162 J. T. Jutson : 



the ridge. There is thus a close resemblance between the two 

 types — the quarry under arid, and the steeji slopes and gulches 

 under normal, erosion; but marked differences also exist. In 

 normal areas, the lower drainage line or channel is wider and 

 more strongly marked than the upper portion running up towards 

 the crest line, and if the upper portion be branched, the branches 

 are separated by a ridge. In the quarry, however, there is a wide 

 scooping out with no distinct bisecting ridge.'i Moreover, the 

 quarry is the clearly marked feature, the lower drainage line often 

 being insignificant and difficult to trace by reason of the tendency 

 of the occasional water flows to spread themselves over a compara- 

 tively wide belt of the lower country. 



The triangular quarries have a moderate range in size. They 

 are from 10 to 15 feet in all dimensions, to a large quarry having 

 a " back " perhaps 100 feet in length measured horizontally along 

 the top. They probably approach equilateral triangles in shape, 

 but the base (the " back ") is, the w^riter lielieves, usually longer 

 than the sides. 



This type of quarry does not, as a rule, form in granite. Much 

 weathered "greenstones" or greenstone schists, with a cap of 

 hard ironstone, appear to be the most suitable rocks for their for- 

 mation. 



Mode of Fonnation. 



The action of the rain in beating upon and undermining decom- 

 posed comparatively soft rocks, which ai'^ associated in definite 

 Avays (to be presently stated) with cei'tain harder rocks, is appar- 

 ently the main factor in producing tlie three types of natural 

 quarries above described. 



The circular quarry originates in most instances at least in the 

 following way : On the face of a " breakaway," the detritus from 

 the hard surface layers and from the rocks below slowly drift 

 towards the bottom, and in doing so the whole face of the " break- 

 away " may at any particular period of time be covered with this 

 detritus or talus to a thickness of about one foot or less.- Owing 

 to the widespread tendency in sub-arid Western Australia to 

 cement all loose surface deposits by mineralised water rising to the 



1 It must be remembered that these remarks only apply to the triaii-ular natural quarries 

 here described, and that gullies with normal branches in every way similar to those formed in 

 wetter climates are numerous in hilly country in the dry areas. 



2 A gfreater thickness than one foot may occur, but under such conditions it is doubtful 

 if a natural quarry would form. 



