Chap. XX.] EIVEES FLOWING THEOUGH GOEGES. 
497 
reexcavated valleys which at no very remote period were filled with coulees 
of basalt. This phenomenon, so strikingly brought before English geolo- 
gists forty-two years ago by the pen and pencil of Mr. G. Poulett Scrope, 
was abundantly verified by Sir Charles Lyell and myself when we ex- 
plored Auvergne together in 1828. 
On the other hand, T maintain that the clearest proofs abound that in 
numberless cases rivers have simply availed themselves of the courses pre- 
pared for them by previous breaks in the rocks, opening depressions along 
which the waters have passed. Take one of the largest of our European 
streams, the Danube, and trace it from its source in the flat plateaux of 
Central Germany, in which it rises, and you see that, whilst it never can 
have been a torrential stream, it simply maintains a steady slow-flowing 
current as it winds through the steep defiles and high cliffs of the hardest 
gneiss and granite, which had been opened out to receive it ; for, even 
now, where the gorges are the deepest and narrowest, and where the 
river must therefore have exerted its greatest power, the buildings of 
Eoman times have been daily bathed by the stream, and not a fragment 
of them has been worn away. 
Then, if we turn to Britain, no one who has examined the tract of Coal- 
brook Dale will contend that the deep gorge in which the Severn flows 
at that place has been eaten out by the agency of the river (there so power- 
less), the more so when that great fissure in the Silurian rocks is at once 
accounted for by their abrupt severance, with an entire unconformity 
between the strata of Wenlock limestone occupying the opposite sides of 
the valley. Now in that part of Shropshire the Severn has not worn 
away the slightest portion of the rocks during the historic era, nor has it 
scooped out a deeper channel : it has only deposited silt and mud, and in- 
creased the extent of land upon its banks. 
The valley of the Avon at Bath^ for example, is also the seat of one of 
those disturbances to which Sir Charles Lyell alluded when he candidly 
said that he had " little doubt that the Bath springs, like most other 
thermal waters, mark the site of some great convulsion and fracture which 
took place in the crust of the earth at some former period " *, — the hot 
waters of that city having ever flowed out of a deep-seated fissure, clearly 
marked by the strata on the one side of the valley having been upheaved 
to a height 200 feet above that which they once occupied in connexion 
with those of the other side. When, indeed, we look to the lazy-flowing, 
mud-collecting Avon, which at Bath passes along that line of valley, how 
clearly do we see that it never deepened its channel ! still more when 
we follow it to Bristol, and observe it passing through the steep gorge of 
hard Mountain-limestone at Clifton, every one must then be convinced 
that it never could have produced such an excavation. In fact, we know 
that, from the earliest periods of history, it has only accumulated mud, and 
* Eeport of British Association, 1864 : Address, p. lxiv. 
2 * 
