ture of their component parts, has not oftener induced agriculturists to have recourse to various artificial mixtures of the materials of rocks which are adjacent to each other, either as regards the surface, or depth below the surface. There are districts in which such mixtures have been practised with the greatest success; but, in general, farmers rely too exclusively on farm-yard manure. Clay, sand, and limestone, are, nevertheless, mineral manures of the greatest value, and have changed the face of whole districts that were before comparatively barren. By such mixtures the constitution of the soil has been improved,-causing the animal and vegetable manure afterwards applied to be more efficacious. The clayey lands of Essex have been greatly improved by the use of chalk. This acts upon the land in several ways. It decomposes any free acids, and some acids in combination, naturally existing in the soil, and which are prejudicial to vegetation, and it acts mechanically, by rendering the soil more pervious to moisture, and affording greater facilities for the roots of plants to expand. When chalk is not at hand, a dressing of sand or fine gravel will produce some of these results, particularly if it contains calcareous matter, in the shape of fragments of limestone or broken shells. Clay, again, is equally efficacious on sandy soils, by increasing the power of the soil to retain moisture, and by enabling the roots to maintain themselves firmly in the ground. In Norfolk and Suffolk, vast tracts of land, which were before incapable of bearing corn, have, by the application of clay, been made to produce good crops of wheat, barley, clover, and turnips. Thus a greater number of cattle are kept upon a given area, and the quantity of animal and vegetable matter returned to the soil is proportionably increased. The mineral manures are, in general, too much neglected, with the exception of lime (the injudicious use of which too often runs into the abuse), and even in those districts where they are applied, they are frequently brought from a distance, when, though not visible on the surface, they exist at a small depth below it, sometimes under the very field for the improvement of which they are required. Now, who, we would ask, is most likely to discover them,-he who never looks deeper into the earth than the bottom of his ditches, or he who studies the position of rocks, with respect to each other, and for this purpose examines every natural section by which they are exposed to view in cliffs and ravines, and every artificial section laid open by mines, wells, and other excavations. Let AB in the annexed diagram (fig. 1.) represent the surface of a district composed, like some of the eastern parts of England, of strata of sand, clay, and gravel, resting upon chalk, all the strata having a slight inclination to the eastward, or towards B: a farmer cultivating the sandy soil at a, knows, for he observes it in riding to market, that at a certain distance from his own farm he shall cross a tract of clay, b, and that, after leaving that, he shall meet with sand and gravel again at c, and that at d he shall quit the plain and reach hills of chalk. But he does not know, and he would probably laugh at the person who communicated the information, that all the strata b, c, and d, are to be found under his own land at a, at a depth proportioned to the thickness and inclination of the strata, so that a bed which, in one part of an estate will be at a considerable depth, may rise in another very near the surface. But these are points to the determination of which the geologist applies himself, and having ascertained, from the nature of the embedded fossils, that the stratum, a, is the crag, and b the London clay,—he knows that by proceeding in a direction contrary to that in which the strata dip, he shall meet with the sands and gravels of the plastic clay, as well as with the chalk rising successively to the surface. be the A knowledge of the stratification of a country will also prove valuable to those engaged in agricultural improvements, by enabling them to drain their land more effectually, and at less expense, than by the ordinary methods. A good soil, as was before observed, may rendered unproductive by resting on a bed of clay, which holds up water, and there may be a porous bed again beneath the clay. Now, if the thickness of the clay is not too great, such land may be freed from excessive moisture by perforating the bed of clay, and leading the surfacewater into the perforations. These The dip or inclination, and also the "faults" or dislocations of the strata of the district, ought to be studied with the same view. faults are fissures which traverse the strata, and produce two opposite effects with respect to drainage, according to the nature of the substances with which they are filled. Sometimes they are pervious to water, and then they act as natural main drains, into which subordinate drains may be turned. In other cases they are filled with clay, and then they act as mutual dams, holding up the water in the strata to a higher level on one side of the fault than on the other. A fault so filled is often traceable on the surface by the land-springs which break out along its course, arising from the pent-up water struggling to escape wherever there is the least resistance; and cases will occur in which, by piercing through the fault, vent may be given to the water which before burst out at several smaller openings, and the land may thus be laid dry at a cheaper rate than by contending with each spring individually. This water, which was injurious in one place, will sometimes be in sufficient quantity, when thus collected into one channel, to improve another part of the estate, by being employed in irrigation, or by being made to drive an overshot wheel. supply of water for both these purposes. Artesian wells have been bored to obtain a The nature of a country is sometimes such that it is destitute of rivers and surface-springs, and then the population are dependent for a supply of water upon that which can be obtained by means of very deep wells, or upon that which falls from the atmosphere, and is collected in ponds or tanks. The water derived from the former source is obtained at a great expense; that "derived from the latter is by no means wholesome, and is liable to fail in dry summers. In such seasons, we have seen the population of an extensive parish almost fighting for the green unwholesome water of a muddy pond; and cases have occurred, in which it has been necessary to drive the cattle so far to drink, that by the time they returned, they were as thirsty as when they started. Few greater benefits could be conferred on a district so circumstanced, than the establishment in it of Artesian wells. These derive their name from the district of Artois, in France, where they are supposed, though it appears erroneously, to have been first used. Artesian wells are nothing more than perforations a few inches in diameter, made through the strata with the ordinary boringtools, and their action arises from the natural tendency of water to find its level. It is only under certain conditions of geological structure that they can succeed; and the structure of the district in which it is proposed to introduce them ought to be thoroughly investigated before any borings are attempted, because, though one of these wells may be made for onethird of the cost of an ordinary well of the same depth, yet it will be perfectly useless, though a water-bearing stratum should be penetrated, if the water will not rise to the surface, the orifice being too small to admit of its being raised by mechanical means. If a reservoir of water were formed on the side of a hill, and a leaden or other closed pipe were laid from it to a hill on the opposite side of the valley, the water might be conducted in this pipe to the same height as that of the source, and wherever a hole was made in the pipe, the water would gush out and endeavour to attain the level of that at the two ends. The geological structure answering to these conditions is shown in the annexed diagram (fig. 2.), which represents the stratification of the vale of London. is a retentive stratum at the surface (in this case the London clay); a a Fig. 2. bb a bed of gravel and sand below it, both of them resting upon the chalk, cc; and the whole series having been thrown, by some subterranean movement, into a curve, concave on the upper side. This, in geological language, is the basin-shaped structure. The porous bed, b b, fulfils the office of the closed pipe before mentioned. The water which falls on the chalk-hills, c c, is absorbed by this porous bed, and prevented from escaping by the bed of clay above, and by the chalk below it; and if the clay be pierced by small borings as at d, e, and f, will rise to the surface, or to heights above the surface, proportioned to the elevation of the source, and to the obstacles which the water meets with in percolating through the bed, bb. There are other modifications of this structure which need not be noticed in a brief sketch like this. There are few persons conversant with rural affairs, who are not aware how much good roads improve the value of property by facilitating the conveyance of the produce of the land to a market, and either for the improvement of old, or the formation of new roads, a knowledge of the strata over which they are formed, is of the utmost importance. The durability of a road depends in a great measure upon the materials employed, and upon the solidity of its foundation. These points are, in general, too much neglected by those to whom the formation of roads is intrusted, the chief objects of attention being the shortening of distance, and the reduction of acclivity; and to obtain these a road is often carried over strata affording an unstable base, and liable to frequent land-slips. Soft and bad stone is also frequently employed, because it is near at hand, when better materials might be brought from a distance by means of canals and railways, at a small extra expense, which would be in the end more economical, because of their greater durability, and because when used, they afford a good instead of a bad road. On the other hand, stone of inferior quality is often brought from a distance of several miles, when better exists much nearer; not quarried, it is true, nor visible on the surface, but easily discoverable by the eye of the geologist. Hardness is not the only essential in a stone employed for the repair of roads. It is required not only to resist friction, but also the crushing force of heavy carriages, moving with considerable velocity. Hence a certain degree of toughness is necessary to durability. Yet when hard and tough materials are both at hand, the latter is often neglected for the former. There are situations in which flints from the chalk, and chert from the green sand, might be obtained for the roads at the same cost. The chert, as the toughest, is the best; yet surveyors of roads, looking only to hardness, or perhaps merely adhering to established custom, almost invariably prefer the flints. Masses of greenstone, again, are of frequent occurrence in the midst of granitic districts. Granite is reduced to powder under the crushing action of wheels much sooner than greenstone, owing to the superior toughness of the latter, which arises from the presence of the mineral called hornblende.. This mineral enters largely into the composition of most of the trappean rocks, a numerous family, which, though differing much in external aspects, all possess, in a greater or less degree, the toughness essential to a good material for roads. We will illustrate this subject by one or two examples. The improvement which has taken place of late in the roads in the neighbourhood of London, must be well known to most of our readers. Much of this has been effected by reducing the surface to a better form, and by applying the materials used in a more judicious manner; but much has arisen from the selection of better materials. Under the old system, these consisted exclusively of gravel raised in the neighbourhood, much mixed with sand and clay, from which it was difficult effectually to clear it. In mineral composition, it consisted chiefly of flint, but the pebbles being for the most part round, could never form so solid a mass as a layer of angular pieces of stone. The first improvement was to substitute for this gravel broken flints from Kent, brought up the river Thames. Possessing hardness only, without tough ness, their brittleness was found to be an objection against them, and granite was substituted. This was found more durable than flint, but less so than a hornblende-rock from Mount Soar Hill, in Leicestershire, which is now chiefly employed, and with manifest advantage. The surface of the counties of Lancashire and Cheshire is occupied chiefly by the new red sand-stone formation, which affords no road materials, but stone of the most friable kind. The roads of this extensive district are now supplied with a trap-rock from Penmaen Mawr on the coast of Caernarvonshire, and the quantity used is so great that even at the low price at which it is supplied, the persons who have taken a lease of these rocks are realizing large profits. Stone of the same kind occurs in many parts of the Snowdonian chain, but nowhere so near the sea, except in Caernarvon Bay; and even the small additional freight which must be paid for its conveyance thence, is sufficient to prevent it from competing successfully with the stone of Penmaen Mawr. Whoever, therefore, shall succeed in discovering within the district thus supplied a mass of trappean-rock, so situated as to be easily quarried, may be assured that he has made a valuable discovery. Mr. Murchison has shown that in a part of Shropshire trappean-rocks have burst through the red sand-stone, and it is by no means improbable that the action may have been prolonged into Cheshire. It sometimes happens that only a small portion of trappean-rock rises to the clay, when there is a large mass at a small depth below the surface; and the surface-appearances are also such as to deceive a person in search of road-materials who was unpractised in geological investigations; for some of the hardest and toughest of these rocks decompose by long exposure to the atmosphere, presenting the appearance of an incoherent bed of earth, on removing which, the unweathered rock will be found to afford a stone well adapted to the repair of roads. Attention to the direction in which the strata dip will frequently save considerable expense, both in the first construction and future maintenance of a road, when it is carried along the side of a hill through a stratified country. Let A (fig. 3.) represent a valley between the hills B and C, composed of many alternations of shale and sand-stone, having a steep dip; so that had the beds of в not been denuded, they would have covered those of the hill c. Let a and b, moreover, be two beds of loose sandstone, on opposite sides of the valley, with beds of clay or shale beneath them. If a road be cut at c, the water percolating through the sand |