530 NEW SOUTH WALES. 
This rise in the Kangaroo Grounds is an index of what takes place 
every few years over the whole country. Our surprise at the amount 
of degradation subsides before such facts; and we rather wonder that 
sandstones so soft and fragile, which have been exposed probably from 
the Oolitic period, still cover the surface to so great an extent as they 
do at the present time.* 
Mr. Darwin derives his principal argument against the hypothesis 
of denudation from the forms of the valleys,—their width, extent and 
ramifications, and yet narrow embouchures. But we find on consi- 
deration that this form is a necessary result of the mode of denudation 
under the circumstances supposed. 
In our account of the valleys of the Pacific islands (page 379), it has 
been shown that the gorges change their character where the slopes 
become quite gradual, from a narrow defile with convergent sides, toa 
broad channel with vertical walls and flat bottom: the cause of this 
change has been explained on page 386. ‘The same cause should pro- 
duce a like effect in Australia. Though it be a repetition, we add in 
this place a brief explanation of the process. A stream, in making a 
descent of two or three thousand feet from the higher summits to the 
level of the sea, gradually deepens its bed by wear. Since the waters 
are increasing in quantity from various sources as they flow onward, 
this deepening of the gorge should be most rapid at its lower extre- 
mity ; and it would continue in progress until the bed in that part 
became so low or gradual in slope, that the waters had lost to a large 
degree their rending force, and any excavation at bottom was made 
up by the material deposited along its course. This fact determines 
a permanent height for the bottom of the lower valley. As the stream 
continues its wearing action in the same manner, the lower valley is 
gradually prolonged upward, retaining nearly the same slope at 
bottom (one or two feet to the mile); consequently the steeper portion 
of the gorge is at the same rate becoming shorter and still steeper. 
Thus the head of the stream may finally become a series of cascades, 
* Tf the annual amount of sediment borne along by the Mississippi were assumed as the 
amount for Cox’s River, the one hundred and thirty-four cubic miles of excavation, esti- 
mated by Major Mitchell, would have been made in less than six thousand years, as we 
learn from a simple calculation. The assumption may be much too large for so small a 
stream, even where there is so much to favour it in a rapid descent, and abundant 
and convenient material for wear and transportation ; yet not more so, than the assumed 
six thousand years is less than the actual period during which the region has been ex- 
posed to degradation. 
