J. T. Jut son — Rounding of Pebbles. 429 



Aet. XXVIII. — Note on an unusual Method of Bounding 

 of Pebbles in sub-arid Western Australia; by J. T. 

 Jutson. 



Introduction. 



The observations recorded in this paper are based on 

 pebbles f onnd at Goongarrie and Comet Vale 1 at the base 

 of cliffs on the western shores of Lake Goongarrie — a 

 "dry" lake or playa — and on adjacent hills. The places 

 referred to are in that part of snb-arid "Western Aus- 

 tralia where the average rainfall does not exceed 10 

 inches per annum. 



Description of the Pebbles. 



For the purposes of description, the pebbles may be 

 divided into two classes: — (a) greenstones, 2 chiefly fine- 

 grained, and (b) soft sediments (comprising grits and 

 shales) and soft decomposed foliated quartz porphyries. 



The greenstone pebbles occur at Goongarrie on the 

 slopes and tops and at the foot of the greenstone hills, 

 which form the western border of Lake Goongarrie, 3 and 

 which rise about 100 feet from the lake country. They 

 vary in diameter from an inch to three or four inches, 

 and may be roughly circular, elliptical or lens-shaped in 

 outline. Some pebbles have the whole surface well 

 rounded, but others have their surfaces angular and un- 

 even. At the foot of the hills and occasionally on small 

 ledges on the hillsides the pebbles are often greenish- 

 gray (the natural color of the rock), and almost devoid 

 of any foreign coat of film such as iron oxide; a fact 

 which makes such pebbles more conspicuous than those 

 elsewhere on the hills, as the latter pebbles are usually 

 more or less coated with a film of iron oxide, which har- 

 monizes with the prevailing color of the surfaces of the 

 rocks around. The pebbles are scattered but fairly 

 numerous ; yet no distinct accumulation anywhere occurs, 

 except possibly on a small scale at the foot of the hills. 



Most of the fine-grained greenstones in the interior of 



1 The township of Goongarrie is 55 miles and that of Comet Vale 63 

 miles north of Kalgoorlie. 



2 " Greenstones " is a common field term in Western Australia for a 

 variety of basic igneous rocks, such as dolerites, gabbros, amphibolites, 

 and other similar rocks. 



3 The normal main lake does not reach to the foot of the hills, where 

 most of these pebbles have been found, but what is practically a westward 

 extension of the main lake does so reach. 



