MARINE BRICK-EARTHS.
Turning to brick-earths of marine origin, we may say that these
constitute by far the largest class of deposits from which bricks are
made in this country, and it will be useful to deal with their origin
in some detail. If we attentively watch the action of the weather on
a friable sea-cliff we notice that large pieces tumble at intervals
on to the beach, and in due time these are washed away by the waves,
thus encouraging more to fall when the time is ripe. This process of
denudation each year takes tens of thousands of tons of sandy clays
and the like from the beaches around our islands. Large pieces of
rock, too, are detached by the weather, and eventually succumb to wave
action. During storms large stones are hurled against the cliffs,
and the general effect of this bombardment is to wear them away, and
reduce them to powder and sand grains with all possible expedition.
No one who has not seen the waves at work at such times can have any
idea of their tremendous power of moving blocks of stone many tons in
weight. During calm weather the slight movement of the waves on the
beach is manufacturing tons and tons of sand. A mass of gravel falls
from the cliff; the finer particles are floated away at the earliest
opportunity; the angular stones have their rough projections knocked
off by striking against each other; and the incessant movement up and
down the beach slope reduces the rough stone to a pebble, all the
time the particles thus shaved off are taken out to sea for greater
or less distances. If the cliffs are of limestone, or similar rock,
both chemical and mechanical methods of denudation come into play, and
considerable quantities of lime, &c., are taken away by the sea water
in suspension and solution. Large quantities of lime are daily added to
the sea through the agency of rivers also.
Now, what becomes of these vast quantities of detritus furnished to the
sea? That depends on the shore currents at the particular locality. If
there is not much of a current, the larger grains of grit and sand are
soon separated from the rest, and fall to the bottom, whilst the clays
are taken farther out to sea before being laid down. But, in any case,
the reader will readily perceive that marine deposits must of necessity
be on a grander scale, and of a much more substantial character, as a
rule, than river, lacustrine, or estuarine deposits. By their mode of
origin, too, they must be more homogeneous, whilst they are frequently
several hundreds of feet in thickness. In their process of deposition
they were not influenced by every storm and freshet; nothing short
of great earth-movements in process of time, or some other equally
grand phenomena, could disturb the even tenour of their existence. How
different to the comparatively insignificant strata formed by the other
methods alluded to!
Take samples of brick-earth of fluviatile origin at intervals and
analyse them; no two analyses will be alike, except by a most
remarkable coincidence--more by accident than otherwise. On the other
hand, take a thick marine clay, and compare its chemical composition
as ascertained at the present time with that of it made, say, 20 years
ago in the same brickyard, and the analyses will, in most instances, be
practically identical--at any rate, so far as they may be of use to the
brickmaker.
A brickmaker using a marine clay possesses innumerable advantages over
another employing brick-earths due to river action. It is no uncommon
thing for a marine clay--say, 300 feet in thickness--to continue
across country for hundreds of miles, stretching from the North of
England to the South, and over into the Continent, save for the slight
break occasioned by the scooping out of the English Channel. The
composition of the Oxford Clay, from which the well-known bricks at
Peterborough are made, does not differ in the slightest degree, so far
as suitability for brickmaking is concerned, from the Oxford Clay of
Bourges or Chateauroux, in the centre of France, or indeed at almost
any other point _en route_. With marine beds it is possible to deal
with the matter on broad lines, but it is not so with any other class
of deposits.
If a marine clay in a specified locality is found to be unsuitable for
bricks at one point, by reason of the presence of too much lime, it
would be a phenomenon if clay along the same geological horizon did
not present the same unfavourable features at every other point within
the district. The homogeneous composition, both from mineralogical and
chemical standpoints, of thick marine clays renders them of special use
to the brickmaker. Having by sundry processes, after infinite labour,
produced a certain class of brick from such an earth, he does not as a
rule have to materially modify those processes as the earth is dug into
to continue manufacturing the same brick. He is dealing with an earth
which, comparatively speaking, is a constant quantity--when the clays
are thick, and no lines of bedding are distinctly visible.
We find that a rooted conviction exists in many brickyards that clays
of marine origin are no good for brickmaking, because (so the opinion
runs) they always contain so much salt. It is wonderful that such
ignorance prevails, when the slightest acquaintance with the subject
would teach otherwise. It is perfectly true that such deposits might
have contained salt during and for some time after deposition, but
it is absurd to suppose that their marine origin has anything to do
with the presence of common salt in the clay at the present time.
Salt is soluble in water, and has been removed from such clays by the
percolation of underground water in 99 cases out of a hundred. Indeed,
as a matter of experience, we find that salt is most commonly found in
beds of lacustrine origin, or those laid down in enclosed portions of
the sea, for reasons we need not enter into at the present moment. Of
course, when material is taken from the sea-shore to make into bricks,
a considerable quantity of salt is manifest, but that is a totally
different thing to the clays deposited--we should not like to say
how many thousands of years ago. Clays of all kinds, however, may be
impregnated with salt (as in parts of Cheshire), owing to the proximity
of other beds containing that mineral; also by the percolation of
underground water with much salt in solution.
To give some idea of the antiquity of the Oxford Clay alluded to--and
that is quite a “young clay” geologically speaking--we may remark
that at the time it was laid down not a single species of animal
existed like those now living. The only mammals found, very small and
very lowly organised, were like kangaroo rats; the birds were more
like flying reptiles than anything else; it was the age of reptiles,
and enormous, unwieldy brutes swam in the water or floundered about
on land; huge sharks abounded, and armour-clad fish of kinds very
different to those now existing roamed the sea; even the “shell-fish”
were not altogether like modern ones; whilst the plants find their
nearest modern analogues in the wilds of Australasia. No elephants,
tigers, lions, bears, or dogs lived then, and the face of Nature wore
a totally different aspect to what obtains at the present time in any
part of the globe.
And this seems a fitting opportunity to the writer to put on record
the fact that many of the most wonderful remains found in the Oxford
Clay and the neighbouring Kimeridge Clay are due to the discoveries
of brickmakers. Without their valuable aid scientists would be quite
unable to clearly depict the life of those remote epochs. We have
mentioned Peterborough; some most interesting remains have been found
in the clays near that town during the past few years. To appreciate
this let the reader visit the fossil reptile gallery of the British
Museum (Natural History), at South Kensington. One of the most recent
acquisitions, set up a year or two ago, is the skeleton of a young
_Plesiosaurus_--without doubt the most perfect specimen in the world of
its kind--from Peterborough. The _Plesiosaurus_ was a large swimming
reptile, with paddles, and a long neck.
We mention these things not only to instil philosophical interest in
such brick-earths, which may be reflected upon after business hours,
but to impart some idea of the extreme remoteness of the epoch from the
human point of view, and to insist on the immensity of the intervening
time throughout which circulating underground waters--even in such an
impervious material as stiff clay--may have exerted chemical action.
The “mineralisation” of the fossils is an eloquent witness of the
effect of such changes. The reader will perceive from this that there
is scant possibility of soluble salts being present in such marine
clays; and the geological circumstances are fully borne out by the
results of hundreds of chemical analyses of thick marine clays.
The invertebrate fossils more particularly testify to the marine
origin of the clays, and are thus invested with considerable practical
interest. The man whose duty it is to determine the persistence, or
otherwise, of valuable marine brick-earths has thus a much easier task
than when called upon to decide the value of a large tract of land for
brickmaking purposes, of fluviatile origin. Finally, brick-earths do
not, except in extremely rare instances, vary materially in character
when dug into horizontally, thus every opportunity is afforded to
the manufacturer for making an unvariable quality brick, tile, or
drain pipe. It should be borne in mind, however, that these clays
often weather a brown colour, which on being dug into changes to a
bluish-black tint, the latter being the unaltered and best portion as
a rule. The only practical advantage the worker of a superficial river
deposit possesses over his neighbour using thick marine clay is in the
great range of variation in materials disclosed in the former kind of
pit. By judiciously mixing the different beds he may be able to live
well where the worker of marine clays, especially where the clay is
too stiff, or contains too much lime, “comes to grief.” A good marine
clay is a great boon, a bad one cannot be remedied other than by the
sacrifice of much money.