1), but to the 

 climate of the region (2), which is characterised by marked sea- 

 sonal rainfall (3). During the hot, dry summer, fissures form 

 in the sub-soil, but do not extend to the soil, owing to its bind- 

 ing of vegetation. During the heavy, winter rains, much of the 

 water sinks straight into the dry ground, which soon be- 

 comes saturated. Relief is afforded by flow along the sub-soil 

 fissures, which soon become widened into definite underground 

 channels. Pressure causes the surface soil to burst at the weak- 

 est spots, with the formation of pot-holes. As the channel is 

 deepened and widened, the roof tends to collapse more or less 

 along the whole length of the original fissure. 



This form of sub-surface drainage (distinct from underground 

 drainage in general) has a certain minor physiographic import- 

 ance. By ignoring the steepest natural slopes it initiates ero- 

 sion in unlikely places, thereby hastening the process of denu- 

 dation. It is probably a factor in the artesian intake of the 

 coastal basin of Western Australia, as by its means, a large 

 volume of rainfall is taken at once into the deeper layers of the 

 piedmont apron of the range during the winter rains. 



REFERENCES. 



1. Jutson, J. T. — An Outline of the Physiographical Geology of 



Western Australia. Geol. Surv. W.A., Bull. No. 61, 1914, 

 pp. 41 -46. 



2. Taylor, T. Griffith — The Australian Environment. Advisory 



Council Sci. and Ind., Commonwealth of Aust., Mem. No. 1, 

 1918, pp. 59-66. 



3. Official Yearbook, Commonwealth of Austr., No. 11, 1918, p. 67. 



EXPLANATION OF PLATE XLV. 



Fig. 1. Trench formed by sub-surface drainage, Greenmount, W.A. 

 Looking downhill. Shows sapling undermined. 



Fig. 2. The same, looking uphill. Shows collapsed roof in fore- 

 ground, earth bridge (remnant of roof) in centre, and con- 

 tinuation of trench in background. 



