[Proc. Eoy. Soc. Victoria, 32 (N.S.), Pt. I., 1919]. 



AuT. V. — A Striking Example of Rock Expansion by Teniper-- 

 ature Variation in Sub-Arid Western Australia. 



By J. T. JUTSON. 



(With Plate II.). 



[Read 10th July, 1919]. 



A rather remarkable example of rock expansion, due to tem- 

 perature-variation, having come under the writer's notice in sub- 

 arid Western Australia, its record may be of value. It occurs 

 about ten miles to the north-north-Avest of Comet Vale, a niining 

 township about sixty miles north of Kalgoorlie, on the Kalgoorlie- 

 Laverton railway line. It is, therefore, situated well in the in- 

 terior of Western Australia, in an area of low rainfall and of great 

 temperature variations. 



The rock is a biotite granite, which outcrops as a bare rounded 

 hill rising to an inconsiderable height above the surrounding 

 plain. This rock is, therefore, constantly exposed to the weather; 

 and, owing to the great variations in daily and nightly tempera- 

 ture, it peels off in layers or bands of various thickness, with the 

 result that the rock assumes the well-known rounded appearance 

 characteristic of granite. This peeling or flaking ofi is the " des- 

 quamation " of Richthofen.i 



The particular occurrence referred to is a surface slab of 

 granite, on the lower slope of the hill, abutting a " gnamma," or 

 natural rock-hole. This slab is 10 feet long, two feet six inches- 

 Avide, and ranges from one and a half to four inches in thickness. 

 It is separated from the parent mass, except at its ends (which' 

 here mean the terminations roughly at right angles to its length). 

 These ends pass into rock of similar character, but the slab rises 

 in a gentle curve toAvards the centime, Avhere it is not resting on 

 anything (except possibly at one point on a loose boulder that has 

 drifted into the cavity). On the uphill and downhill sides of the 

 slab, at the centres of the respective sides, the height of the lowe»- 

 face of the slab from the surface of the solid rock below is seven 

 inches and four and a-half inches respectively. The slab is cracked 



1. Hume, y^. F., "Professor Walther's Erosion in the Desert Considered."' 

 Geol. Mag., Decade VI., Vol. I., Nos. 595-6 (1914), p. 21. 



