Mineralogy and Geology. 291 
difficulty in the fingers; and besides, it is much fissured. Even t 
‘harder fossiliferous Wollongong rock has been described as sometimes 
failing 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 de- 
the age of the deposits,—crumbling readily, and often breaking without 
he 
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 pre- 
vailing rock along the bed of the stream and in the enclosing hills, has 
scarcely a representative fragment among the debris. The sandstone 
blecks are worn to sand or earth by the torrent, while the harder basalt 
ts slowly rounded. On the plains of Puenbuen, similar facts were ap- 
parent. The hills contain sandstone and basalt, but only the latter 
a as boulders or pebbles over the plains, or along the strea 
elow, ; 
stances of this along the coves of Port Jackson, where the crystalliza- 
tion of the saline spray reduces the rock to its original sand; and in 
the interior of the country there are large caves, formed apparently 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. e roof is arched, and appears to be con- 
stantly crumbling, while the bottom is covered with a fine dry ash-lik 
sand, into which the feet sink several inches. The Same operation is 
going on along the summits of the Illawarra range ; and one huge block 
as found so hollowed out in this way as to 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, 
allude toa 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 favor is farther sustained by the char- 
acter of the streams. We have alluded to the great extent of the floods, 
and the rapid rise of the rivers attending them. e stream of the 
Kangaroo Grounds, when visited by the writer, was a mere brook, ford. 
able in any part, and it flowed along with quiet murmurings. How 
different when the brook becomes a river thirty feet deep, driving on in 
@ broad torrent, and flooding the valley; and this had been its condi. 
tion but a few weeks before. If, as has been shown, the transporting 
ol 
