but that its extreme height, are liable only to trifling variations; 3dly, That both the mean and extreme depth of the sea are invariable; and 4thly, It may be consistent with due caution to assume that the grouping together of the land in great continents is a necessary part of the economy of nature; for it is possible that the laws which govern the subterranean forces, and which act simultaneously along certain lines, cannot but produce at every epoch continuous mountain chains, so that the subdivision of the whole land into innumerable islands may be precluded.

"If it be objected that the maximum of elevation of land and depth of sea are probably not constant, nor the gathering together of all the land in certain parts, nor even perhaps the relative extent of land and water: I reply, that the arguments about to be adduced will be strengthened, if in these peculiarities of surface there be considerable deviations from the present type. If, for example, all other circumstances being the same, the land is at any one time more divided into islands than another, a greater uniformity of climate might be produced, the mean temperature remaining unaltered; or if at another era there were mountains higher than the Himalaya, these, when placed in high latitudes, would cause a greater excess of cold. Or if we suppose that at certain periods no chain of hills in the world rose beyond a height of 10,000 feet, a greater heat might then have prevailed than is compatible with the existence of mountains thrice that elevation.

"However constant may be the relative proportion of sea and land, we know that there is annually some small varia

tion in their respective geographical positions, and that in every century the land is in some parts raised, and in others depressed by earthquakes, and so likewise is the bed of the sea. By these and other ceaseless changes, the configuration of the earth's surface has been remodelled again and again since it was the habitation of organic beings, and the bed of the ocean has been lifted up to the height of some of the loftiest mountains.

"If we now proceed to consider the circumstances required for a general change of temperature, it will appear from the facts and principles already laid down, that whenever a greater extent of high land is collected in the polar regions, the cold will augment; and the same result will be produced when there is more sea between or near the tropics; while, on the contrary, so often as the above conditions are reversed, the heat will be greater. If this be admitted, it will follow, that unless the superficial inequalities of the earth be fixed and permanent, there must be never-ending fluctuations in the mean temperature of every zone; and that the climate of our era can no more be the type of every other, than one of our four seasons of all the rest.

"To simplify our view of the various changes in climate which different combinations of geographical circumstances may produce, we shall first consider the conditions necessary for bringing about the extreme of cold, or what may be termed the winter of the great year,' or geological cycle, and afterwards the conditions requisite to produce the maximum of heat, or the summer of the same year.

"To begin with the northern hemisphere, let us suppose

those hills of the Italian peninsula and of Sicily, which are of comparatively modern origin, and contain fossil shells identical with living species, to subside again into the sea. from which they have been raised, and that an extent of land of equal area and height, varying from one to three thousand feet, should rise up in the Arctic ocean, between Siberia and the north pole. In speaking of such changes I shall not allude to the manner in which I conceive it possible they may be brought about, nor to the time required for their accomplishment-reserving for a future occasion not only the proof that revolutions of equal magnitude have taken place, but that analogous operations are still in gradual progress. The alteration now supposed in the physical geography of the northern regions would cause additional snow and ice to accumulate where now there is usually an open sea ; and the temperature of the greater part of Europe would be somewhat lowered, so as to resemble more nearly that of corresponding latitudes of North America; or, in other words, it might be necessary to travel about 10° further south to meet with the same climate which we now enjoy. No compensation would be derived from the disappearance of land in the Mediterranean countries; but the contrary, since the mean heat of the soil in those latitudes is probably far above those which would belong to the sea, by which we imagine it to be replaced.

"But let the configuration of the surface be still further varied, and let some large district within or near the tropics, such as Mexico, with its mountains rising to the height of twelve thousand feet and upwards, be converted into sea,

while lands of equal elevation and extent rise up in the arctic circle. From this change there would, in the first place, result a sensible diminution of temperature near the tropic, for the soil of Mexico would no longer be heated by the sun, so that the atmosphere would be less warm, as also the neighboring Atlantic. On the other hand, the whole of Europe, Northern Asia, and North America would be chilled by the enormous quantity of ice and snow, thus generated on vast heights on the new arctic continent. If, as we have already seen, there are now some points in the Southern hemisphere where snow is perpetual down to the level of the sea, in latitudes as low as central England, such might assuredly be the case throughout a great part of Europe, under the change of circumstances above supposed; and if at present the extreme range of drifted icebergs is the Azores, they might easily reach the equator after the assumed alteration. But to pursue the subject still further -let the Himalaya mountains, with the whole of Hindostan, sink down, and their place be occupied by the Indian Ocean, while an equal extent of territory and mountains of the same vast height rise up between North Greenland and the Orkney Islands. It seems difficult to exaggerate the amount to which the climate of the Northern hemisphere would then be cooled.

"Let us now turn from the contemplation of the winter of the great year,' and consider the opposite train of circumstances which would bring on the spring and summer. To imagine all the lands to be collected together in equatorial latitudes, and a few promontories only to project beyond

the thirtieth parallel, would be undoubtedly to suppose an extreme result of geological change. But if we consider a mere approximation to such a state of things, it would be sufficient to cause a general elevation of temperature. Nor can it be regarded as a visionary idea that amidst the revolutions of the earth's surface, the quantity of land should, at certain periods, have been simultaneously lessened in the vicinity of both the poles, and increased within the tropics. We must recollect that even now it is necessary to ascend to the height of fifteen thousand feet in the Andes under the line, and in the Himalaya mountains which are without the tropic, to seventeen thousand feet, before we reach the limits of perpetual snow. On the northern slope, indeed, of the Himalaya range, where the heat radiated from a great continent moderates the cold, there are meadows and cultivated land at an elevation equal to the height of Mont Blanc. If, then, there were no arctic lands to chill the atmosphere and freeze the sea, and if the loftiest chains were near the line, it seems reasonable to imagine that the highest mountains might be clothed with a rich vegetation to their summits, and that nearly all signs of frost would disappear from the earth.

"When the absorption of the solar rays was in no region impeded, even in winter, by a coat of snow, the mean heat of the earth's crust would augment to a considerable depth; and springs, which we know in general to be an index of the mean temperature of the climate, would be warmer in all latitudes. The waters of lakes, therefore, and rivers, would be much hotter in winter, and would never be chilled in