ORIGIN OF VALLEYS. 529 
and basalt, but only the latter appears as boulders or pebbles over the 
plains, or along the streams below. 
This Sydney sandstone does not even require running water to 
promote degradation. In many caverns along cliffs, the rock gra- 
dually falls to powder by a species of efflorescence. There are nume- 
rous instances of this along the coves of Port Jackson, where the crys- 
tallization of the saline spray reduces the rock to its original sand ; 
and in the interior of the country there are large caves, formed appa- 
rently by this same process, though probably from the crystallization 
of nitrates. Near Puenbuen, these caves are from six to twenty feet 
deep, and from four to forty long. ‘The roof is arched, and appears to 
be constantly crumbling, while the bottom is covered with a fine dry 
ash-like sand, into which the feet sink several inches. The same ope- 
ration is going on along the summits of the Illawarra range; and one 
huge block was found so hollowed out in this way as to be a mere 
shell, which sounded under the hammer like a metallic vessel. 
These various facts bring before us some idea of the yielding nature 
of the rock which the waters have to contend with in the denudation 
of this country, and they also illustrate the various processes at work. 
We allude to a single other mode of degradation before passing: it is 
the action of growing trees and their roots, both in opening fissures 
and tumbling blocks down the precipices. It is a cause influencing 
very decidedly the characters of cliffs, and at the same time preparing 
the rock for decomposition and wear. 
The credibility of the view we favour is farther sustained by the 
character of the streams. We have alluded to the great extent of the 
floods, and the rapid rise of the rivers attending them. The stream of 
the Kangaroo Grounds, when visited by the writer, was a mere brook, 
fordable in any part, and it flowed along with quiet murmurings. 
How different when the brook becomes a river thirty feet deep, driving 
on in a broad torrent, and flooding the valley ; and this had been its 
condition but a few weeks before. If, as has been shown, the trans- 
porting power of running water zncreases as the sixth power of the velo- 
city, and a stream of fifteen miles an hour has more than ten times the 
transporting power of one moving ten miles an hour, and more than a 
million times that of a stream of two miles an hour,* we can compre- 
hend how inadequate must be the conceptions of this force which we 
derive from viewing the streams at low water. 
* Wm. Horxins, Ox the Transport of Erratic Blocks, Trans. Camb. Phil. Soc. viii. 
1844, p, 221. 
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