244 On the Excavation of Vallies. [Aug, 
ravine or river course consists of volcanic basalt or breccia, and the other of the 
primitive or tertiary rocks of the district 
We have supposed two, three, or four successive river channels to exist above 
each other in this way. But the erosive power of a stream varies with the height 
from which it falls, and the greater hardness or softness of the rock subjected to 
its action. The volcanic basalt, by filling up a large portion of the channel, may 
transfer a fall or cascade some miles farther down the stream, to a place perhaps 
where the gentleness of the slope formerly rendered the erosive action extremely 
small. A powerful agent thus operating at a new spot, may quarry out the latest 
bed of the stream to a depth greater than that of the oldest. It is in this way that 
the secret of these channels beneath channels has been brought to light; the old 
beds of rolled pebbles and sand which mark the earlier channels, with the gneiss 
below and the basalt above them, being exposed at some places on the side of the 
existing river courses, at the height of 60 or 70 feet above the present surface of 
the water. 
Messrs. Lvell and Murchison were able to distinguish four orders or series of 
river beds of this description, not all in visible connection at one locality, but all 
sufficiently marked by characteristic circumstances. The distinction was ascer- 
tained in more ways than one, but chietly by the nature of the gravel or rolled 
pebbles, or bodies inclosed in it, which pointed out the ancient course. Thus one 
bed of pebbles, 6 or 8 feet thick, was found, consisting entirely of fragments of 
primitive or tertiary rocks, without a single bit of basalt or breccia, shewing that 
this very ancient water course had been excavated before the neighbouring cones 
had commenced their eruptions. Another bed was found, containing some pieces 
of basalt, but only of that species which is known from its composition to belong 
to the oldest products of these volcanoes. A third bed contained newer volcanic 
materials, and was distinguished by including bones of the extinct quadrupeds 
found elsewhere in the alluvial soil. The fourth bed was that of the present 
stream. 
Now, even the newest of these channels have in some instance, cut their way 
down 150 or 200 feet beneath the natural surface of the rock. The older ones had, 
in all probability, produced excavations equally great, though not always at the 
same spots. We have farther proof of the great lapse of time between one eruption 
and another, in a fact mentioned by M. Scrope, that the older lava had in some 
cases acquired a surface of soil, which had been covered with forests when the 
newer eruptions took place, as attested by the remains of wood found in the tuff or 
agglutinated volcanic ashes. 
Let us now consider what are the inferences to which these facts lead. First , we 
might perhaps account for the scooping out of an original valley in the primitive 
rock, by the agency of the "Deluge ; but when we find this original valley three 
times filled up, and three times successively excavated, and have evidence other- 
wise, that long periods elapsed between each of these operations, it is plainly ab- 
surd to suppose that these excavations, divided by long periods of time, were pro- 
duced by one deluge of temporary duration. Mr. Buckland’s argument, too, we 
must remember, if it proves that there was a deluge, is quite as strong to prove 
that there was but one. It is obvious then that the erosive power of streams has 
cut out vallies of great depth, through rocks of a very durable kind: it has, in 
short, performed the work which some writers have referred to the agency of a 
flood, simply because they were unwilling to admit that the streams themselves 
could effect in any supposable length of time, what we find they have actually ac- 
complished within a period which may be called extremely limited, compared with 
the lapse of ages embraced by the calendar of geology. 
But a query occurs, whether those volcanic eruptions and the rocks they pro- 
duced, are postdiluvian or antediluvian. Now from the lapse of time required to 
effect the changes we have described, some or all of the eruptions ought to be an- 
tediluvian ; but what is remarkable, not the slightest trace of diluvial action is to 
be found upon the volcanic mountains or vallies of Auvergne. So at least say 
Messrs. Lyell and Murchison, both upon their own authority, and that of other 
scientific observers. Mr Scrope observes, that the undisturbed and perfect state 
of the cone of loose scoriae whence the lava of Chaluzet flowed, demonstrates that 
no denuding wave has passed over the spot since the eruption took place. Messrs. 
Tyeil and Murchison also remark, that the upper part of the lava currents retain 
their original asperities, and are no where strewed over with sarid or pebbles, and 
that no traces are found on the wide basaltic plateaux of boulders or travelled 
fragments from the east, the west, or any point of the compass. 
