The only other drafting of sheep is that of the lambs after they are weaned. If intended for market, the wether and ewe lambs are separated, and marked into lots, that bear their dis tinctive marks with ruddle. If, on the other hand, they are to be kept on the farm, they are marked with tar with the marking iron, according to their sex; the ewe lambs being marked on the near and the wether lambs on the far rib. Many farmers mark their lambs on the ears also, to show that they have been bred on the farm, in contradistinction to those which may have been bought in; or some of them are so marked to distinguish them as the produce of particular and favourite ewes which breed good tup or ewe lambs. The mark in the ear is made with an instrument like the nippers which the shoemakers use to punch the holes through which the shoe-ties are passed, with this difference, that the shepherd's punch presses upon a piece of horn while making the hole through the ear of the lamb, and not upon metal. The ewe lamb's near ear is bored, and the far ear of the wether lamb. The hole can be placed farther towards the tip or the sides of the ear, or two holes can be placed upright or aside each other, all according to the desire of the farmer to particularise his stock. Cutting a small triangular piece out of the anterior or posterior margin of the ear, is a favourite mark for lambs, particularly for the shepherd's own little flock, to distinguish it from his master's. Removing wholly the tip of the ear, as some people do, very much injures the placid countenance of the sheep. The wether and ewe lambs are allowed to graze together during the summer on good pasture, but the tup lambs must of course be kept away from the ewe lambs.

Efficient drafting is of the utmost importance in the ewe flock, for on that depends the future character of the whole sheep stock. Whether the tups which are to serve the ewes have been bred on the farm, or bought or hired in, they must be put to such of the ewes as show a deficiency in the point or points which the particular tup possesses in an eminent degree. Having drafted the ewes well, there will be comparatively little difficulty in arranging them for the ram.

GEOLOGY VIEWED IN RELATION TO AGRICULTURE.

By W.

MACGILLIVRAY, A. M., &c. (Continued from p. 440.)

SECTION III. General View of the Vegetation of the Globe, and of the Causes which influence its Development.

WHEN a volcanic island emerges from the deep, scarcely has its fissured and porous mass cooled down to the temperature of the surrounding media, when traces of an obscure vegetation begin to manifest themselves upon its surface, which soon becomes covered with lichens of various kinds, one generation succeeding another, until of their decayed and partially decomposed fragments, a soil is prepared for the reception of mosses. These in their turn decay, and supply a stratum of soil fitted for the growth of other vegetables, the seeds of which, wafted by the currents of the atmosphere, or conveyed by other means, vegetate into tufts of verdure. Meanwhile, the weather acting upon the surface of the rock, and the roots of the plants penetrating into their fissures, an augmentation to the mass of soil is preparing, so that in time, becoming possessed of the necessary qualities, it is capable of supporting a vegetation of a higher order; and the once bleak and ominous mass is converted into a green and smiling island, the resort of aquatic birds. and marine animals. In like manner, the coral shoals, composed of depositions of carbonate of lime, elaborated by myriads of minute animals, and reared upon the summits of submersed mountains, are gradually invested with a vegetation resembling that of the neighbouring islands. In the Savannahs of the Orinoco, Humboldt found small platforms of granite, often 800 feet in circumference, elevated a few feet above the surface, and, although of an origin coeval with the emersion of the plains, manifesting in a striking manner the progress of vegetation. Lichens were observed cleaving the stone, and collected into crusts; little portions of quartz-sand nourished succulent plants; and, lastly, layers of black mould formed by the decay of roots and leaves, were deposited in the hollows, and shaded by tufts of evergreen trees.

In all these cases we are not sensible of any new creation.

The plants which appear are of the same species as others which we find scattered over portions of the earth's surface varying in extent and circumstances; and, as we have no evidence of a spontaneous generation or a development ex novo, we must suppose that their germs have been transported by the ordinary means of nature. There are numberless indications of the destruction of formerly existing species, but nowhere have we seen a new plant coming into existence. Nature, as man has seen her, has made no fresh display of creative energy.

The walls of caves are crusted with plants of the lowest order, which disclose their sombre hues and unimposing forms only to the curious inquirer. If a mine be opened into the bowels of the earth, we find its galleries in the course of time covered with peculiar forms of vegetable life. The depths of the ocean conceal unmeasured fields of a diversified and luxuriant vegetation, sometimes shooting up into stems of extraordinary length. Even the snows of the polar regions and alpine heights are not destitute of organic existence, as they have been found to contain the singular Protococus nivalis. Certain tribes prefer an elevated, others a low situation; some thrive in moist, others in dry places; the sand of the sea-shore, the gravel of the mountain slope, the dusty soil of the desert, even the rocky precipices, have their peculiar plants; and so curiously have the qualities and constitution of plants been varied, that, in a general point of view, it may be said that every kind of soil and situation has been furnished with an appropriate vegeta

tion.

But although the empire of Flora may be said to extend from pole to pole, and from the summits of the loftiest mountains to the depths of the ocean, the earth is not covered with a continuity of verdure. The polar regions of both its hemispheres, and the most elevated points of all its zones, are almost entirely destitute of vegetation; while vast plains occur, more especially within the tropics, where a few scattered plants with difficulty maintain their existence; and a large proportion of rocky surface is either naked and sterile, or crusted with plants which the curious eye hardly recognises as such.

Of the agents which modify the appearance of the vegeta

tion, heat is that which produces the most sensible effects. In a cold region, whether in the vicinity of the poles, or within the tropics, toward the summits of the higher mountains, the aspect of the vegetation is characterized by contracted and depressed forms. In the intertropical zones, and in certain parts of the temperate regions of the globe, where a high degree of temperature is constantly maintained, we find a luxuriance which we in vain look for elsewhere. But heat is less beneficial to vegetation in proportion to the deficiency of moisture. Thus, in the parched regions of central Africa, the caravans pass for many days in succession over plains of sand almost entirely destitute of plants, while the springs which occur at distant intervals are surrounded by lofty trees. The valley of the Nile, once the granary of the nations, would be an arid waste were it not for the inundations of the river. During the season of drought, the savannahs of South America are parched wildernesses, traversed by clouds of burning sand; but no sooner has the thunder rolled and the first showers fallen, than the fountains of life burst forth, the reviving sap again ascends in the blackened trunks of the scattered palms, and before the rainy season has closed, an ocean of verdure stretches to the horizon.

Those countries where heat and moisture are combined during the greater part of the year, furnish the most luxuriant vegetation. The banks of the great rivers of South America, and even of the Mississippi, display masses of perpetual verdure, of which those who have not seen them can form only a very inadequate conception. At the falls of the Orinoco, according to Humboldt, a single trunk, decorated with parasitic plants, affords nearly as great a diversity of vegetable forms as a whole field in the more northern parts of Europe. It might be supposed that excess of heat should prove an obstacle to the development of plants; and in the case of unconsolidated lavas and other products of volcanoes, it is certain that the heat would prevent vegetation, even were other circumstances favourable; yet, in many parts of the world, plants grow vigorously by the side of hot springs, and even in their waters, which are yet of a temperature approaching that of boiling. On the other hand, cold, although inimical to vegetation, may

exist during the greater part of the year at a very low degree, without destroying it. Thus, in Greenland and Lapland, when the snow, which has covered the ground from October, has disappeared in June, the peculiar vegetation of these countries appears uninjured. Many species of plants, such as saxifrages, ranunculi, willows, lichens and mosses, are peculiarly fitted for exposure to cold, and habitually occur on the limits of perpetual snow.

The chemical proportions of the atmosphere vary so little in the different regions of the globe, that in general their effect seems to be inappreciable. The sulphureous vapours of volcanoes, however, are destructive of vegetation; but the exhalations of marshy districts, so injurious to animals, or at least to man, have no deleterious effect on plants. Light is one of the most powerful agents in producing the perfect development of vegetables, and, without it, although many species may attain their ordinary size, they never produce seeds capable of germinating. Yet numerous plants, especially of the lower orders, thrive in, and are even peculiar to, caverns and excavations.

There was a period when it was imagined that the same specific forms of plants occur in all or in most parts of the world: but more accurate observation has shewn that, while some species have a very wide range, others are limited to particular spots, and that, in general, each natural district of the globe has a vegetation peculiar to itself. Thus, the vegetation of New Holland is entirely different from that of Great Britain, and the specific forms which present themselves on the lake of Geneva are unlike those that occur on the lake of Valencia. At the same time, abrupt transitions are rare, and the flora of one district or country is blended with that of another, certain species only appearing in the mass, which are characteristic of the region. Thus, the great mass of vegetation in Great Britain is similar to that of France; but in passing from the pole to the equator, species are found to disappear, and to be substituted by others in such a manner, that the extreme points, the shores of Greenland and the banks of the Amazon, for example, present a complete contrast. It would be difficult or impossible to point out a plant of general distribution; and, on the other hand, many species seem to be confined to particular