If a soil contain more than 5 per cent. of carbonate of lime, it is called a _marl_; if more than 20 per cent., it is a _calcareous_ soil. _Peaty soils_, of course, are those in which the vegetable matter predominates very much. To estimate the lime, a quantity of the soil should be burned in the air, and a weighed portion, 100 or 200 grains, diffused through half a pint of cold water mixed with half a wine glassful of spirit of salt (muriatic acid), and allowed to stand for a couple of hours, with occasional stirring. The water is then poured off, the soil dried, heated to redness as before, and weighed: the loss is nearly all lime.[11] [11] Unless the soil happen to contain a large quantity of magnesia, which is rarely the case. The quantity of vegetable or other organic matter is determined by drying the soil _well_ upon paper in an oven, and then burning a weighed quantity in the air: the loss is _nearly all_ organic matter. In stiff clays this loss will comprise a portion of water, which is not wholly driven off from such soils by drying upon paper in the way described. SECTION III.—OF THE DIVERSITIES OF SOILS AND SUBSOILS. Though the substances of which soils _chiefly_ consist are so few in number, yet every practical man knows how very diversified they are in character—how very different in agricultural value. Thus, in some of our southern counties, we have a white soil, consisting apparently of nothing else but chalk; in the centre of England a wide plain of dark red land; in the border counties of Wales, and on many of our coal-fields, tracts of country almost perfectly black; while yellow, white, and brown sands give the prevailing character to the soils of other districts. Such differences as these arise from the different proportions in which the sand, lime, clay, and the oxide of iron which colours the soils, have been mixed together. But how have they been so mixed—differently in different parts of the country. By what natural agency?—for what end? Again, the soil on the surface rests on what is usually denominated the _subsoil_. This, also, is very various in its character and quality. Sometimes it is a porous sand or gravel, through which water readily ascends from beneath or sinks in from above; sometimes it is light and loamy like the soil that rests upon it; sometimes stiff and impervious to water.