528 NEW SOUTH WALES. 
1. Their high, precipitous, or vertical walls of stratified sandstone, 
and their flat areas at bottom—excepting where the descent of the 
stream is rapid. 
2. Their frequent great breadth towards their head, while below, 
they are often very narrow, like a large bay with a small entrance. 
3. The absence of all traces of the fragmentary material which 
could have filled these valleys. 
The idea that running water was the agent in these operations 
appears not so ‘‘ preposterous” to us, as it is deemed by Mr. Darwin; 
and we think it may be shown that Major Mitchell was right in attri- 
buting the effects to this cause. The extent of the results is certainly 
no difficulty with one who admits time to be an element which a 
geologist has indefinitely at command. But the subject admits of full 
explanation, as we believe, without making any improbable supposi- 
tion on this point. We need but refer to a former page, in which we 
have discussed the subject of valley-making by denudation among the 
Pacific islands, to show that New Holland, after all, is not the most 
remarkable land in the world for valleys of denudation. 
We should consider that the rock material is far more yielding 
than that of basaltic Tahiti. Indeed the whole rock, from the upper- 
most layer to the deposits below the coal, is remarkably fragile, con- 
sidering the age of the deposits,—crumbling readily, and often break- 
ing without difficulty in the fingers; and besides, it is much fissured. 
Even the harder fossiliferous Wollongong rock has been described as 
falling to pieces of itself when exposed to the air. Moreover there are 
occasional clayey or argillaceous layers which are still softer; and 
many of those of the coal formation are not firmer than the material of 
a common clay bank. ‘The denudation of such material requires no 
preparatory decomposition, as with many igneous rocks, but takes 
place from wear alone, and with but slight force in the agent. 
It is obvious for the same reason that the material carried off by 
denudation ought not to appear in fragments through the lower 
country. A short journey along a rapid stream would reduce even 
large masses to powder. ‘The plains of the Kangaroo Valley are 
covered in places with basaltic pebbles or boulders ; but the sandstone, 
which is the prevailing rock along the bed of the stream and in the 
enclosing hills, has scarcely a representative fragment among the 
debris. ‘The sandstone blocks are worn to sand or earth by the tor- 
rent, while the harder basalt is slowly rounded. On the plains of 
Puenbuen, similar facts were apparent. ‘The hills contain sandstone 
