Rocks and Their Origins

CHARACTERS OF CLAY AND SHALE

The question of what is a true Clay has been much discussed, especially by agriculturists, in recent years[939]. The material, as a rock, is regarded as a massive kaolin, and, if pure, should have the following percentage composition:—silica 46·3, alumina 39·8, water 13·9. Some Pipe-clays, white and uncontaminated, closely approximate to this ideal. True clays are very plastic when moistened, and shrink on drying, forming a compact mass the particles of which do not fall apart. When thoroughly dried, however, and placed in water, lumps of clay break up readily; the water creeps in along their capillary passages and expels trains of air-bubbles as it goes. This fact has been utilised in the extraction of fossils from a matrix of stiff clay. If the clay thus reduced to powder is now "puddled" by the finger, it again forms a closely adherent plastic mass.

The individual spaces between adjacent particles in a clay are very minute, and this accounts for its practical impermeability to water; but the total pore-space or "porosity" may amount to more than fifty per cent. of the volume of the rock. Unless earth-pressures have brought the mass into the condition of shale or slate, the tiny flaky kaolin particles, and the associated very small grains of other minerals, have not shaken themselves down into a closely aggregated state. When moistened, however, and again dried, the surface-tension of the film of water about any group of grains, increasing as evaporation thins the film, draws the grains nearer to one another, and a considerable shrinkage of the mass results. Alternate wetting and drying tends to make a clay less obdurate and sticky, by increasing the number of separate aggregates of grains. The passages between these aggregates are no longer so minutely capillary, and a clay soil becomes by this process distinctly "lighter" from the farming point of view.

The larger cracks caused by shrinkage greatly increase the evaporation of water, by exposing new surfaces, which penetrate deeply into the clay. Often the mass shrinks so as to develop hexagonal structure, from the drying surface downwards Fig. 8).

The natural "flocculation" of clays, the process by which compound grains are formed in place of individual soil-particles, is assisted by the action of water bearing certain salts in solution. Calcium carbonate is an excellent flocculator, and this fact has long led farmers to place burnt lime or powdered limestone on their lands. Sodium carbonate, on the other hand, is brought up in some dry regions by capillary action, and exercises a reverse effect, keeping the minute particles apart from one another, and thus promoting thorough clayiness in the clay.

Experiment has shown that fineness of grain is responsible for most of the characters of a clay, and from this point of view the small size of kaolin flakes as compared with grains of other minerals will account for the "clayiness" of this particular mineral when it constitutes a rock. Clays, however, when shaken up in a column of distilled water, cause what seems to be a perpetual cloudiness, since it remains after the great bulk of the clay has settled down. Flocculation by salts alone removes it. Some authors have urged that a colloid substance, amounting perhaps to only one or two per cent. of the whole clay, imparts this distinctive character. Such colloids are believed to arise during the decomposition of aluminous silicates under tropical and probably alkaline influences; but they are not known to be associated with the processes by which kaolin is formed from felspars.

A. D. Hall[40] points out that the cloudiness is probably due to the extreme minuteness of certain of the particles. True clayiness thus depends on the proportion of grains smaller than ·002 mm. in diameter. Yet Hall and Russell look to other causes to explain the continued suspension of such particles in the water, and they suggest the presence of potassium and sodium silicates of the zeolite group, which liberate by hydrolysis a little alkali in contact with a large bulk of water. Free alkalies prevent flocculation, and so encourage suspension of the particles.

To the ordinary observer, a rock possesses the properties of clay, and is a clay, if it contains more than forty per cent. of particles less than ·01 mm. in diameter. But such rocks are found, on chemical analysis, to contain a large amount of kaolin, and the old view, that clays are massive kaolins, is thus substantially correct.

None the less, clays are notably impure, and in many there is a large admixture of quartz sand. The kaolin, derived originally from the decay of other silicates, is rarely freed from a variety of minerals and rock-fragments that were associated with it in its place of origin. Grains of quartz and unaltered felspar a tenth of a millimetre in diameter distinctly "lighten" a clay soil, on account of their relative coarseness. A sandy clay is styled a Loam, and a fine-grained loam furnishes the ideal soil for the general purposes of a farmer. It does not retain water too long upon its surface, nor does it dry too quickly after rain. Much of what we call boulder-clay proves to be in reality a loam.

T. Mellard Reade and P. Holland[41] have shown that even in clays of marine origin there may be a considerable proportion of very fine quartz sand.

Calcium carbonate, usually occurring as fine rock-dust derived from limestone, or as minute shell-fragments, may be mingled with clay to form a Marl. The term is not a quantitative one, and may be applied to any clay that shows a brisk effervescence with cold acids. Though unpleasantly sticky when wet, marls flocculate themselves naturally by supplying calcium carbonate in solution to waters that pass through their crevices (see p. 80).

The stratification of clays may be invisible throughout considerable masses, unless sandy beds are intercalated among them. Yet, when a lump of clay is dried and then placed in water, as previously described, it will often break up along parallel planes, which show that there is a regular arrangement of its particles. The fact that so many of these particles are platy becomes emphasised under the pressure of subsequent sediments, whereby the platy surfaces of the particles are brought into planes parallel with one another. The clay then becomes a Shale, with regular planes of fissility, which are parallel to those of bedding. A certain amount of deformation of the rock accompanies this change, flow being set up parallel with the bedding, and included fossils becoming sometimes flattened. This deformation is especially noticeable in the case of plant-remains. Shales may in time attain the density and fissile structure of true slate.

The colours of clays and shales are of considerable interest. Blackness is often due to organic matter, and especially to fragments of plants, which retain their woody structure and their carbonaceous character when protected by clay from oxidation.

The bluish tint of clays is due to finely divided iron pyrites (iron disulphide), which may occasionally appear as distinct crystals or nodules of one or other of its forms, pyrite or marcasite. On oxidation, limonite arises, which colours the mass brown, as is seen in the upper part of many clay-pits. The occurrence of iron pyrites often dates back to the time at which the clay accumulated. N. Andrussow[42] points out that in the Black Sea there is an enormous supply of decaying organic matter provided by the floating organisms of the upper layers. This rains continually down towards the floor. The portion that reaches depths of over 100 fathoms escapes from the voracity of free-swimming organisms and arrives at the region where bacteria alone abound. These bacteria act on dissolved sulphates, and also largely, according to Andrussow, on the albumen of the decaying matter. In both cases, sulphuretted hydrogen is produced. Andrussow treats the reduction of the marine sulphates as a minor process, due to the need that the bacteria have for oxygen in the deep waters, which are insufficiently supplied. The sulphuretted hydrogen attacks the salts of iron, and iron disulphide results.

Here we have an excellent illustration of how, in deep basins, with imperfect vertical circulation, black pyritous muds may arise, devoid of ordinary fossils. The depths of the Black Sea are practically poisoned by the abundance of sulphuretted hydrogen. But numerous cases of shales are known to us where iron pyrites replaces the shells of ammonites or forms complete casts of bivalves, and has accumulated also in concretions and crystalline groups. Such pyrites is probably of secondary origin, or arose from the reducing action of decaying organic matter on ferrous sulphate in solution in the sea.

The oxidation of iron pyrites in shales gives rise to aluminium sulphates, such as alums. Sometimes sufficient heat is evolved during this oxidation to set on fire carbonaceous matter present in the rock.

Pink-purple and green are common colours among shales, and imply that the iron is in two different states of oxidation. When the colour varies thus in successive bands, we may believe that a climatic change promoted the formation of ferric salts on the land surface when the pink layers were being formed, while ferrous (less oxidised) salts predominated when the green particles were washed into the basin. B. Smith[43] suggests that the organic matter and humic acids which are swept down in times of flood may temporarily prevent oxidation from occurring in shallow lakes and pools. Dry seasons would thus lead to the deposition of pink clays, while wet seasons would furnish green ones. The green colour in shales is mostly due to chlorite or to glauconite.

Subsequent deoxidation has been invoked to account for the green colour of certain shales. Organic matter may have been responsible, and the green spots in purple slates have been attributed to the decay of entombed organisms, the reaction having spread outwards from a centre.

Clays, owing to their impermeability, preserve fossils excellently, and the oldest shells and corals in which the original aragonite has escaped conversion into calcite occur in clays and shales of Mesozoic age (see p. 22).

ORIGIN OF CLAYS

Something has been said on this matter in the foregoing paragraphs. It is now recognised that a pure china-clay or a pipe-clay, that is, a pure kaolin-earth, does not arise from the sifting of the products of surface-denudation. The alkali felspars decompose as they lie in exposed layers of granite and gneiss, but the kaolin thus formed under the acid action of atmospheric waters is relatively small in quantity, and cannot escape from its coarser associates, such as undecomposed felspar and quartz, until it is carried away far from land. Even then, as the records of H.M.S. "Challenger" show[44], marine muds may contain more than fifty per cent. of detrital quartz-grains, and quartz is always the most abundant mineral among the larger particles of the mud.

Where, however, decomposition of the granitoid rock has been exceptionally thorough, kaolin may be present in sufficient quantity to predominate over other materials. The product washed from the surface then gathers as a white clay even in lakes, and further artificial washing may extract from it an actual kaolin-earth or china-clay. In such cases, the rock has become rotted throughout in consequence of subterranean action. Hydrofluoric acid as well as other gases have been at work, as is shown by the secondary minerals associated with the kaolin; and the appearance of white powdery kaolin in unusual abundance on the surface is due to the local exposure of a mass that was long ago made ready in the depths.

The sifting action, however, of running waters, and especially of the sea upon a shore, ultimately causes clayey matter to be carried away into regions where it is slowly deposited. The flocculating action of the salts dissolved in sea-water greatly assists the precipitation of clay before it has reached some two hundred miles from land. However, just as sandstone begets sandstone, clays or shales exposed upon a coast produce new clays close to shore. The estuary of the Thames and many "slob-lands" serve as examples. Off Brazil, red clays arise[45] from the large quantity of "ochreous matter" carried from the coast. Modern green marine muds are found to contain glauconite, a silicate common in the English Gault clays, and formed by interactions in the sea itself. Modern blue muds[46] are recorded down to 2800 fathoms, and contain organic matter and iron disulphide.

Much has been written by the observers on the "Challenger" and by others on the red clay of truly abyssal depths, which is attributed to the decay of wind-borne volcanic dust, and of igneous matter erupted on the sea-floor, rather than to any direct transport by water from the land.

Clays may also accumulate on a land-surface from fine volcanic ash, which decomposes through the action of percolating waters.

SLATE

The relations between shale and Slate are so obvious that slate may readily be regarded as a very well-compacted mud. The clayey material in it, like that of muds, may be ordinary detritus or of volcanic origin; its colours repeat those of shales. Its essential character, however, is the possession of a "cleavage," that is, of well-developed planes of fissility, which are often inclined to those of bedding. The bedding may be indicated by bands of different coarseness or constitution, and these may show crumpling due to pressure that has been exerted on the mass. The cleavage, however, may run right across these bands, and the rock, as a rule, splits far more cleanly along the cleavage-planes than a shale does along its planes of bedding.

The early and historic observations on slaty cleavage have been excellently reviewed by A. Harker[47], who also provides an independent investigation. Reference may also be made to a later treatise by C. K. Leith[48], which contains numerous illustrations, and to a discussion by G. W. Lamplugh[49]. D. Sharpe and H. C. Sorby, between 1847 and 1853, developed the theory that rock-cleavage was due to compression in a direction perpendicular to the planes of cleavage and to expansion along them. As Harker points out, it is unlikely that the expansion balances the compression. The density of slate, about 2·7, is a good indication that the "porosity," or percentage of pore-space, has been reduced, while the mineral changes, soon to be referred to, are also in favour of greater density. C. Darwin[50] laid stress on the connexion between cleavage and the development of flaky minerals, such as micas, along the cleavage-planes, the structure ultimately passing into that known as "foliation" (see p. 145). H. C. Sorby urged that compression brings platy particles into parallel positions throughout the mass, so that the plates, which may consist of kaolin, mica, or chlorite, come to lie with their broad surfaces perpendicular to the direction of compression. At the same time, any constituents capable of deformation become compressed in this direction, become expanded in a direction perpendicular to it, and are themselves converted into lens-like forms or plates. T. Mellard Reade and P. Holland[51] have emphasised the part played by crystallisation at the close of the process of compression. They urge that the platy minerals, mica and chlorite, are produced during the alteration of the rock, and can spread with ease in directions perpendicular to that of compression; they thus give rise to slaty cleavage at a late stage in the deformation of the rock. These authors, it will be seen, have developed one of Darwin's principal propositions, as to the close connexion between rock-cleavage and foliation, and, in opposition to Sorby, consider the platiness of the original constituents to be of less importance.

In support of their view, in regard to the late stage at which cleavage is induced, it may be noted that the crystals of pyrite and magnetite that sometimes occur in slates and in the allied foliated schists have developed at an earlier date as knots which oppose the cleavage or the foliation[52].

Darwin observed that mineral differences sometimes occur along bands parallel with the cleavage-planes. In such cases, the difference may be largely one of grain, shearing having broken down the minerals into a finer state along certain bands of movement[53]. Shearing of the rock may occur along any of the cleavage-planes, which are superinduced planes of weakness, and parts of the slate thus slide over others, just as the mineral flakes slide over one another in the directions in which expansion of the rock is possible. Where traces of the original stratification remain, it is easy to see if rock-shearing has occurred.

Beds of different composition naturally take on cleavage in very different degrees. Sandy layers show the compression that has taken place by contorting; but they cleave very poorly, and in proportion to the amount of mud present in them. Where clayey and sandy layers alternate, and the direction of the cleavage is oblique to them, it is refracted, as it were, on passing from one layer to the other; it is more highly inclined to the bedding in the sandy layers and less so in the clayey layers. Hence a cleavage-surface forms a fold resembling the shape of an italic S as it traverses each harder bed. Harker[54] and Leith[55] discuss the cause of this from somewhat different points of view. It is probable that such cleavage-planes as develop within the hard bed are approximately perpendicular to the direction in which the compressive force acts, because there is in such beds little possibility of lateral creep of the material along the bedding-planes. In the softer layers, we have to deal, not only with a tendency towards the rotation of platy particles until their flat surfaces are perpendicular to the direction of pressure, but also with a tendency of the same particles to flow along the bedding-planes. The resultant arrangement gives rise to a cleavage nearer to the bedding-planes than that in the more sandy layers.

Sometimes, after the cleavage is established, compression folds it, just as strata may be folded. Still greater compression may obliterate it and establish a new cleavage, and all gradations towards this result are traceable. The cleavage layers, again, may be wrinkled into a series of sharp folds, thrust over in one direction, and parting may then take place along the ridges of these folds, which furnish a second series of planes of weakness in the rock. This type of separation has been styled a strain-slip cleavage, and by Leith a fracture-cleavage, in distinction from ordinary or flow-cleavage. Shearing may take place along it, and the true or flow cleavage-planes become thus broken across and faulted.

Commercial slates should exhibit none of these structures that interfere with genuine cleavage. An argillaceous rock of uniform grain, compressed evenly over a considerable district, is required for successful slate-quarries. Yet all quarrymen will admit that the material varies from point to point, and that the best slate runs in "veins." Some of the coarser slates, with irregular surfaces, and with splashes of colour, such as are provided by limonite, are sought after for their picturesque effect; while slates which do not split readily enough for roofing purposes may have their use for flags, mantel-shelves, and billiard-tables.

ARGILLACEOUS ROCKS IN THE FIELD

Obviously, nothing can be more different than the features of a country made of clay, when acted on by denudation, and those of one where slate prevails. In the former case, low rounded hills rise, without any definite arrangement, above hollows where rushes spring amid the grass. The streams are muddy, and they readily cut their way down to base-level, meandering thenceforward in a clay-alluvium. Shales provide bolder features, but crumble rapidly where the climate permits of frost and thawing. They may be protected by more resisting rocks, but provide oozy surfaces underground, over which the higher masses may slide disastrously (Fig. 9).

Shale-beds, when uplifted and folded, slip away in flakes from one another, supplying very ragged and irregular material to the taluses, and exposing shimmering surfaces when damp with rain (Fig. 10). Among hilly lands, the passes will often be found to be due to bands of shale, which are cut down by weathering far sooner than the rocks on either hand. In central England, the Lias shales, despite the presence of some limestones, have been worn down almost to a plain, wherever the overlying Middle Jurassic limestone has been removed.

Slates, with their ragged edges and resistance to rain, play their part in wilder mountain-scenery. Frost-action destroys them, producing taluses that slip frequently towards the valleys; but the residual crags assume more serrated forms, in contrast with the smooth covering of the lower slopes. The cleavage, when steeply inclined to the horizontal, promotes the cutting of gullies down the mountain-sides, and the intervening ribs of rock may easily be mistaken for uptilted strata. The entrance to the Pass of Llanberis at Dolbadarn is a fine picture of slate-scenery. Eventually, mountains formed of slate assume hog-backed and rounded forms, but they still, where notched by streamlets, yield sheer cliffs and picturesque ravines.

ON BOULDER-CLAY

The material known as Boulder-Clay presents such distinctive features, and is so prevalent in our islands, that it deserves a few separate remarks. From a coating a foot or two in thickness, it swells in places to a hundred feet or more, and may form the important round-backed hills to which Maxwell Close reserved the name of drumlins.

It consists essentially of mixed materials, unsifted by water, huge boulders of various rocks occurring side by side with angular fragments and pebbles of all sizes, set in a groundwork of loamy clay (Fig. 11). Sands and gravels are often associated with the boulder-clay, and result from the local washing of the mass in copious floods of water. The blocks are here on the whole more rounded, and the sandy part of the loam predominates.

Blocks of shale and limestone, and even of sandstone and quartzite, occurring in the boulder-clay, bear the characteristic striations that we now recognise as due to glacial action. The sand and small stones have, in fact, been held against the larger ones by solid ice, and have cut and grooved their surfaces. Shales and schists have gone to pieces and have provided the clayey groundwork. The whole of the material has been at one time embedded in and moved forward by glacier-ice.

Though Louis Agassiz developed his glacial theory from studies in Switzerland, he possessed an imagination that ran before the knowledge of his time. Swiss glaciers are now so limited that they are of very little use to us when we seek to explain the origin of boulder-clay. In arctic and antarctic lands, however, we meet with continental glaciers, many miles in width, moving across lowlands, in virtue of the pressure from some great snow-dome, to which additions are continually being made behind them. Even when fed by diminished snow-fields, like those in Spitsbergen, these glaciers dominate the landscape and form the principal rock-masses over hundreds of square miles. Such glaciers gather into their lower portions all the loosened material on the hill-slopes and valley-floors. With the tools thus supplied, further material is plucked from jointed or fissile rocks as the mass moves forward. Freezing and thawing at the base of the great ice-sheet, as water flows here and there beneath it, further disintegrate the rocky floor. The broad ice-sheet sinks in a mass of broken rock and sludge at one point, and at another drags this mixed material forward as an abrading agent. The lower half of such a glacier, or the whole thickness of it near its front, where surface-melting has removed the higher layers, is in reality an agglomerate of stones and mud held together by an ice-cement (Fig. 12). When an epoch of advance is over, when the ice-sheet stagnates and its frozen constituent melts away, it becomes more and more like a boulder-clay as time goes on. True boulder-clay then forms its surface, while ice remains plentiful below.

Since the stony matter is not evenly distributed, some parts of the surface sink more quickly than others, through loss of a greater portion of their former bulk. Roughly circular pits or "kettle-holes" appear, in which water gathers. The water running from these washes across a part of the boulder-clay, bears off the mud, and leaves bands of sand and gravel. The clayey portion thus removed may accumulate as a fine deposit in other outlying pools, and is interstratified, when the flow of water is temporarily increased, with coarser and more sandy layers. Ultimately, the frozen water of the groundwork drains away, and only the stones and clay of the ice-sheet remain upon the field. They form, however, a very important residue, weathering in steep cliffs and pinnacles in the dry air of the arctic lands. The boulder-clay thus left shows a sharply marked boundary where the edge of the stagnating ice-sheet lay. It is, in fact, the surviving part of the complex sheet, and now undergoes moulding, like other rocks, by atmospheric agencies (Fig. 13).

Many interesting features of the hills called drumlins cannot be discussed here. Their arrangement with their longer axes in the direction of the movement of the ice shows that they were moulded in large measure within the ice itself, and came to light as it melted away from above downwards. They may be regarded as originating in tough and mixed materials, ice and stones and clay, from the lower layers of the ice-sheet, which became associated with the purer upper ice in certain episodes of the flow. Such mingling may occur at an ice-fall, or where shearing over an obstacle takes place. In the former case, the upper ice descends into the lower layers; in the latter, masses from below are pushed up into higher levels. As the forward flow proceeds, the masses representing the lower and stone-filled layers are treated just as "eyes" of coarser material are treated in a fluidal lava or in a rock deformed by metamorphic pressures. The purer and more plastic ice moves past and round them, and they assume an elongated form[56]. When final stagnation and melting have gone on, these masses are still separated from one another as rounded hills. Their bases have settled down upon the ice-worn surface, but their flanks and crests retain traces of the moulding action of the purer portions of the complex body styled an ice-sheet.

In recent years great interest has been aroused by researches on boulder-clays of ancient date, especially those of Permo-Carboniferous age[57]. These compacted deposits contain abundant striated boulders, and rest on glaciated rock-surfaces, which have a surprisingly modern aspect when laid bare by denudation. The grey-green Dwyka Conglomerate that is so widely spread throughout South Africa forms "kopjes" on the borders of the Great Karroo, with spiky crests and irregularly weathered cliffs; but its original deposition as a boulder-clay has been amply verified. It has now, moreover, been paralleled by a very similar rock discovered by A. C. Coleman in the Huronian beds of Canada.