120 J. T. Jutson: 



western border of the lake, and there reach a height of about 100 

 feet. 



The lower ones, composed of the altered sediments and porphy- 

 ries, occur mostly along the western margin of the main portion of 

 the lake. Both groups are receding westward. 



(2) Eock Floors. — The rock floors may be divided into three 

 groups, natnely — (a) those of the main portion of the lake, (b) those 

 of the arms of the lake between the regular sand ridges, and (c) 

 those associated with the quartz reefs. The rock basins are sepa- 

 rately described. 



(a) The rock floors of the main portion of the lake occupy a 

 considerable area of its western part, being traceable for a mile 

 from west to east along a line just north of the Boddington Island. 

 They are either almost wholly devoid of any detritus or possess a 

 mere film, or a thickness of two or three inches of fine silt. In 

 places they are slightly furrowed along the strike of the rocks, but 

 this does not destroy the " billiard table " character of the floors. i 



The actual plane of the surface may be slightly inclinedi or undu- 

 lating, but precise levelling is needed to determine the directions of 

 slope. So horizontal, how^ever, are some of the rock surfaces that 

 Avhen rain falls (unless it be long continued) it simply rests on 

 such surfaces without flow. This feature was observed by the writer 

 in a part of the lake outside its main portion. 



(b) The east-west trending arms of the lake associated with the 

 regular sand ridges are rock-floored wholly or partly (except that a 

 veneer of quartz debris from quar-tz reefs may lie upon them in 

 places). Towards the eastern ends of the arms the rock floors are 

 not always visible, as they may be buried under fine silt, which is 

 believed to be noAvhere in the arms more than a foot thick. The 

 arms at their western ends in some instances break up into smaller 

 irregular arms, which in places pass gradually into the " lowlands " 

 previously referred to. 



(o) The rock floors associated with the quartz reefs occur in the 

 western portion of the lake, and mostly beyond the western boundary 

 of the main portion of the lake. Although tliey may be regarded as 

 portions of tlie lake, they yet form a number of more or less inde- 

 pendent areas, owing to the occurrence of numerous and prominent 

 quartz reefs whicli are from two to twenty feet high. These reefs, 

 under exposure to the atmosphere, break up fairly rapidly, and con- 

 sequently have a mass of debris around them which tends to cover 



1 See Jutsoii, J. T., Geofj. Journ., December, 1917. 



