for it, and next by tilling and manuring in a suitable manner.  A clay soil, in which, in addition to the predominating alumina, there is a fair proportion of lime, may be regarded as the most fertile for all purposes; but we have few such in Britain, our clays being mostly of an obdurate texture, retentive of moisture, and requiring much cultivation, and containing, moreover, salts of iron in proportions and forms almost poisonous to plants.  But there are profound resources in most clays, so that if it is difficult to tame them, it is also difficult to exhaust them.  Hence a clay that has been well cultivated through several generations will generally produce a fair return for whatever crop may be put upon it.  Limestone soils are usually very porous and deficient of clay, and therefore have no sustaining power.  Many of our great tracts of mountain limestone are mere sheep-walks, and would be comparatively worthless except for the lime that may be obtained by burning.  On the other hand, chalk, which is a more recent form of carbonate of lime, is often highly productive, more especially where, through long cultivation, it has been much broken up, and has become loamy through accumulation of humus.  Between the oldest limestone and the latest chalk there are many intermediate kinds of calcareous soils, and they are mostly good, owing to their richness in phosphates, the products of the marine organisms of which these rocks in great part, and in some cases wholly, consist.  For the growth of cereals these calcareous soils need a certain proportion of silica, and where they have this we see some of the finest crops of Wheat, Trifolium, Peas and Beans in these islands.  If we could mix some of our obdurate clays with our barren limestones, the two comparatively worthless staples would probably prove remarkably fertile.  Although this is impossible, a consideration of the chemistry of the imaginary mixture may be useful, more especially to the gardener, who can in a small way accomplish many things that are impracticable on a great scale.  Sandy soils are characterised by excess of silica, and deficiency of alumina, phosphates and potash.  Here the mechanical texture is as serious a matter as it is in the case of clay.  The sand is too loose as the clay is too pasty, and it may be that we have to prevent the estate from being blown away.  It is especially worthy of observation, however, that sandy soils are the most readily amenable of any to the operation of tillage.  If we cannot take much out of them, we can put any amount into them, and it is always necessary to calculate where the process of enrichment is to stop.  It is not less worthy of observation that sandy soils can be rendered capable of producing almost every kind of crop, save cereals and pulse, and even these can be secured where there is some basis of peat or loam or clay with the sand.  The parks and gardens of Paris, Versailles, and Haarlem are on deep sands that drift before the wind when left exposed for any length of time with no crop upon them; and not only do we see the finest of Potatoes and the most nutritious of herbage produced on these soils, but good Cauliflowers, Peas, Beans, Onions, fruits, and big trees of sound timber.