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Staffordshire, some of which had, a few years before, been covered with lime, and others with burned marl and cinders, which substances had in every case been buried to the depth of several inches below the turf, just as if (as the farmers believe) the particles had worked themselves down. After showing the impossibility of this supposed operation, Mr Darwin affirms that the whole is due to the digestive process by which the common earth-worm is supported; since, on carefully examining between the blades of grass in the fields above mentioned, he found there was scarcely a space of two inches square without a little heap of cylindrical castings of worms—it being well known that worms swallow the earthy matter, and that having separated the nutritive portion, they eject at the mouths of their burrows the remainder in little intestine-shaped heaps. The same observer notices a still more remarkable instance of this kind, in which, in the course of eighty years, the earthworm had covered a field, then manured with marl, with a bed of fine rich mould, averaging thirteen inches in depth! Indeed there is no portion of the earth's crust that has not a tendency to be transformed into soil—whether by mechanical, chemical, or vital agency—save where the restless winds are ever shifting the loose material, the rains ever washing away, or the volcano ever scattering abroad its scorching showers of cindery and clinkery material.
And when the minute agencies by which these soils are formed, and their consequent slow rate of augmentation, are taken into consideration, it must be evident, thin and insignificant as they may appear in comparison with the underlying strata, that many of them must be of venerable age, and long antecedent to the historic era. The turf with which the Eomans, two thousand years ago, faced the circumvallations of their encampments, had depastured the herds of the native ages before the invaders set foot on these islands; and long before savage man erected his wigwams on British soil, that soil had been gradually accumulating under the conjoint influences of mechanical disintegration, chemical change, and vital aggregation. When digging through some fertile loam, with here a landshell and there a tooth, here a bronze spear and there a flint arrow-head, here the wood-ashes of some savage fire and there a circle of calcined cooking-stones, how little do we reflect on the long lapse of time of which these relics give evidence, or of the high antiquity of the earthy envelope that contains them! To the antiquarian and geologist this mere epiderm or scarf-skin of the earth's crust is a thing of deep and enduring interest, connecting the past with the present, and man and his works with the manifold forms that preceded him.
Such are the soils composed of inorganic and organic materials. Let us now turn to the subsoils, or purely mineral accumulations that lie beneath them. The former, being partly composed of organic material, and being exposed to the direct influences of air and rain, consist of many varieties—loams, sandy loams, clayey loams, unctuous clays, bog-earths, vegetable moulds, and the like; while the latter, unaffected by surface influences, consist mainly of clays, sands, gravels, and other rocky debris. Between the two, or immediately underlying the fertile soil, there frequently occurs what is termed by farmers a "sole " or "pan "—a thin hard layer of ferruginous or calcareous earth inimical to vegetation, and formed by matter dissolved from the soil and carried down by the percolation of rain-water. Beneath this "pan," where it occurs, lies the true subsoil, and may consist either of the disintegrated underlying rock-formation, of boulder-clay, drift-sand, or gravel, or it may be the miscellaneous silts of some dried-up lake, or the clay-silts of some carse or upraised estuary. These subsoils all vary, less or more, with the locality; here decomposed trap-rock, dry and friable—there the decomposed outcrops of sandstones and shales, tough and less pervious to water; here carse-clays and lake-silts soft and wet, but highly capable of amelioration—there boulder-clays, tenacious, retentive, and defying all culture; here drift-sands and gravels, dry and irregular in composition—there marine sand-silts, compact, variously retentive, and homogeneous in aggregation. In fact, the subsoils, properly so called, are merely those portions of the "superficial accumulations " of the geologist which are placed beyond admixture with organic matters, and consequently remain unconverted into dark, pulverulent, and fertile soils. Butthough not themselves converted into soils, many of them are capable of profitable admixture with the soils we cultivate, and a knowledge of their presence and nature is ever of advantage to the agriculturist. Here a tenacious and too retentive clay may be rendered friable and pervious by an admixture of sand, there a light sandy soil may be improved by an admixture of clay; here a soft vegetable mould may be consolidated by an admixture of loamy earth, and, vice versa, a poor silicious soil may be rendered sufficiently fertile by a corresponding admixture of vegetable mould. Soil and subsoil are intimately connected in formation and history; and with a sufficient knowledge of geology, the former could in many instances be rendered deeper, richer, and more fertile, and this by the mere admixture and treatment of the mineral matter that lies beneath it.
And if the soils in many instances give evidence of their high antiquity, and carry us back through thousands of years of slow and gradual accumulation, how much more must these subsoils, whether lake-silts, estuary-silts, river-drifts, or glacial accumulations! Though, geologically speaking, they are the most recent and superficial of all formations, yet, chronologically, they carry us far beyond human times, and to the days of the glacier and ice-sheet that enveloped all the higher latitudes of the northern hemisphere. From then till now, all that has been borne and laid down by the glacier and iceberg over the rocky formations—all that has been formed and accumulated by frost, rain, and wind, by river, lake, and wave—belong to these Post-tertiary formations, which are thus invested with a geological interest not inferior to that of any of the older systems. Though seldom noted on our maps and little studied in detail, these soils and subsoils are really of high geological interest, whether as regards their mode of formation, their composition, the fossil relics they contain, or their indispensable presence for the manifestations of vegetable life and agricultural fertility. In their composition and mode of aggregation we detect the agencies that have been at work from the glacial period down to the present day; in the relics they contain we can trace the local extinction of life-forms, and often the successive stages of the human inhabitants, thus giving them an antiquarian as well as geological interest; and in their relative natures, science can detect the means of their admixture and amelioration, and thus confer upon them, and especially upon the soils we cultivate, new and increased fertility and amenity. But it is less from an economical than from a scientific point of view that we now regard them—directing the attention of the general reader to a subject of interesting study in every field through which he takes his walks, and in every superficial opening that has been made by the spade and pick-axe of the delver and ditcher.
MAN'S PLACE IN THE GEOLOGICAL EECOED.
THE GEOLOGICAL RECORD—NATURE OF ITS CHRONOLOGICAL STAGES— DIFFICULTIES AND IMPERFECTIONS—COMPARATIVE RECENTNESS OF MAN'S PLACE—NATURE OF THE EVIDENCE—PREJUDICES TO BE COMBATED — MAN'S EARLIER LIFE - COMPANIONS IN SOUTHERN AND WESTERN EUROPE—THEIR REMOVALS AND EXTINCTIONS— TRACES OF HIS OWN RACE—PRE-HISTORIC AND HISTORIC—AGES OF STONE, BRONZE, AND IRON—SHELL-MOUNDS, CAVE-DWELLINGS, LAKE - DWELLINGS, ETC.—MAN IN OTHER REGIONS — GENERAL QUESTION OF MAN'S ANTIQUITY—HOW TO BE SOLVED.
Unlike the periods of human history, those of Geologyhave no definite expression in years and centuries. We speak of eras and epochs, of cycles and systems, but these are merely relative terms. They have no definite duration; the one merely precedes the other, and the larger may include many recurrences of the lesser within its limits. In speaking of geological time this is all that is signified; in fixing the dates of geological events this is all that can be fairly asserted. The Primary merely precedes the Secondary, the Secondary the Tertiary, and the Tertiary the events of the Current epoch. We may subdivide these greater stages into narrower limits, and talk of Laurentian, Cambrian, Silurian, Devonian, Carboniferous, Permian, Triassic, Oolitic, Cretaceous, Tertiary, and Eecent rock-systems, and this is no doubt restricting events to more precise bounds, but it gives no definite idea of duration, nor tells us how long the Chalk preceded the Tertiary, or the Tertiary the occur