Section Iv.—of Geological Formations, and the General Characters of the

SOILS THAT REST UPON THEM.

The thousands of beds or strata of which I have spoken as lying one over the other in the crust of the globe, have, partly for convenience, and partly in consequence of certain remarkably distinctive characters observed among them, been separated by geologists into three great divisions—the primary, which are the lowest and the oldest; the secondary, which lie over them; and the tertiary, which are uppermost, and have been most recently formed. The strata, in these several divisions, have again been subdivided into groups, called formations. The following table exhibits the names and thicknesses of these formations, and the mineralogical characters of the rocks of which they severally consist.

I. TERTIARY STRATA.

1. The London and Plastic clays, 500 to 900 feet thick, consist of stiff, almost impervious, dark-coloured clays,—chiefly in pasture. The lower beds are mixed with sand, and produce an arable soil, but extensive heaths and wastes rest upon them in Berkshire, Hampshire, and Dorset.

II. SECONDARY STRATA.

2. The Chalk, about 600 feet in thickness, consists in the upper part (see diagram, No. 3, p. 88) of a purer chalk with layers of flint; in the lower, of a marly chalk without flints. The soil of the upper chalk is chiefly in sheep-walks, that of the lower chalk is very productive of corn.

3. The Green Sand, 500 feet thick, consists of 150 feet of clay, with about 100 feet of sand above, and 250 feet below it. The upper sand forms a very productive arable soil, and the clay impervious, wet and cold lands chiefly in pasture. The lower sand is generally unproductive.

It is an important agricultural remark, that where the clay (plastic clay) comes in contact with the top of the chalk, an improved soil is produced, and that where the chalk and the green sand mix, extremely fertile patches of country present themselves. (See pages 86 and 87.)

4. The Wealden formation, nearly 1000 feet thick, consists of 400 feet of sand, covered by 300 of clay, and resting upon 250 of marls and limestones. The clay forms the poor wet pastures of Sussex and Kent. On the sands below the clay rest heaths and brushwood; but where the marls and limestones come to the surface, the land is of better quality, and is susceptible of profitable arable culture.

5. In the Upper Oolite, of 600 feet in thickness, we have a bed of clay (Kimmeridge clay) 500 feet thick, covered by 100 feet of sandy limestones. The clay lands are difficult and expensive to work, and are chiefly in old pasture. The sandy limestone soils above the clay are also poor, but where they rest immediately upon, and are intermixed with the clay, excellent arable land is produced.

6. The Middle Oolite of 500 feet consists also of a clay (Oxford clay) dark-blue, adhesive, and nearly 1000 feet thick, covered by 100 feet of limestones and sandstones. These latter produce good arable land where the lime happens to abound; the clays form close heavy compact soils, most difficult and expensive to work. The extensive pasture lands of Bedford, Huntingdon, Northampton, Lincoln, Wilts, Oxford, and Gloucester, rest chiefly upon this clay, as do also the fenny tracts of Lincoln and Cambridge.

7. The Lower or Bath Oolite, of 500 feet in thickness, consists of many beds of limestone and sandstone, with about 200 feet of clay in the centre of the formation. The soils are very various in quality, according as the sandstone or limestone predominates. The clays are chiefly in pasture,—the rest is more or less productive, easily worked, arable land. In Gloucester, Northampton, Oxford, the east of Leicester, and in Yorkshire, this formation is found to lie immediately beneath the surface, and a little patch of it occurs also on the south-eastern coast of Sutherland.

6. The Lias is an immense deposit of blue clay from 500 to 1000 feet in thickness, which produces cold, blue, unproductive, clay soils. It forms a long stripe of land from the mouth of the Tees, in Yorkshire, to Lyme Regis in Dorset. It is chiefly in old, and often very valuable pasture.

9. The New Red Sandstone, though only 500 feet in thickness, forms the surface of nearly the whole central plain of England, and stretches north through Cheshire to Carlisle and Dumfries. It consists of red sandstones and marls,—the soils on which are easily and cheaply worked, and form some of the richest and most productive arable lands in the island. In whatever part of the world the red soils of this formation have been met with, they have been found to possess in general the same agricultural capabilities.

10. The Magnesian Limestone, from 100 to 500 feet in thickness, forms a stripe of generally poor thin soil from Durham to Nottingham, capable of improvement as arable land by high farming, but bearing naturally a poor pasture, intermingled with sometimes magnificent furze.

11. The Coal Measures, from 300 to 3000 feet thick, consist of beds of sandstones and dark-blue shales (hard clays), intermingled (interstratified) with beds of coal. Where the sands come to the surface, the soil is thin, poor, hungry, sometimes almost worthless. The shales, on the other hand, produce stiff, wet, almost unmanageable clays;—not unworkable, yet expensive to work, and requiring draining, lime, skill, capital, and a zeal for improvement, to be applied to them, before they can be made to yield the remunerating crops of corn they are capable of producing.

12. To the Millstone Grits of 600 feet or upwards in thickness the same remarks apply. They are often only a repetition of the sandstones and shales of the coal measures, forming in many cases soils still more worthless. When the sandstones prevail, large tracts lie naked, or bear a thin and stunted heath; where the shales abound, the naturally difficult soils of the coal shales again recur. These rocks are generally found on the outskirts of our coal-fields.

13. The Mountain Limestone, 800 to 1000 feet thick, is a hard blue limestone rock, separated here and there into distinct beds by layers of sandstones, of sandy slates, or of blue shales like those of the coal measures. The soil upon the limestone is generally thin, but produces a naturally sweet herbage. When the limestone and clay (shale) adjoin each other, arable land occurs, which is naturally productive of oats, yet, when the climate is favourable, capable of being converted into good wheat land. In the north of England a considerable tract of country is covered by these rocks, but in Ireland they form nearly the whole of the interior of the island.

14. The Old Red Sandstone varies in thickness from 500 to 10,000 feet. It possesses many of the valuable agricultural qualities of the new red, consisting, like it, of red sandstones and marls, which crumble down into rich red soils. Such are the soils of Brecknock, Hereford, and part of Monmouth; of part of Berwick and Roxburgh; of Haddington and Lanark; of southern Perth; of either shore of the Moray Firth; and of the county of Sutherland. In Ireland, also, these rocks abound in Tyrone, Fermanagh, and Monaghan; in Waterford, in Mayo, and in Tipperary. In all these places, the soils they form are generally the best in their several neighbourhoods, though here and there,—where the sandstones are harder, more siliceous and impervious to water,—tracts, sometimes extensive, of heath and bog occur.

III.—PRIMARY STRATA.

15. The Upper Silurian system is nearly 4000 feet in thickness, and forms the soils over the lower border counties of Wales. It consists of sandstones and shales, with occasional limestones; but the soils formed from these beds take their character from the general abundance of clay. They are cold, usually unmanageable, muddy clays, with the remarkably inferior agricultural value of which the traveller is immediately struck, as he passes westward off the red sandstones of Hereford on to the upper silurian rocks of Radnor.

16. The Lower Silurian rocks are also nearly 4000 feet in thickness, and in Wales lie to the west of the upper silurian rocks. They consist of about 2500 feet of sandstone, on which, when the surface is not naked, barren heaths alone rest.

Beneath these sandstones lie 1200 feet of sandy and earthy limestones, from the decay of which, as may be seen on the southern edge of Caermarthen, fertile arable lands are produced.

17. The Cambrian System, of many thousand yards in thickness, consists in great part of clay slates, more or less hard, which often weather slowly, and almost always produce either poor and thin soils, or cold, difficultly manageable clays, expensive to work, and requiring high farming to bring them into profitable arable cultivation. Cornwall, western Wales, and the mountains of Cumberland, in England; the high country which stretches from the Lammermuir hills to Portpatrick, in Scotland; the mountains of Tipperary, and a large tract on the extreme south of Ireland,—on its east coast, and far inland from the bay of Dundalk,—are covered by these slate rocks. Patches of rich, well cultivated land occur here and there on this formation, with much also that is improvable; but the greater part of it is usurped by worthless heath and extensive bogs.

18. The Mica Slate and Gneiss systems are of unknown thickness, and consist chiefly of hard and slaty rocks, crumbling slowly, forming poor, thin soils, which rest on an impervious rock, and which, from the height to which this formation generally rises, are rendered more unproductive by an unpropitious climate. They form extensive heathy tracts in Perth and Argyle, and on the north and west of Ireland. Here and there only, in the valleys or sheltered slopes, and by the margins of the lakes, spots of bright green meet the eye, and patches of a willing soil, fertile in corn.

A careful perusal of the preceding sketch of the general agricultural capabilities of the soils formed from the several classes of stratified rocks, will have presented to the reader many illustrations of the facts stated in the preceding section; he will have drawn for himself—to specify a few examples—the following among other conclusions.

1. That some formations, like the new red sandstone, yield a soil almost always productive; others, as the coal measures and millstone grits, a soil almost always naturally unproductive.

2. That good, or better land at least, than generally prevails in a district, may be expected where two formations or two different kinds of rock meet,—as when a limestone and a clay mingle their mutual ruins for the formation of a common soil.

3. That in almost every country extensive tracts of land on certain formations will be found laid down to natural grass, in consequence of the original difficulty and expense of working. Such are the Lias, the Oxford, the Kimmeridge, and the London clays. In raising corn, it is natural that the lands which are easiest and cheapest worked should be first subjected to the plough; it is not till implements are improved, skill increased, capital accumulated, and population presses, that the heavier lands will be rescued from perennial grass, and made to produce that greatly increased amount of food for both man and beast, which they are easily capable of yielding.

The turnip soils of Great Britain are in many districts, it may be, but indifferently farmed; and the state has reason to complain of much individual neglect of known and certain methods of increasing their productiveness; but the next great achievement which British agriculture has to effect, is to subdue the stubborn clays, and to convert them into what many of them are yet destined to become, the richest corn-bearing lands in the kingdom.