« PreviousContinue »
The word “clay" is very commonly used, but often misapplied. It is incorrectly used in the term “a clay soil,” so frequently met with in books and newspapers; for although a soil may be a clayey one from having much clay in it, if it is fit for cultivation it is not clay. Clay is essentially a hydrous silicate of alumina, the varieties being formed by the inclusion and intimate admixture of various substances in different but small proportions. The purest clay is the fine white china clay, or kaolin, used for the manufacture of porcelain, which is altogether silica, alumina and water. It results from the decomposition of the felspar of granitic rocks, which, consisting of a silicate of alumina and potash, is acted upon by the carbonic acid of the atmosphere and rain water combining with its potash to form carbonate of potash, a soluble compound that is carried off by water action, leaving the insoluble silicate of alumina that constitutes the clay. Kaolin was brought to Europe from China, where its name was given from the hills of Kao-ling in Chiang-Usi, at the beginning of the last century, and in 1755 it was discovered in England. In this country it is chiefly obtained from Cornwall, where granite forms several large areas of the county. China clay, from the absence of iron and other substances that act as fluxes, is most refractory, and hence it is used for small crucibles for chemical analyses in which high temperatures are necessary. Pipe clay is a similar white clay, but containing a little free silica. It is worked in the Isle of Purbeck and at Newton Abbot in Devonshire. Ordinary potters' clay is one less pure than china clay, and coarser clays with varying proportions of oxide of iron are used for the making of tiles, drain pipes, bricks, etc. Clays for terracotta are selected clays, and are frequently mixed with fine white sand and the powder of broken pottery. The usual colours of clays are white, blue, brownishyellow, and reddish-brown. White clays are almost pure kaolin ; blue clays contain, with other mineral substances in a state of minute division iron in the form of protoxide: and the yellow and brown clays are so coloured by the peroxide of iron. The presence of the protoxide of iron in blue clays occasions the change of colour to red on “burning," the iron taking more oxygen to form the reddish-brown hydrated peroxide, and, when the water of hydration has been quite driven off, the red anhydrous peroxide or sesquioxide of iron. Of the older geological formations, the Coal Measures contain a clay of a very refractory character, and hence it is used for making fire-bricks, and called “fire-clay.” It is the thin bed of clay underlying each seam of coal, and doubtless represents the soil in which the coal plants grew. In the Permians there are also beds of clay and marl, and the Triassic clays form the subsoil of a large area in Cheshire, used for dairy farming and the production of the famous Cheshire cheese. The blue clay of the Lower Lias is sometimes used for brick and tile making, but not extensively, and the indurated clays or shales of the Upper Lias, at Whitby, yield our chief supplies of alum. In the Lower Oolites are the Fullers' earth, and in Wiltshire the Bradford clay. Fullers' earth has not the same plasticity that ordinary clay possesses, and when put into water falls into powder. This is owing to its containing a smaller percentage of alumina, the composition of Fullers' earth being, approximately, silica 53 : alumina, Io; water, 24; and the remainder magnesia, lime and iron oxide. Its power of absorbing grease and oily matter renders it of great value in the woollen manufacture.
The Middle Oolites contain a very thick clay, called the Oxford clay, from its forming a large area in Oxfordshire. It is a stiff blue, and sometimes brown clay, attaining in some places a thickness of six hundred feet, and extending from the coast of Dorsetshire to the coast of Yorkshire. It forms land much better adapted for pasture than tillage. Near Peterborough, Oxford, and other places, it is worked for bricks and tiles.
In the Upper Oolites there is the highly bituminous clay, called the Kimmeridge clay, that has a maximum thickness of nearly seven hundred feet. Besides bituminous matter, the Kimmeridge clay contains much iron pyrites and selenite. Though not forming very fertile land, timber trees, especially oaks, grow well upon it, and the clay is used in some localities for brick and tile making. It underlies much land in Dorsetshire, Somersetshire, Wiltshire, Berkshire, Oxfordshire, Norfolk, Lincolnshire, and Yorkshire.
The wide-spreading clay of the great vales of the Weald of Kent, Surrey, and Sussex is also very thick, in one place a thousand feet. Though it forms rather cold land, hops are largely grown upon it, and in past times it supported dense forests of oaks. It is worked for brick and tile making, and at Ditchling, in Sussex, it yields very fine terra-cotta clays, from which large quantities of that material are made. Another though minor clay of the Weald, the Wadhurst clay, gave the iron-stones from which the famous Sussex iron was manufactured in the last and the preceding centuries, when the southern county was the chief seat of the iron trade. It has a maximum thickness of about one hundred and eighty feet.
At Speeton in Yorkshire, and at Atherfield, and various places in the south of England, are other Cretaceous clays. At Nutfield, in Surrey, the Lower Greensand contains a very valuable bed of fine Fullers' earth, which is extensively worked. The Gault clay is of considerable importance. It is a stiff blue clay, in some places two hundred feet thick, and having very long though narrow outcrops. It is seen in the Isle of Wight, Dorsetshire, Wiltshire, Surrey, Sussex, Kent, Berkshire, Oxfordshire, and Cambridgeshire, and in Kent is very largely worked for tiles as well as bricks. It contains much iron pyrites, the fossils of the Gault being often coated with that mineral. The clay of the Woolwich and Reading series is worked for bricks in several localities both in the eastern and western parts of the area occupied by this formation. In the Isle of Wight it constitutes what was called at one time the “Plastic clay,” and from its somewhat mixed and varied colours, the “Mottled clay.” It contains in some places leaves and lignites. To the thick clay which extends from Hungerford in Berkshire to the coast of Essex, although overlain in many places by gravels, brick-earth, drift, and alluvium, the name “London clay " is given. It underlies London and nearly the whole of Middlesex and Essex, and has a maximum thickness of upwards of four hundred feet. It is thickest in the Isle of Sheppy, where it contains much iron pyrites. In the metropolitan area many septaria, aggregations of calcareous matter, are found in it, and crystals of selenite are also common. At Highgate a peculiar bituminous substance called Highgate resin, or copaline, occurs. The London clay is a very stiff clay, brownish-yellow where it has been exposed to air and water action near the surface, but of a blue-grey
colour at lower depths. It forms good pasture lands and supports fine timber. It is not much used for brick making, as the brick earths upon it are much more suitable for that purpose from containing more sandy matter. The London clay contains very little impurity besides iron oxide. From 1750 grains of dry London clay I have only obtained ten grains, or o:57 per cent of non-argillaceous and non-ferruginous matter. The economically very important though very impure clay, the Brick-earth of river valleys, is widely spread in the Thames valley, and from it immense quantities of bricks are annually made, both east and west of London. At Sittingbourne, at Grays, at Crayford, at Ilford, at Acton, and at Southall are extensive brick works on deposits of this material, which is of post-Pliocene age, and therefore very much newer than the London clay over which it lies. It is a clay containing a certain proportion of fine sand, that renders it less stiff than ordinary clay, and therefore better adapted for the stock bricks used for ordinary brickwork. It may consequently be said, speaking broadly, that the brick-earth, not the London clay, is the material of which London is built. At Bovey Tracey, in Devonshire, there is an interesting and valuable clay of doubtful Miocene age. It encloses so much lignite that that substance has been obtained from it for purposes of fuel. The clay itself furnishes excellent potter's and pipe clay, of which large quantities are obtained from this deposit. On higher levels, and widely distributed in England, is the Boulder clay, which, in Lancashire and other places, forms the usual brick-making material, and near London also, at Finchley, it has long been worked for bricks. The southern clay is bluish in colour and encloses masses, of various sizes and shapes, of chalk,