112 J. T. Jutson; 



fined channels, at times flow, and these are quite unable to move: 

 the rock debris 3 . 



Such debris seems to travel slowly along by mere gravitational 

 drift, aided or brought about by the removal of the underlying, 

 soil by rain and wind, on account of which the fragments topple 

 forward. The "clawing" action of rain, which the writer has 

 already described 4 , is a potent factor in this respect. 



As a general rule there is no particular arrangement of the 

 rock debris except in the case of the remarkable mosaics, or 

 "desert pavements," and in the example to be now described 

 of the drift of the rock debris in parallel lines. 



Gravitational Drift of Rock Debris in Parallel Lines. 



The example of this parallel drift occurs on the floor of 

 Lake Goongarrie, a " dry " lake immediately to the east of the 

 mining township of Goongarrie, which is 55 miles north of 

 Kalgoorlie. The bed rock is a compact, almost black, shale,, 

 somewhat indurated, which breaks easily under the hammer in 

 hand specimens, but which, when it forms a floor, as it does here, 

 is quite firm. The shale is well laminated and strikes about 

 N. 10° W. ; it is "practically vertical, but dips if anything to the 

 west. At the precise locality referred to, it is quite free from, 

 debris, except for the quartz detritus to be presently described.. 

 In the shale is a quartz reef, with approximately the same strike 

 and dip as the shale itself. This reef is a foot or more thick, and 

 some yards in length. It projects from two to four feet above 

 the shale. The reef is breaking down into fragments of various 

 size, and these fragments drift over the shale away from the reef.. 

 This drifting material protects, to some extent, the shale from 

 erosion, so that the ground rises into a low hillock, culminating 

 in the quartz reef, a few feet above the surrounding ground. 

 It is on the eastern side of this reef that the example referred 

 to occurs, and the greatest inclination (which however, is only a 

 few degrees) of the ground on this side of the reef is to the 

 east, and at right angles to the strike of the shales. Water 

 falling on the hillock on the eastern side of the reef therefore 

 flows across and not with the strike of the shales. 



3. See Jutson, J. T. — "Sheet-flows, or Sheet-floods, nnd their asso- 

 ciated phenomena in the Niagara District of sub-arid south- central "West- 

 ern Australia." Am, Journ. Science, Vol. XLVIII (1919), pp. 435-439. 



4. Proc. Roy. Soc. Vict., Vol. XXXII. (N.S.), Part I: (1919), pp. 20-21. 



