Another clay is called the Oxford clay. This is of a bluish colour, which alters on exposure to the-air, probably from a change in the oxidation of the iron which it contains. This clay is favourable to the growth of grass, and some of the richest pastures in Wiltshire and Oxfordshire have it for a subsoil, over which the decomposition of the roots and leaves of the grasses has formed a layer of vegetable mould of the highest degree of fertility. In the fens of Lincolnshire, the Oxford clay is covered by a coat of peat, formed by the decomposition of aquatic plants, which have accumulated wherever the water had no natural exit. When these fens were laid dry by an extensive system of draining, the peat was converted into a rich soil by the admixture of the clay which was found under it.
The Oolite formation contains much carbonate of lime, cemented by an unctuous earth into various aorta of stone. The soil which lies over them, and which is of nearly the same nature, but broken and disunited, is various in its qualities. Sometimes it is of great fertility, and sometimes nearly barren, according as the impalpable matter in it abounds and contains a duo proportion of the different earths, or it resembles a loose chalky sand, in which moisture is retained with diffi culty. In the first case, it produces every kind of grain in abundance nith moderate cultivation ; in the latter, it requires a great outlay of manure, which readily disappears, and then it is justly called a poor hungry soil.
On the red-sandstone is found a Boil which is usually of the finest quality. The fine loose soils of Devonshire and Somersetahire are of this description. It unites most of the requisites of a good soil, both iu its texture, neither too close nor too loose, and in the impalpable matter in its composition. It is peculiarly adapted to the growth of potatoes and all roota which form the basis of a judicious cultivation. When it contains a proper portion of calcareous earth, it may be reckoned amongst the most fertile soils ; and where this . is deficient, the addition of lime or chalk is the beat means of improving it. Tho calcareous earth seems greatly to add to the effect of the usual manures, so that a much smaller portion is required to produce good crops.
Each distinct formation gives rise to a great variety with respect to fertility, even where the basis remains the same : but it is of great importance to the farmer to ascertain the general nature of the rocks and strata on which his farm is incumbent.
Tho alluvial soils formed by the deposit of a variety of earths in a state of great division, and mixed with a considerable portion of organic matter, form by far the most productive lands. They will bear crop after crop with little or no addition of manure, and with a very slight cultivation. These soils are found along the course of rivers which traverse extensive plains, and which have such a current as to keep very fine earth suspended by a gentle but constant agitation, but not sufficiently rapid to carry along with it coarse gravel or sand. Wherever there is an obstruction to the current and an eddy is formed, there the soil is deposited in the form of mud, and gradually accumu lating, forms those alluvial soils which are so remarkable for their fertility when carefully protected from the inroads of the waters. In these soils the impalpable matter greatly predominates ; but the intimate mixture of the earths with organic matter prevents their con solidating into a stiff clay ; and the gases which are continually evolved from the organic matter keep the pores open, and give scope to the growth as well as the nourishment of the roots. It is in the alluvial soils principally that an accurate analysis is useful ; because the pro portion of their constituent parts varies in innumerable degrees. It may be laid down as a general rule, that the most fertile of these soils are those in which the earths are nearly in equal proportions, silica, however, being the most abundant, with about 10 per cent. of organic matter; a greater proportion of this last would form too loose and spongy a soil to bear good crops of corn, especially of wheat. But 4 per cent. of vegetable matter, with a good mixture of earths, and some phosphate of lime from the decomposition of bones and marine shells, produces a very good wheat soil The rich warp•lsnds along the Humber are artificial alluvial soils, and although they contain but a small proportion of organic matter, are highly fertile after their first deposition, but it is observed that they gradually become more tena cious and difficult of cultivation as this humus is carried off by the crops ; and that it is soon necessary to add animal and vegetable manures to supply its deficiency.