calculation, but it lies quite beyond the power of the mind to contemplate with steadiness.

Abandoning all further attempts to determine the probable antiquity of the Strata, and the several races of Life, in measures of solar time, we may refer to some more limited trials to assign a date to the origin of the present physical aspect of nature— the present action of the sea on its coasts, and of the rivers on their beds. In the speculations of De Luc concerning natural chronometers this period is described as posterior to the existence of our continents, and as having a fixed chronology; it is now regarded as the latest of the Pleistocene periods; in the northern zones of the world it is the Postglacial period; and it includes, according to all observation and opinion, the age of the human race.

Herodotus, the first author who ventured an estimate of this kind, was naturally conducted to it by his inquiries regarding the ancient history of Egypt. This fertile country, 'the gift of the Nile,' offered him, in the real and fabulous narrative of its governors, a long series of centuries of elapsed time, and in the periodical floods and corresponding rise of its surface and growth of its delta, natural chronometers by which in some degree to check the traditions of the priests.

The conclusion of Herodotus does not however

directly apply to the land of Egypt. The statement is to the effect that if the Nile were turned into the Arabian gulf, that part of the sea would be filled up by sediments in 20,000 years; indeed, according to his own opinion, in 10,000 years. Such being the present tendency of the river to deposit sediment, he justly concluded that, in the long lapse of earlier time, the Nile may have filled up the greater Egyptian gulf which lay in its course.

De Luc, one of the most ingenious and laborious geologists at the close of the last century and the commencement of the present, devoted much attention to those operations of the sea and rivers which promised to afford some measures of effect applicable to the problems of past time. The growth of new land on the sea-shore, the waste of the old surfaces by the waves, the filling up of lakes and the wearing away of valleys,-these, diligently studied, led him to the general conclusion that the actual state of our continents is not ancient; that it is not very long since they were given to the dominion of men; that not many ages have elapsed since the continental parts of our globe were abandoned by the ocean.

The limited growth of new lands-still yearly on the increase by deposition of sediment-the small extent to which lakes have been filled up by the

rivers which enter them, and the truncation of hills sloping to the sea by cliffs not far from the point where the slope once met the sea-level, may be mentioned as data of this kind. Undoubtedly they all tend to convince us of the comparatively short period of time which has elapsed since the sea began to waste, or to be filled up, and the rivers began to wear away the upper parts and to fill up the lower parts of the valleys; but it is difficult to translate this conclusion into centuries or thousands of years. Perhaps such a case as that of the Derwent River flowing into the Lake of the same name in Cumberland, and augmenting year by year the mass of sediments at the upper end, may be found worthy of special attention, because of the abundance of rains on the slaty mountains around, the shortness of the course of the river, and the very slight degree in which cultivation, quarrying, and mining, have, till within a few years, altered the natural character of the surface. The requisite measures of the lake, the delta, and the sediment brought by the river, would present no great difficulty.

Perhaps it may be sufficient to take one example, the best at present known, to which computation has been applied, for determining the number of years in which a river has been running and excavating for itself a channel in rocks of one definite charac

ter. This example is found in the recession of the

Falls of Niagara'.

The river St Lawrence, in traversing the space of 32 miles between Lake Erie and Lake Ontario, has a fall of 330 feet. The general floor of the whole country is limestone resting on shale; the limestone appears in the stream, covered in the banks on each side by alluvial sand from 10 to 140 feet thick for the first 25 miles. At this point the great Falls occur; the river being precipitated over the solid and projecting rock of limestone, in one tremendous cascade 158 to 164 feet deep, into the subjacent shale, which is deeply excavated below the general level of the channel, and also worn into a recess forty feet within a perpendicular line dropped from the limestone edge of the cataract. Below this point the river flows in the deep chasm which it has worn for itself, seven miles, to Queenstown and Lake Ontario.

The Falls recede, not regularly, but by sudden steps, in proportion as the subjacent shale is worn away, and leaves the crown of limestone unsupported. In the course of forty years they have thus receded fifty yards, Adopting this as the rate of recession for the whole of the channel below the Falls, we have 9856 years for the time which has elapsed Lyell, Principles of Geology, I. 277.

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since the epoch when the Falls began their backward progress from Lake Ontario.

This epoch, however, is not of necessity the same as that of the origin of the river action, which may have gone on for some unknown time previously. It does not then give us the desired information of the length of the Postglacial period, or the date, as De Luc might have expressed it, of the birth of our continents. But it seems to point in the same direction as all the other natural chronometers, and to compress within a few thousand years the later part of the Pleistocene Period, when the main features of the Land, the Rivers and Lakes, and Plains and Mountains, had been finally redeemed from the power of the sea, and peopled by the now existing races of plants and animals.

CHANGES OF CLIMATE.

Few inferences have obtained a more general assent among geologists than that which affirms the change of climate during the progress of life on the globe. The evidence on which reliance has been placed has been sometimes adopted on light grounds, sometimes rejected for fresh and better testimony; but the conviction of almost every writer has been deliberately recorded in favour of the prevalence of much higher temperatures during early geological