drained and when the fertility of the mountainous district was probably enhanced by the igneous matter ejected from below, and poured down. upon the more sterile granite. During these changes which appear to have taken place in the miocene epoch, the mastodon, rhinoceros, elephant, tapir, hippopotamus, together with the ox, various kind of deer, the bear, hyana, and many beasts of prey, ranged the forest, or pastured on the plain, and were occasionally overtaken by a fall of burning cinders, or buried in flows of mud, such as accompany volcanic eruptions. Lastly, these quadrupeds became extinct, and gave place to the existing creation. There are no signs during the whole time required for this series of events of the sea having intervened, nor of any denudation which may not have been accomplished by currents in the different lakes, or by rivers and floods accompanying repeated earthquakes, during which the levels of the district have in some places been materially modified, and perhaps the whole upraised relatively to the surrounding parts of France.'

Notwithstanding that 'there are no signs of the sea having intervened,' Lyell says that these ' lacustrine strata, which must have filled a hollow, have been denuded and shaped out into hills and valleys on the site of the ancient lakes.'

trary to the

submarine and

in favour of

the subaerial

theory.

Here Lyell is not only in direct contradiction This is conto his submarine theory of the formation of valleys, but also to his anti-catastrophe system, and his 'causes now in operation' system; and observe how he changes from one agent to the other. 'During the earlier scenes of repose' he fills his lakes with deposits, and his deposits with fossils, by causes now in operation.' But when volcanoes come on the scene to fill the lakes

6

currents in the different

with deposits, he calls in
lakes, and floods accompanying repeated earth-
quakes; though, as we shall see hereafter,
active volcanoes are safety-valves against earth-
quakes. His very fossils are now entombed by
fire instead of water; that is, when they are
'occasionally overtaken by a fall of burning
cinders, or buried in flows of mud, such as ac-
company volcanic eruptions.'

the lakes, he darkly refers to

canic eruptions when the lakes

Then, to drain

the era of vol

were drained.'

But a little after he says: The subterranean movements may then have continued until they altered the relative levels of the country, and caused the waters of the lakes to be drained off, and farther accumulation of regular freshwater strata to cease.' But he forgets that he had already dammed the water out of the lakes by sediment; and with regard to the relative levels

Why valleys

grow wider as

of the country,' the Loire and the Allier still run through the deposits where the lakes were, qua Troja fuit, with doubtless no farther alteration of level than the enormous one produced by these rivers themselves and rain. But for the shaping of these deposits into hills and valleys' after the lakes were drained,' Lyell gives no reason at all.

6

But grant subaerial denudation and deposit, and the filling in of the lakes with drift and alluvium, the storing these with fossils of land animals, and the cutting of valleys through the deposits, are all matters of course and plane sailing. The wash of rain and rivers first fill the ancient lakes with drift and alluvium, and the same floods which form the drift store it with land animals. But while this was doing, they descend. and ever since it was accomplished, rain and rivers have been eroding the dams or barriers which originally formed the lakes. And as these barriers were gradually worn away, channels were cut longitudinally through the deposits which filled the lakes. Then come disintegration and the lateral wash of rain to widen these channels into valleys. And the lower parts of these valleys, speaking longitudinally, and the higher parts, speaking laterally, being gradually and in succession carlier exposed to these wasting causes,

supposing equal hardness of material, disappear earlier. This is the cause of the increasing width of valleys as they descend; and the very exception proves the rule. For the different parts of the same valley are invariably wide directly as the softness of those parts, and narrow directly as their hardness. This would not be the case if valleys were igneous cracks.

These causes are as hard at work now as ever they were. And in future ages rain and rivers will abstract from these valleys or ancient lakes every particle which in former ages they deposited there. This would not be the case if the Adriatic or any arm of the sea were filled up, because the sea would keep the mouths of the rivers at one level, and prevent them cutting channels through their own deposits.

Many people consider a valley to be like a teacup, or the happy valley of Rasselas,* without an outlet. But valleys are like inclined trough's, open at the lower end. A valley is, in my opinion, a water-slope or a system of water-slopes, converging to one outlet at the sea. In the case of cups or crevices without outlets, elevation of the land, rain instantly begins to

formed in the original

* This was a mistake of mine in the first edition, the great Dr. Johnson contrived an outlet for the water of the Happy Valley.

fill them with water and alluvium. Should either of these reach the lowest part of the brim, rain erodes that brim, makes an outlet, and proceeds to let off the water, or to clear out the alluvium which it has before deposited. This holds true from the largest of the American lakes (for rivers are subterranean rain, that is, rain reappearing) to the smallest puddle on a road.

In the first announcement in the papers of the great exploit of the indomitable explorer Captain Speke, it was stated on the authority of natives, that the Lake Nyanza had more than one outlet. I ventured to suggest that the native information would be found incorrect: and started the principle that owing to the erosion of the barrier or dam, a lake can only have one outlet. The principle was much discussed, and two lakes, Black Loch in Dumfries, and Loch Davie in Arran, were pointed to as having two outlets. I visited both, and reported that, me judice, each had only one outlet.

As might be expected, the ancient alluviums formed in hollows on dry land are the grand museums of the remains of land animals, as strata deposited in the sea, though equally by the wash of rain from the land, are of the marine creation. I would here call attention to these facts, which I shall allude to again in the eighth