Features and Growth of Lake Goongarrie. 119 



about eight to tliirtv feet high, and, going westward, pass into 

 mere embryonic ridges and thence into the surface deposits of the 

 " lowlands." The regular ridges trend approximately east and 

 west; are all fairly parallel to one another; have in some instances 

 fairly steep sides, which may indicate. scouringi; and form penin- 

 sulas and islands as well as bounding some almost closed arms of the 

 lake. 



The ridge sands consist of small and well "rounded grains, chiefly 

 of quartz and subordinately of ironstone. Gypsum occurs in and on 

 the margin of the lake, but gypsum dunes have not been noticed,, 

 although they may pos.sibly have been overlooked. 2 



The vegetation already described holds the sands to some extent, 

 but the ridges can hardly be regarded as fixed. 



The blown sands mostly do not rest directly on the bedrock, but^ 

 overlie a detrital deposit from one to three feet or more thick, which 

 consists of a mixture of fine clayey material, fine and coarse sand, 

 and nmch detrital angular vein quartz, the fragments of which are 

 usually under one incli in size. This dejiosit, Avhich lies on the 

 bedrok, is evidently due to water action and gravitional drift, and 

 is no doubt the counterpart, and probably a continuation of, the 

 third class of detrital deposits (other than sand ridges and lake 

 silts) described above. 



Rock floors, which in places are covered with wliite quartz detritus 

 from adjacent reefs, or with a veneer of silt, bound the irregular 

 ridges, and also at their western ends, the regular ones; but towards 

 their eastern ends such regfular ridges nmy be abutted upon by the lake 

 silts, as well as by rock floors. 



The Rock Cliffs, Rock Floors, and Rock Basins 

 of the Lake. 



The " lake " or " lake area," as here used, includes the lake as 

 a whole; that is, the area where water may lest on bare floors 

 (either of rock or of silt), although the surface may be broken by 

 islands. The " main portion of the lake " is that part of the sur- 

 face which is almost free from obstructions, and on which after 

 heavy rain a practically continuous sheet of water might lie. Its 

 outline is easily followed. 



(1) Eock Cliffs -Tha higher rock cliffs, composed of greenstones, 

 occur along the eastern borders of the '^ high " lands, that is, the 



1 See Cornish, V. "On the Formation of Sand Dunes," Geo<>-. Joiirn., ix. (1897), p. 288. 



2 "Kopi" (powdered yypsinn) occurs, but not as definite ridges. 



