on such topics. If new land be raised from the sea, the greatest alteration in its physical condition, which could ever arise from the influence of organic beings, would probably be produced by the first immigration of terrestrial plants, whereby the tract would become covered with vegetation. The change next in importance would seem to be when animals enter, and modify the proportionate numbers of certain species of plants. If there be any anomaly in the intervention of man, in farther varying the relative numbers in the vegetable kingdom, it may not so much consist in the kind or absolute quantity of alteration, as in the circumstance that a single species, in this case, would exert, by its superior power and universal distribution, an influence equal to that of hundreds of other terrestrial animals.

If we inquire whether man, by his direct removing power, or by the changes which he may give rise to indirectly, tends, upon the whole, to lessen or increase the inequalities of the earth's surface, we shall incline, perhaps, to the opinion that he is a levelling agent. He conveys upwards a certain quantity of materials from the bowels of the earth in mining operations; but, on the other hand, much rock is taken annually from the land, in the shape of ballast, and afterwards thrown into the sea, whereby, in spite of prohibitory laws, many harbours, in various parts of the world, have been blocked up. We rarely transport heavy materials to higher levels, and our pyramids and cities are chiefly constructed of stone brought down from more elevated situations. By ploughing up thousands of square miles, and exposing a surface for part of the year to the action of the elements, we assist the abrading force of rain, and destroy the conservative effects of vegetation.

But the aggregate force exerted by man is truly insignificant, when we consider the operations of the great physical causes, whether aqueous or igneous, in the inanimate world. If all the nations of the earth should attempt to quarry away the lava which flowed during one eruption from the Icelandic volcanoes

in 1783 and the two following years, and should attempt to consign it to the deepest abysses of the ocean, wherein it might approach most nearly to the profundities from which it rose in the volcanic vent, they might toil for thousands of years before their task was accomplished. Yet the matter borne down by the Ganges and Burrampooter, in a single year, probably exceeds, in weight and volume, the mass of Icelandic lava produced by that great eruption.

CHAPTER XIII.

Effects produced by the action of animal and vegetable life on the material constituents of the earth's crust-Imbedding of organic remains in deposits on emerged land-Growth of Peat-Peat abundant in cold and humid climates— Site of many ancient forests in Europe now occupied by peat-Recent date of many of these changes-Sources of Bog Iron-ore-Preservation of animal substances in Peat-Causes of its antiseptic property-Miring of quadrupeds— Bursting of the Solway Moss-Bones of herbivorous quadrupeds found in ́peat -Imbedding of animal remains in Caves and Fissures-Formation of bony breccias-Human bones and pottery intermixed with the remains of extinct quadrupeds in caves in the South of France-Inferences deducible from such associations.

We now come to the second subdivision of the inquiry explained in the preceding chapter,-the consideration of the permanent modifications produced in the material constituents of the earth's crust, by the action of animal and vegetable life.

New mineral compounds, such as might never have existed in this globe but for the action of the powers of vitality, are annually formed, and made to enter into deposits accumulated both above and beneath the waters. Although we can neither explain nor imitate the processes of animal and vegetable life whereby those substances are produced, yet we can investigate the laws by virtue of which organic matter becomes imbedded in new strata,-sometimes imparting to them a peculiar mineral composition,-sometimes leaving durable impressions and casts of the forms of animate beings in rocks, so as to modify their structure and appearance.

It has been well remarked by M. Constant Prevost, that the effects of geological causes are divisible into two great classes; those produced on the surface during the immersion of land beneath the waters, and those which take place after its emersion. Agreeably to this classification we shall consider, first, in what manner animal and vegetable remains become included and preserved in solid deposits on emerged land, or that part

VOL. II.

Р

of the surface which is not permanently covered by water, whether of the sea or lakes; secondly, the manner in which organic remains become imbedded in sub-aqueous deposits.

Under the first division we shall treat of the following topics-1st, the growth of peat, and the preservation of vegetable and animal remains therein;-2ndly, the preservation of animal remains in stalactite, and in the mud of caves and fissures ;-3dly, the burying of organic remains in alluvium and the ruins of land-slips ;-4thly, of the same in blown sand;-5thly, of the same in volcanic ejections, and alluvions composed of volcanic productions.

The growth of Peat and the preservation of Vegetable and Animal Remains therein.

THE generation of peat, when not completely under water, is confined to moist situations, where the temperature is low, and where vegetables may decompose without putrifying. It may consist of any of the numerous plants which are capable of growing in such stations: but a species of moss (sphagnum palustre) constitutes a considerable part of the peat found in marshes of the north of Europe; this plant having the property of throwing up new shoots in its upper part, while its lower extremities are decaying*. Reeds, rushes, and other aquatic plants may usually be traced in peat, and their organization is often so entire, that there is no difficulty in discriminating the distinct species.

In general, says Sir H. Davy, one hundred parts of dry peat contain from sixty to ninety-nine parts of matter destructible by fire, and the residuum consists of earths usually of the same kind as the substratum of clay, marl, gravel, or rock on which they are found, together with oxide of iron. "The peat of the chalk counties of England," observes the same writer, "contains much gypsum; but I have found very little in any

* For a catalogue of the plants which contribute to the generation of peat, see Dr. Rennie on Peat, p. 171-178. Dr. Macculloch's Western Isles, vol. i, p. 129.

specimens from Ireland or Scotland, and in general these peats contain very little saline matter*." From the researches of Dr. Macculloch, it appears that peat is intermediate between simple vegetable matter and lignite, the conversion of peat to lignite being gradual, and being brought about in a great lapse of time by the prolonged action of water †.

Peat is sometimes formed on a declivity in mountainous regions where there is much moisture, but in such situations it rarely if ever exceeds four feet in thickness. In bogs, and in low grounds into which alluvial peat is drifted, it is found forty feet thick and upwards, but in such cases it generally owes one-half of its volume to the water which it contains. It nas seldom, if ever, been discovered within the tropics, and it rarely occurs in the valleys even in the south of France and Spain. It abounds more and more in proportion as we advance farther from the equator, and becomes not only more frequent but more inflammable in northern latitudes; the cause of which may probably be that the carbonic acid and hydrogen, which are the most inflammable parts, do not readily assume the gaseous form in a cold atmosphere.

There is a vast extent of surface in Europe covered with peat, which in Ireland is said to extend over a tenth of the whole island. One of the mosses on the Shannon is described by Dr. Boates to be fifty miles long, by two or three broad; and the great marsh of Montoire, near the mouth of the Loire, is mentioned by Blavier as being more than fifty leagues in circumference. It is a curious and well-ascertained fact that many of these mosses of the north of Europe occupy the place of immense forests of pine and oak, which have many of them disappeared within the historical era. Such changes are brought about by the fall of trees and the stagnation of water, caused by their trunks and branches obstructing the free drainage of the atmospheric waters, and giving rise to a marsh. In a warm climate such decayed timber would immediately be *Irish Bog Reports, p. 209. + System of Geology, vol. ii. p. 353.

Rev. Dr. Rennie, ibid. p. 260.