suppose that while this, as it may be termed, preliminary configuration of the surface has been taking place, the land has become tenanted by various trees, shrubs, and plants affording means of subsistence to different forms of animal life; while the streams also have been occupied by colonies of freshwater testacea; and let us now trace what would be the action of the rivers. To use the words of Sir Charles Lyell,[1] "when we are speculating on the excavating force which a river may have exerted in any particular valley, the most important question is, not the volume of the existing stream nor the present levels of its channel, nor even the nature of the rocks; but the probability of a succession of floods at some period since the time when the valley may have been first elevated above the sea."
Now in the first place, all rivers whose banks are not artificially protected, and whose channels are not kept clear, are of necessity more liable to floods than those in civilized countries, which bear much the same relation to rivers flowing through uncultivated lands, as domesticated animals do to wild. We have, moreover, ex hypothesi, a fruitful source of floods in a greater rainfall and in a more rigorous winter climate. The marvellous effects of such floods in excavating channels, and in transporting materials, can only be estimated by those who have seen their results, or have studied the accounts given of them. When we read of a small rivulet on the Cheviots,[2] swollen by heavy rain, having transported several thousand tons of gravel and sand into the neighbouring plain, and having carried blocks of stone, weighing upwards of half a ton, two miles down its course, while another block weighing nearly two tons was transported the distance of a quarter of a mile, we may form some conception of the effects of even a flooded brook. The blocking of a stream by ice or fallen trees, so as to keep back its waters, and thus form a lake, which is suddenly drained by the breaking of the barrier; a heavy fall of rain; or a rapid fall of snow on ground hard frozen, and therefore impervious, are common causes of floods; and such as we may presume to have prevailed in our hypothetical case. What, therefore, would be the effect of such floods?
The first effect would no doubt be to cause the streams to overflow their banks, and spread over the bottom of the valleys in which they usually flowed. The shallower the valley the greater
