430 J. T, Jut son — Rounding of Pebbles. 



Western Australia are rather minutely jointed and under 

 the influence of the weather they break up into thousands 

 of angular fragments, averaging perhaps in size, in all 

 dimensions, about three inches. The faces of the joint 

 planes near the surface of the ground are generally cov- 

 erd with a film of iron oxide, giving a brownish-red 

 appearance to the rocks. The tops and slopes of the 

 hills are littered with these fragments. It is this coating 

 of iron oxide that plays an important part in regard to 

 the pebbles. 4 



The pebbles of the sediments and porphyries, which 

 are usually soft and easily broken, have been observed 

 at the foot of cliffs on the western shores of the lake and 

 at the eastern end of an island in the lake. The pebbles 

 are associated in places with smooth rounded " knife- 

 edges' ' of the rocks in situ. The pebbles are generally, 

 but not always, thin and flat ; are roughly circular, ellip- 

 tical or lens-shaped in outline ; and attain three of four 

 inches in length. The flat surfaces are in places irreg- 

 ular and rough, and in some pebbles, both flat and other- 

 wise, the rounded outlines do not extend over the whole 

 surface. 



The shapes assumed by the pebbles are controlled by 

 the shapes of the original rock fragments. This point 

 has been well brought out for practically all pebbles by 

 H. E. Gregory. 5 



Causes of the Rounding. 



Two hypotheses, which immediately suggest them- 

 selves to account for the rounding, may be considered. 

 They are: — (a) abrasion through the agency of the 

 waters of a lake or an arm of an ancient sea; and (b) 

 abrasion due to stream action. Both are rejected and 

 for the following reasons : — 



(1) — The pebbles are always found close to rocks in 

 situ, with which they are identical in composition and 

 structure, it being quite clear that the pebbles are de- 

 rived from these rocks. In one place, successive thin 

 bands of rock of different texture and composition have 

 been observed to have corresponding pebbles immedi- 



4 Oxide of iron films on rocks affect rock weathering in other ways, 

 which however, the writer proposes to discuss in other papers. 



5 " Note on the Shape of Pebbles," this Journal, vol. 39, pp. 300-304, 

 1915. 



