20 STUDIES, SCIENTIFIC AND SOCIAL chap. 



the essential conditions for the formation of vertical escarp- 

 ments, since by the weathering away of the softer beds 

 the harder strata above them remain unsupported and 

 break off, and thus the vertical or sometimes overhanging 

 character of the precipices is kept up. 



If we look at a large-scale map of this part of Australia, 

 we see that the rivers Grose, Cox, and other tributaries of 

 the Nepean which drain the sandstone plateau, have great 

 numbers of diverging branches which almost interlace 

 with each other, as so often occurs among the streams of 

 a nearly level well-watered district. Now, bearing in 

 mind what has been said of the permanence of water- 

 courses once formed, we can see that these many-branch- 

 ing streams must have flowed on the surface of the plateau 

 at the epoch of its first elevation ; that surface itself being 

 perhaps considerably above the present surface, which has 

 certainly been lowered by denudation during its long 

 existence as dry land, probably during the whole of the 

 tertiary period. From the time that these streams began 

 to penetrate the sandstone plateau as far as the first hard 

 bed, miniature cliffs would be formed by the wasting away 

 of the softer beds beneath it, and the continual movement 

 backward thus produced would widen the valleys till those 

 of many of the smaller tributaries became united together. 

 Thus age after age the valley would widen and deepen, 

 always preserving its precipitous rock-walls due to the 

 alternation of hard and soft layers. 



The deepening of these great valleys would probably be 

 aided by subterranean denudation due to the presence of 

 salt and alum, which Mr. Clarke states are found at several 

 places in these strata. The solution of these salts by per- 

 colating water would form cavities and water channels, and 

 the subterranean streams would eat away the softer beds, 

 forming caverns, the roofs of which would in time fall in, 

 and the d^hris be gradually disintegrated by atmosjDheric 

 agencies and then carried away by floods. This mode of 

 denudation was seen actually at work by the late Sir 

 George Grey, during his exploration of the Glenelg River 

 in North-West Australia. He describes a nearly level 

 table-land covered with numbers of sandstone pillars of 



