CLAYS, SHALES, AND SLATES
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.
[Illustration: Fig. 8. Shrinkage-cracks in Clay, with footprints of
birds in the foreground. Tundra of Mimer Bay, Spitsbergen.]
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.
[Illustration: Fig. 9. Landslide of Limestone over Shale. Near
Luc-en-Diois, Drôme, France. The scale is shown by the main road
passing among the blocks.]
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).
[Illustration: Fig. 10. Weathering of Shale. Granite mountains behind.
Above La Grave, Lautaret Pass, Isère, France.]
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_.
[Illustration: Fig. 11. Boulder-Clay, Crich, Derbyshire.]
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.
[Illustration: Fig. 12. Arctic Glacier charged with stones and clay.
Side of the Nordenskiöld Glacier, Billen Bay, Spitsbergen. The top of
the ice appears in the left-hand upper corner of the picture.]
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.
[Illustration: Fig. 13. Arctic Glacier and Boulder-Clay. The Sefström
Glacier, Ekman Bay, Spitsbergen, in 1910, with boulder-clay in
foreground, marked by kettle-holes, and deposited by an advance of the
glacier over Cora Island in 1896.]
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.