CHAPTER VI. 1. THE GEOLOGICAL NATURE OF THE SOIL.-2. THE DEGREE OF Nature of the soil in relation to THE geological nature of the soil bears very important relations to the sanitary condition of a country,—its configuration and pathological geological characters having a close connection, within a certain conditions of limit, with certain pathological conditions of man. certain man. Clayey soils. The compact nature of clayey soils prevents it from admitting both heat and air. It permits the superficial retention water and moisture of every kind. "It generates, how partially soever, those marshy or undrained spots, or wet woods, or moist meadows, which are the sources of malaria, and, consequently, of the various diseases confounded with the vague term unhealthiness." The atmosphere on clayey soils is generally more cool than on other soils, because during the heating process they give out a large quantity of the contained water in the form of vapour, which acts beneficially on the heated air; the air is, however, very damp and foggy, and its effect on the system is very relaxing. The inhabitants of such soils are most subject to colds, coughs, and rheumatism. Clayey soils absorb about ten or twenty times more water than sandy soils, and twenty or thirty times less than surface soils. Clay soils absorb heat less than sandy soils, and cool down more rapidly also; they allow less quantity of air to pass through them. soils. Gravelly soils are, when dry, very healthy; they are easily Gravelly permeated by air, water, and light, and quickly get dry; they are easily drained, and prevent the accumulation of vegetable putrescent matter. soils. Ferruginous soils absorb heat rapidly, and give it out also Ferruginous rapidly. Sir Ranald Martin considers them most unhealthy and deadly. From a priori reason I consider that this is not the case, but I shall say more on the subject when we come to consider the ferruginous soils of Western Africa. Sandy soils in the tropics absorb heat greatly, and are very Sandy soils. slow in its radiation in many cases. The heat during the day is very great, and the radiation being slow, at night it is very oppressive, unless other climatic circumstances lessen it: these conditions may be modified by covering these soils with green vegetation. Sand absorbs very little water; it passes very rapidly through it, and, cœteres paribus, it is generally healthy; but sometimes it contains organic decomposing matter, or the sand is the superficial covering of a clay mould, in which cases sandy soils may become very unhealthy; in which latter condition, the air is generally moistened by the evaporation from the clayey subsoil. Chalky soils are very healthy, especially when not mixed chalky soils. with marl; they are easily permeable to light and water, the latter of which they readily absorb, and slowly return it to the atmosphere; the air is generally dry and bracing. light-colour The colour of the soil, also, has some important effect. Dark and Dark soils absorb and radiate heat more powerfully than light- ed soils. coloured soils; but the latter reflect light more, and are hotter from the rays of the sun being also reflected. Dr Parkes has given the following " General Observations on Parkes' obthe Healthiness of Soils:"__* 1. The Granitic, Metamorphic, and Trap Rocks.—Sites on these formations are usually healthy; the slope is great; Dr Parkes' "Practical Hygiene," p. 256. servations on the healthiness of soils. water runs off readily; the air is comparatively dry; vegetation is not excessive; marshes and malaria are comparatively unfrequent, and few impurities pass into the drinking water. Such regions are often elevated, strong currents of air are more frequent, and the particles derived from the dried stools are carried away. When these rocks have been weathered and disintegrated, and when they often yield a red or dark-coloured soil, they are supposed to be unhealthy. Such soil is certainly absorbent of water. 2. The Clay Slate.-These rocks precisely resemble the granite and granitoid formations in their effect on health. They have usually much slope; are very permeable; vegetation is scanty, and nothing is added to air or drinking water. They are consequently healthy; water, however, is often scarce, and, as in the granite districts, there are swollen brooks during rain, and dry water courses at other times swelling rapidly after rains. 3. The Limestone and Magnesian Limestone Rocks.-These so far resemble the former that there is a good deal of slope and rapid passing off of water. Marshes, however, are more common, and may exist at great heights. In that case, the marsh is probably fed with water from some of the larger cavities, which, in the course of ages, become hollowed out in the limestone rocks by the carbonic acid of the rain, and form reservoirs of water. The drinking water is hard, sparkling, and clear. Goitre is more common, and, it is said, renal calculus. Of the various kinds of limestone, the oolite is the best, and magnesian is the worst; and it is desirable not to put stations on magnesian limestone if it can be avoided. 4. The Chalk. The chalk, when mixed with clay and permeable, forms a very healthy soil. The air is pure, and the water, though charged with carbonate of lime, is clear, sparkling, and pleasant. Goitre is not nearly so common, nor apparently calculus, as in the limestone districts. If the chalk be marly, it becomes impermeable, and is then often damp and cold. The lower parts of the chalk, which are underlaid by gault clay, and which also receive the drainage of the parts above, are often very malarious. 5. The Sandstones.—The permeable sandstones are often very healthy; both soils and air are dry; the drinking water is, however, sometimes impure. If the sand be mixed with much clay, or if clay underlies a shallow sand rock, the site is sometimes damp. In choosing such a site, the water should be always carefully examined. The hard millstone grit formations are very healthy, and their conditions resemble those of granite. 6. Gravels of any depth are always healthy, except when they are much below the general surface, and water rises through them. Gravel hillocks are the healthiest of all sites; and the water, which often flows out in springs near the base, being held up by underlying clay, is very pure. 7. Sands. There are both healthy and unhealthy sands. The healthy are the pure sands, which contain no organic matter, and are of considerable depth. The air is pure, and so is often the drinking water. Sometimes the drinking water contains enough iron to become hard, and even chalybeate. The unhealthy sands are those which are composed of silicious particles (and some iron), held together by a vegetable sediment. It is nearly impermeable to water, but water dissolves gradually the vegetable matter, and acquires a brownish yellow colour, and, if it comes from six feet in depth, has a marshy odour. It is most unwholesome, and causes intermittent and visceral engorgements. Chemical and microscopic analyses will detect this condition. In other cases sand is unhealthy, from underlying clay or laterite near the surface, or from being so placed that water rises through its permeable soil from higher levels. Water may thus be formed within three or four feet of the surface; and in this case the sand is unhealthy, and often malarious. Impurities are retained in it, and effluvia traverse it. Merely digging for water in the wet season will cause the discovery of these conditions. In a third class of cases the soils are unhealthy because they contain soluble mineral matter. Many sands contain much carbonate of magnesia and lime salts, as well as salts of the alkalies. The drinking water may thus contain a large quantity of carbonate of soda, and even lime and magnesia salts and iron. 8. Clay, Dense Marls, and Alluvial Soils generally.-These are always to be regarded with suspicion. Water neither runs off nor runs through; the air is moist; marshes are common; the composition of the water varies, but it is often impure with lime and soda soils. In alluvial soils there are often alternations of thin strata of sand and sandy impermeable clay. Much vegetable matter is often mixed with this, and air and water are both impure. If such spots must be chosen (for a station), thorough subsoil draining, careful purification of water, and elevation of the houses above the soil, are the measures which must be adopted. The deltas of great rivers present these alluvial characters in the highest degree, and should not be chosen for sites. If they must be taken, only the most thorough drainage can make them healthy. 9. Cultivated Soils.-Well cultivated soils are often healthy, nor at present is it known that the use of manure in any form has been hurtful. Irrigated lands, and especially rice fields, which give not only a great surface for evaporation, but also send up organic matter into the air, are hurtful. Where a country is densely populated, and where agricultural pursuits are carried on extensively and to a high degree, the health of the country must of necessity be greatly improved; malaria is prevented from being generated, and the death rate is much diminished. But when a land is first cleared and the soil broken up for cultivation, the exposed land might be the source of malaria for some time; it is always better that it should be done in the heat of the day. |