432 J. T, Jutson — Rounding of Pebbles. 



pebbles seems to be the direct action of the rain in beating 

 npon the surfaces of the rocks (in the case of the green- 

 stone when the iron crust has been broken) and wearing 

 them away without any rock particles to act as abrasion 

 tools. Under this action the corners are most likely to be 

 first removed and hence the rounded appearance. There 

 may also be some latent structure in the rock which, de- 

 spite the angular jointing, assists rounded weathering; 

 but this would hardly seem to apply to the thin-bedded 

 sediments and decomposed foliated porphyries. The 

 unrounded irregular under surfaces of many of the sedi- 

 ment and porphyry pebbles may be accounted for by the 

 fact that the rain cannot beat directly on such surfaces. 

 Where, however, a pebble has been rounded on its whole 

 surface, this appears to be due to all portions having 

 been directly exposed at different times to the rain, ow- 

 ing to the rock fragment which became the pebble having 

 changed its position by reason of the slow gravitational 

 movement, which all loose fragments are subject to, and 

 which is especially well illustrated in sub-arid Western 

 Australia. 



The greenstone pebbles are also more abundant and 

 freer from iron oxide films at places such as the base of 

 a hill or on a tiny bench on the hillside, where the flow 

 of the surface rain water has been more concentrated 

 than usual, or where it may have lodged for a little while. 

 The flow, however, has been so insignificant that it can- 

 not have any direct abrasive power akin to normal 

 stream action, but it seems to act in keeping the surfaces 

 of the rock fragments largely free from the deposition 

 of iron and hence assisting their rounding by the rain. 

 At the foot of the hills, percolating meteoric water may 

 ooze out and act in the same way. Certainly in other 

 places than those mentioned the pebbles have more iron 

 oxide films, and their disintegration must therefore be 

 retarded. It is the occurrence of the grey pebbles at the 

 foot of lake cliffs that suggests, until a closer investiga- 

 tion has been made, that they are normally waterworn 

 pebbles due to their abrasion by stream, lake or old sea 

 waters. 



Wind action has probably had some slight effect on the 

 pebbles, but that need not be discussed here. 



Fragments of decomposed rocks from the oxidized 

 zone thrown out from mining shafts sunk on the West- 



