166 J' T. Jutson: 



or he quite apart from lakes); and possibly on the under surfaces 

 of granite and quartz boulders. The process may be acting on 

 other rock outcrops, but it is probably masked by the stronger 

 erosive action at such other outcrops of other processes, such as 

 insolation and the action of rain. 



The Process of " Exsudation " as here Defined. 



According to Hunieji^ " exsudation " is a name given by Fut- 

 terer, and comprises several "desert evaporation effects"; but in 

 this paper the term will be restricted to those processes by which, 

 under certain conditions, flakes or grains are mechanically broken 

 off from the parent rock, or by which the latter, if soft and decom- 

 posed, may crumble almost to powder. These results are appar- 

 ently chiefly due to the crystallization of salts contained in solu- 

 tions brought to the surface by capillary attraction, and the evapo- 

 ration there of the water. The deposition of the salts exerts pres- 

 sure on the rock, with the result that flakes or grains may be forced 

 off or a soft rock may crumble. Walther2 has described the effect 

 on the rocks in arid areas of the crystallization of salts from 

 evaporating underground water, and a valuable series of observa- 

 tions and experiments as to the disintegration of building stones 

 in Egypt has been made by Lucas, ^ who regards such crystallization 

 of salts as the main agent of such disintegration. 



In sub-arid Western Australia, the operation of "exsudation" 

 is best seen in cliffs — usually at or close to the base of such cliffs — 

 of weather-resisting rocks, at the edge of a " dry " lake. Amongst 

 the resistant rocks forming these cliffs the " greenstones " are the 

 most abundant; and such cliffs are frequently high, steep and 

 prominent features on the borders of the lakes. " Hard," prac- 

 tically undecomposed granite also occurs, but the cliffs so far seen 

 by the writer are usually low and insignificant. 



These rocky cliffs frequently rise from a rock floor at the edge 

 of the lake of such an extremely smooth level character that the 

 writer has applied to such a floor the name " billiard-table rock- 

 floor." The cliffs may be nearly vertical for some height, ranging 

 from a few feet to perhaps 20 feet, beyond which they recede at 



1 Hume. W. F. — Professor Walther's Erosion in the Desert considered. Geol. Mau. Decade 

 VI , vol. i. (1914), V. If). 



1 Walther, J. -Das (iesetz der Wustenhildiing. 2 ed., Leipzit;-, 191-2, pp. T2S-l-2f>. 



3 Lucas, A.— VVte Disinteri ration of Bwlding Stones in Kriyitt. Survey Department, Cairo, 

 1902. 



