The Elements of Agricultural Geology: A Scientific Aid to Practical Farming ...

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Preface

SoMJB thirty years a^ the author of this volume — them working as a lad ou his father's farm among the hilb of Kyle, in Ayrshire — read Geikie's " Story of a Boulder," and acquired therefrom a taste for geological science, which grew stronger as he grew older. The influence of geological structure, not only in forming the features of the country, but also in modifying the nature of the soils and the farming carried out thereon, struck him very early in his observations, and at least twenty-five years ago he began to systematically study the subject and to collect information thereon. As part of this investigation the consultation of many books was necessary, and in recent years three London libraries have been ransacked — including that of the Geological Society — to find any information bearing on the application of geology to agriculture. In addition, collectors' lists were examined, and in the result a respectable private library of books and pamphlets bearing upon this special line of study has been accumulated.

The greater part of the information, however, for which the author is indebted to books, has been derived from the volumes of the Jcnvmal of the Royal Agricultural Society of England, extending over 60 years, embedded in which are many Reports bearing directly on the subject. Outside of the English language, the only publication of any value known to the author is Risler's " Geologic Agricole," in French, of which great use has been made, for, strange to say, no German scientist has ever tackled the matter, so far as he can find out.

Vlll PREFACE.

While many writers have thus been laid under contribution, as is shown by the footnotes, the more important sections of the volume can be claimed as the result of the author's own investigations, carried on for a great number of years. The method he followed was to procure the sheets of the Geological Survey maps for any district to be studied over or even passed through, and to cut them into small squares and paste them on the leaves of notebooks. With these in his pocket, he has worked over hundreds of farms in various parts of the United Kingdom, watched and noted the various kinds of farmiug passed on railway journeys or on drives, and collected samples of soil for examination where the maps indicated typical surface deposits.

All the information so gathered has been sifted and arranged in the light of his own lifelong practical experience. For the last twenty years he has been farming on the London clay in Essex, and this volume has been prepared in the intervals of leisure in the management of a large liolding of mixed husbandry — in the winter evenings, on wet or stormy days, in the summer mornings between sunrise and the beginning of the day's work, and so on ; the preparation of the manuscript taking ten years in its accomplishment.

It is now presented to the public who are interested in scientific farming, as an honest endeavour to help on the development of the most ancient and the most important of all human occupations — agriculture.

ONaABFABK HaLL,

Onoar, Essex,

Chapter I. In The Beginnimq

When I was a very small boy at school the elementary geography I first grappled with informed me that the earth was an " oblate spheroid," and that this was a proof that it had at one time been a molten mass. When I grew up I had the curiosity to find out what on earth an oblate spheroid is, and I discovered it to be a globe slightly flattened at the poles or spindle-ends, and that this flattening was due to it whirling round in space while in a liquid state.

The ancients believed that we lived on a huge flat cake, and that if anyone went to the edge he would fall over, while now a twentieth century philt)sopher informs us that the earth is not. an oblate spheroid after all, but tlxat it is shaped like a boy's spinning top, dwindling down to a point at the south pole, as we can see proved with our own eyes from tlie shape of Africa and South America.

Apart from this, scientists tell us that unnumbered ages ago this earth, in common with the whole planetary system, and probably the whole visible universe, existed in the state of incandescent gas or vapour, diffused in irregular patches throughout that mysterious entity we call " Space," and that at leaat the portion corresponding to our Solar System had a rotary motion from west to east. This theory — known as '^ KanVs Nebular HypotJiesis^^ — most satisfactorily and logically explains the first appearance of this world of ours as read in the light of the phenomena all around us, and is most in conformity with the state of matters at tlxat far-off beginning when '* the earth was without form and void." The sixty-odd elementary bodies which compose all matter with which we are cognisant, existed then in the simple uncombined gaseous state,- or in such compounds as may remain stable and unchanged at a temperature very much above white heat. In process of time the heat contained in these clouds of nebular matter or incandescent gas radiated off into the outer regions with the result that eventually the material condensed into the liquid form, and by the ordinary action of the attraction of gravity, j^Ztw the initial rotary motion, gathered together into the masses forming the suns and planets with which the universe is filled, while the various nebulse known to astronomers are remnants of the gaseous matter which is still in the process of cooling to form new worlds.

In the molten state the earth was surrounded by an atmosphere of various gases, the future water in the form of steam — or its component oxygen and hydrogen gases — composing the greater portion of it. The liquid globe of the earth itself consisted pretty much of molten lava, or of some material — or rather perhaps of a mixture of materials — similar to that of which granite, diorite, lava, basalt, and rocks of a similar nature, are the soUditied and perhaps partly altered forms. We do not know what was the character of the rock or rocks which formed the crust of the earth when the surface of the heated mass first cooled.* Some held at one time that granite claimed the highest antiquity, but we now know that granite may be a comparatively recent, as well as a primary igneous (plutonic) rock. Bearing in mind the fact that volcanic rocks — which are recently igneous in contradistinction to the older or " plutonic" forms — take the forms of trachyte, obsidian, phonoUte, pumice, volcanic ash, as well as dolerite, basalt, trap, and lavas generally, we are at liberty to assume that the original crust of the earth, formed on the first cooUng of the mass, was made up of as diverse materials as the ejectamenta of a modem volcano. It is doubtful if the primitive rock now exists anywhere sufficiently near the surface of the modern world to be accessible,! owing to the denudations, re-deposits, metamorphisms, upheavals, and depressions which have constantly been going on for an unthinkable length of time before the advent of Man ; though the "Fundamental Gneiss," or granite-gneiss of ArchiPan formation, is by some held to be the lowest, or oldest form of rock, or at least the oldest rock in the British Islands. J Prestwich says that it is hardly possible at present to draw the line where the original mother rocks end and the derivative strata commence.g

♦ Rutley : " Study of Rocks," p. 7.

t Stockbridge : ** Rocks and Soils," p. 5.

+ R«imsay : "Physical Geology and Geography of the British Islands," 3rd Edition, p. 44.

§ Prestwich : " Geology," Vol 11, p. 19.

Dawson is of opinion that this formation of the Lower Laurentian is the first that ever was formed, as it is the lowest we have access to, and is of exceptional character — aqueo-igneous. It had in the initial stages formed the "slaggy" crust, and shows bedding from cooling in layers under water in the later stages, with molten matter injected into the interstices.*

The time that has elapsed since this first crust was formed has been estimated at various figures by different authorities. Lord Kelvin at one time allowed that it might be as much as 100,000,000 years, he then reduced his estimate to 20,000,000 years. On the other hand, geologists calculated that at the present rate of denudation of one foot all over the land surface in from 4,000 to 5,000 years, the 70,000 feet of sedimentary strata would represent 86,000,000 years, though some — ^as Wallace, Houghton, and Dawson — reckoned 28,000,000 years as not far off the truth.t These allow, however, that organic life did not appear till about 15 or 16 millions of years ago.

Opinion among scientific men is thus divided as to the time that has elapsed since the globe became habitable. Some geologists require an almost immeasurable number of years to allow for the slow deposition of the strata with their contained fossils, reckoning that changes were as slowly accomplished then as now ; while on the other hand, physicists calculate from the known rate of the cooling of the earth, that it became habitable only about 15J millions of years ago, and that all the geological changes — or at least the deposition of all the fossiliferous rocks— must have taken place within this time.

As the age of the earth does not particularly concern the object of this book, we may leave it now and let the scientists interested in the matter fight it out among themselves.

Molten material of all kinds contracts on cooling, and if the arrangement of the mass does not allow of contraction, then it will wrinkle up or crack and break. This has happened with the crust of the earth, and the gradual contraction of the mass of the globe from cooling has given rise to wrinkles and creases and fissures, which formed the first hills and hollows — the larger of the latter being filled kter on with water to form the first seas. It is, indeed, held by many geological authorities of eminence that these primal folds have remained in existence ever since ; in other words, that

* Dawson : *' Salient Points in the Science of the Earth," p. 18. t Ibid., p. 4ia.

the larger continents and oceans were then formed, and have remained more or less constant down to the present time ; the depressions below sea-level necessary for the vast accumulations of sedimentary strata now forming the dry land above sea-level being only comparatively local and temporary. In this way the Atlantic (North and South), the Pacific, and the Indian Ocean basins are believed to be the " primary grooves " formed on the first cooling and contraction of the crust of the earth, with the corresponding elevations represented by the larger or more elevated continental masses.

In the course of ages the heat continued to be still further radiated outward until the. temperature of the outer crust became lower than 212° Fah., when the water, which would previously have existed in immense clouds of steam and vapour, became condensed to form the oceans, seas, and lakes. Once these were formed a new agent came into action, and a new set of circumstances took effect. This was water and its eroding power when in motion.

In the earliest stages of the existence of our globe it is computed that it must have spun round on its axis every three hours, and that the slowing down of the speed to the present diurnal 24 hours was due to the action of the tides — like a huge frictionbrake round the earth j while cls the moon was then nearer the earth the tides were believed to be as much as 36 times their present magnitude.* The amount of erosion carried on round the shores of the primeval seas by such a mass of swirling and rushing water must have been enormous, and possibly some of the beds of the Laurentian — that great group of sedimentary rocks of five miles in thickness — may owe their existence to the deposition in the greater depths of the oceans of material ercded off the shores by these immense tides.

No one who has not given some attention to the subject, can adequately realize the tremendous wearing or eroding power of ordinary running water when exercised for an immense length of time. A river in time of flood is muddy, and the muddiness is due to the solid particles of earthy or rocky material carried along in suspension, while in addition to this there is always a proportion of solids in a state of solution even when the water is clear ; and all this material must have been derived from the surface soil or exposed rock by the water washing over and through the sakne. The amount of eroded material carried down to the sea has been ♦ King: " The Soil," p."l7i calculated for the principal rivers of the world, and, as an illustration, it may be stated that in the case of the Thames, the debris Trashed away is equal to a lowering of the surface of the whole Thames basin to the extent of one foot in 1,000 years, at the present rate of waste.

When the globe was cooled down sufficiently to allow of the deposit of the water in the seas, the evaporative action of the sun's heat came into force, and the vapour thus formed was in the higher regions of the atmosphere cooled and re-deposited as rain, which collected into streams running down to the sea to be re-evaporated and re-condensed into rain in a continuous cycle ever since. When the earth itself was hotter, and possibly the sun's rays stronger, the evaporation and condensation would be much greater than we now have at this period of the world's history ; that is, tropical and exce&sive rains would be the normal state of the weather, with a corresponding excessive erosion of the land's surface by the floods, together with a tropical growth of plants.

The mass of the earth is always radiating off some of its heat continuously into space ; that is, the cooling process is still going on. The internal temperature has no appreciable effect on the surface now, excepting in such phenomena as volcanoes, hot springs, and features of this sort, while the process of radiation has been materially hindered or helped by the arrangement of the strata at a particular spot. Flat strata have retarded the escape of the heat, and vertical strata have helped the same, while likewise allowing of a more easy percolation of the rain. Thus the cooling has gone on unequally in different regions. One instance may be given of that of Astley pit in Cheshire, where the rise in temperature is 1® in every 35 feet of descent, but the observed average of many mines in different parts of the country is 1^ in 64 feet.* This, of course, has very little effect on the surface conditions now, though there is no doubt that at one period in the earth's history it must have excercised a vast influence on the animal and vegetable life of sea and land, in much the same way that "bottom heat" acts in a gardener's forcing frame in the modern world.

It is believed that vegetable and even animal life was possible

when the " water under the earth " had got cooled down to about

190® Fah., yet estimations made indicate that the temperature of

the earth's surface at the close of Archaean time was still about

* Cooley : *' Physical Geography," p. 81.

100^ Fah.,* the present mean annual surface temperature being 49^ Fah.t

All the known land must have been under the water at one time or another, whether a " primal fold " or not, and in most cases for a long time. Tennyson was not a geologist, but he stated facts better than any of the clan when he wrote : —

" There rolls the deep where gi'ew the tree, earth, what changes hast thou seen !

There where the long street roaus, hath been The stillness of the central sea.

The hills are shadows, and they flow From forni to form, and nothing stands ; They melt like mist, the solid lands,

Like clouds they shape themselves and go."

When any given area of land first appeared above the waters it must have been nothing more or less than a hump or boss of igpeous rock, similar in appearance to a volcanic island, but varying in size from a small reef to a huge continent. The rain falling on such immediately collected into streams and began to run seawards, gradually wearing out water-courses and valleys as the time went on. The greater part of this eroded material was carried out to sea and deposited in layers, forming the various strata which compose the whole series of stratified rocks, but a small portion would always be deposited on the level parts of a river course, or at the mouth of the same, forming the basis of the level alluvial or estuarine soils of the world.

By the time this stage was reached other forces had come into action : such as the power of oxygen to chemically combine with every substaace oxidizable at ordinary temperatures, the solvent power on limestone of water containing carbonic acid gas, and the effect of rising and falling temperature on all exposed material. Iron in particular of all the ingredients composing the rock-forming minerals has an afltoity for oxygen, forming the protoxide and peroxide of iron (rust) and thus breaking up the cementing power of the original iron in the rock and allowing the other mineral matter to collapse or form friable debris which in turn helped to form the mineral basis of soils : while the same happened to any mineral '""" or rock containing carbonate of Ume when acted on by water with

C O2 in solution.

Again, when the temperature became cool enough to allow of

• Stockbridge : ** Rocks and Soils," p. 7. t Prestwich : " Geology," Vol. XL, p. 537.

the action of frost, the freezing of the water in the seams and pores of exposed surfaces caused the breaking off of scales and even slabs and chunks, this going to still further augment the amount of debris.

On a level surface, or where there was little washing power of water, this debris accumulated and thus the bulk of the first soils was formed, and the formation has been going on in the same way to this day.

The above is a short summary of the process of formation of the solid crust of the earth with the interdependent arrangement of sea and land, cloud and sky. The origin and formation of soils, with all that is involved therein, rightly began only after the globe had cooled down sufficiently to allow of the existence of water, clouds, rain, frost, and animal and vegetable life of low grade ; when, in fact, the formation of the geological strata began. Indeed, the formation of a soil and a bed of sediment have been large^ due to the action of the same agents, and much that will appear hereinafter will apply equally to both phenomena.

There must have been an endless succession of soils formed and destroyed in the course of geological time, for every land surface which supported plants and animals must have had some sort of soii thereon, while any geological change must have meant the destruction of the same. But few records are left of these old land surfaces, though there must have been such from very remote geological time. Some of these ancient soils have been preserved, however ; fossilized, as it were, in some cases.

A good example is found in most coalpits. Underneath the beds of coal there is usually found a layer of a particular kind of clay — used now, in the arts, under the name of "fire-clay," because bricks and other things made out of it stand the continuous action of fire much better than those made from ordinary clay. From the fact that it is often full of the roots of coal plants (Stiginaria) it is held that this is the soil on which the coal plants grew w^hen it was existing as mud at the bottom of a marsh or lagoon. A peculiarity of this ancient soil is that it is deficient in potash, soda, and other elements of fertility which are conspicuous and necessary ingredients in the greater part of the soils of the present time,* but tlie absence of which prevents fusion or

• Shaler : " Origin and Xiiture of Soils." United States Geological Survey Annual Report, 1891, p. 32*2.

vitrification when exposed to intense heat, as would be the case with ordinary soik. Every seam of coal must have had its corresponding soil : in the South Wales coalfield there are 100 seams with the corresponding soils on which the trees and plants grew * while in Nova Scotia Dawson noted 68 root-bearing soils in a thickness of 1,400 ft. of coal strata.t

In the railway cuttings through the chalk near Tring there is a still better example of an ancient soil. The older chalk rock has formed dry land at one time, sufficiently long to allow of the denudation of the same, and the accumulation of a soil thereon of brown loam about six inches thick — representing the insoluble residue of probably 10 to 20 ft. of weathered chalk carried off in solution. This old surface has been submerged again so gently as not to disturb the soil on the same, and the deposition of chalk has recommenced and covered it up to the depth of many feet. Finally, the elevation of the whole to form part of the Chiltem Hills has allowed of the second denudation and formation of the soil of the present day — ^now farmed and cultivated, and fonning a retreat for weary members of Parliament.

That the old soil, dating from the middle of the Upper Cretaceous period, is still a soil proper — ^though "fossilized" for ages — IB shown by the fact that whitethorn and other plants have taken root in it, and grow where its section is exposed on the sides of the railway cutting.

Another example yet again : in the opinion of some geologists the laterite or bed of " red bole " which occurs in connection with the deposits of basalt in Antrim and the north of Ireland generally, is also the remains of an ancient soil. The top of the lower basaltic beds was weathered to form this soil material, possibly mingled with volcanic ejectamenta, on which in later ages another layer of molten basaltic lava was poured out, thus embedding it and burning and metamorphosing it into material like red brick clinkers ; the associated deposits of lignite and beauxite representing the fossihzed vegetable matter.J

As a matter of fact many such old soils must be in existence if we knew where to look for them and recognised them. We can see that their preservation must have been due to an accident, however.

• Woodward : ** Old Land Surfaces," p. 4. Geological Magazint^ Nov., 1871.

J Green: "Physical Geology," p. 146.

when we realize that the depression of a land surface below sea or lake level most have been accompanied in the vast majority of cases with a destruction — or, at least, jumbling up — of all loose top accumulations from the action of waves or currents. When we come to think of it, however, there must liave been soils corresponding to all the strata we find forming the sedimentary accumulations of the crust of the earth, or at any rate to the most of them. The greater proportion of the strata represent the debris or washings from off land surfaces carried down by rivers and floods, while this debris in its origin is ejcactly the same material as that which goes to form soils when left to accumulate on the dry land, and as the whole of it was not carried out seawards to form beds, some remained behind to form a soil-covering. The existence of fossil land plants, and also of animals, is proof of the existence of a soil yielding food for the same, and while every bed may not have had a special land surface and soil corresponding to it— as one set of land conditions may have existed right through the time several diflferent strata were being deposited, — it is certain that groups of related or similar beds must have had the same or similar land surfaces, with soils which in many cases left no trace behind when the conditions were altered.

Some of these old land surfaces, however, with their soils containing roots are reckoned "formations" in the ordinary geological sense, and classed in lists as such. In addition to the two examples just given, we may instance the brashy Purbeck " Dirt Beds " of the Upper Oolite, identical in composition with the present surface soil,* and containing the silicified trunks of cycads and stools of conifers preserved in 8itu;f the OUgocene " Leaf Beds" of Antrim, Mull, and Skye ; and the " Forest Beds" and submerged forests of the Pliocene ; while a closer examination of formations from this point of view would result in discovering the traces of others scattered up and down through the long series of deposits. An " uncomformity " is the most Ukely place to find such, because it represents a denuded surface on which a soil might have existed before re-depression gave an opportunity for depositing other rocks on the same, as exemphfied in the case of the chalk near Tring above mentioned. Every deposit of carbonaceous material, however, — such as tlie Kimmeridge series,

* Green: ** Physical Geology," p. 144.

t Woodward : ** Old Land Surfaces," p. 3. Geological Magazine.^ Nov. 1S71.

the Triassic coal of Virginia, the lignite deposits, and so on — as also all mammalian and other fossil remains of air-breathing animals, are indications of old land surfaces with soils thereon.

The discovery and study of these primeval soils would be certain to lead to interesting results. Fossil plants are minutely studied by professional geologists — the study of the soils on which those plants grew ought to be just as important.

The age and position of a given series of rocks are partly known from the associated fossil plants, and as we know that in the existing conditions of things certain plants often have a predilection for certain soils, and certain soils are formed from the d6bris of certain specific rcx^ks, we may infer that there were the like conditions of things in the far past, and that if fossil plants (and even animals) could be collated to the soils on which they grew, and these soils to their parent rocks, there would be many interesting points in Historical Geology brought out.

The Great Ice Age.— Yet further again, there has been one powerful agent which has sculptured out and tremendously modified the surface contour and moulded the face of the country in many places into its present general appearance. This has been the action of ice with its concomitants — ^the expansive force of freezing water, together with the effect of great floods of water from the after melting of that ice. There has been, to put it shortly, a covering of glaciers^ more or less all over the northern parts of these Islands, as also of the Continent and North America, existing during what has been called the " Great Ice Age."

Whatever theories are held by various authorities respecting the existence and duration of a " Great Ice Age " or " Glacial Epoch," there is no doubt about the fact that there are large accumulations of surface " Drift " of various kinds over the greater proportion of the northern parts of the United Kingdom which, to some extent, " mask " the underlying " Solid " formations. So much is this so that many writers have gone so far as to say that any attempt to trace a soil to its original rock is futile* and that where this series of beds occur the rock of the district has no influence on the surface soil whatever. This may, perhaps, be true in some cases, as on a great continent like North America, where, according to Merrill,t the whole of the original soil formation of New England