_Nature of the Cements._—The nature of the cements that serve to consolidate the hardpan mass is substantially the same as those already mentioned in the discussion of sandstones (chapt. 4, p. 55); with the addition of those formed, usually in connection with siliceous solutions, by the acids of the humus group. The latter class of hardpans is especially conspicuous in the case of swampy ground and damp forests, where “moorbedpan” and reddish “ortstein” (the latter particularly developed in the forests of northern Europe, where it has been studied in detail by Müller and Tuxen[65], are characteristic.) The latter gives for a characteristic sample of the reddish hardpan underlying a beech forest in Denmark a content of from 2.20 to 4.40% of ulmic compounds, and shows that the color is due to these and not, as had been supposed, to ferric oxid, which is present only in minute quantities. [65] See “Studien über die natürlichen Humusformen,” by Dr. P. E. Müller. _Bog ore, Moorbedpan, Ortstein._—It is otherwise with moorbedpan, which often consists of a mass of bog iron ore permeated more or less with humous substances, which impart to it the dark brown tint so often seen also in the “black gravel” spots of badly-drained land. On the whole, however, ferric cements are much less frequently found in hardpans than in sandstones formed above ground. _Clay substance_ washed from the surface into the subsoil by rains (chapter 10, p. 161) always helps materially to render the hardpan impervious when afterwards cemented, a much smaller proportion of the cementing material sufficing in that case to form a solid layer. In such cases however the cement is rarely of a calcareous nature, since lime prevents the diffusion and washing-down of the clay. It is mostly siliceous or zeolitic; if the former, acid will have little or no effect upon the solidity of the hardpan; while if zeolitic, acid will pretty promptly disintegrate it. The presence of humus acids in the cements, if not apparent to the eye, is readily demonstrated by immersing the hardpan fragment in ammonia water or a weak solution of caustic soda; when if humus acids are the main cementing substance the fragment will fall to crumbs, or be softened to an extent corresponding to the amount of the humus present. Calcareous hardpan is, of course, readily recognized by its quick disintegration by dilute acid, with evolution of carbonic gas. In “alkali” soils containing sodic carbonate (“black alkali”) there is commonly found at the depth of two or three feet an exceedingly refractory hardpan resulting from the accumulation of puddled clay (see above chapt. 4, p. 62) in the subsoil, or sometimes even on the surface of depressed spots. This hardpan, easily destroyed by the use of gypsum and water, is described more in detail in chapter 22, on alkali soils; it blues red litmus paper instantly.