10 GEOLOGY OF THE SON VALLEY, ETC. 



A rock of the character of that just described is essentially and 

 , . , . . inevitably of subaerial origin. The peculiar mix- 



Subaenal origin. * , ° r 



ture of pebbles with fine sand is impossible 

 as a subaqueous deposit, for the current capable of moving the one 

 would sweep away the other. In the case of a stream flowing on 

 the surface of the land different conditions prevail ; there is at every 

 point a certain level, determined by the conditions of the stream, 

 below which it cannot erode. Pebbles may consequently be swept 

 over a surface of sand, which the current is unable to attack because 

 it lies at too low a level, and subsequently become covered by a fresh 

 accumulation of sand. The frequent occurrence of current bedding 

 in the sandstones, and the manner in which shale beds give way 

 laterally and vertically to beds of sandstone, all point to the same 

 conclusion, that these deposits were formed suba^rially by streams 

 flowing over the surface of dry land. 



The arenaceous lower stage passes upwards into the argillaceous 

 middle stage. The rocks composing this 

 are mostly shaly, though sandstones also occur ; 

 the prevailing colour is red varying from dull brick to bright 

 vermilion, as a rule pure and free from any tendency towards violet. 

 As is common in red rocks a pale green colour is common occurring 

 as a mottling or in patches of larger size, the boundaries between 

 the two colours being quite independent of the bedding planes. In 

 the eastern exposures, where the sections are most complete, some 

 pale grey shales are found near the top of this stage. 



The upper sandstones are only well seen in the two easternmost 

 exposures, they resemble those of the basal 

 stage, being white, much false-bedded and, in 

 the Jungel outlier, containing scattered pebbles. 



The thickness of the series as a whole varies much in different 

 exposures the greatest being seen in the most 



Variations of thickness. . . 



easterly, where it is estimated by Mr. Vreden- 

 burgat about 1,500 feet, and as an unknown thickness of beds, above 

 the uppermost seen, has been remeved by denudation, the original 

 ( 10 ) 



