from which it is called the "chalky boulder clay," but in the north the clay is brown and much more homogeneous. On still higher levels, on the summit of the chalk downs, there are to be found argillaceous deposits, giving beds of clay, of no great thickness it is true, but yet in some places furnishing useful material for bricks or tiles.

It will thus be seen that both the Palæozoic, the Secondary and the Tertiary rocks contain clays, and that they are very widely distributed in this country, and so varied in character as to be suited to a variety of purposes. And it may be safely predicted that this most useful and cheaply procurable substance, with improved methods and appliances for its manufacture into building materials and ornamental objects, will be more and more used, and beds of fine clay will therefore become very desirable features of a landed estate.

J. LOGAN LOBLEY.

CHAPTER LXXXV.

LIME.

BY PROFESSOR J. LOGAN LOBLEY, F.G.S.,

Professor of Physiography and Astronomy, City of London College; Lecturer on Geology, Crystal Palace School of Engineering: Vice-President of the City of London College of Science Society: Member of the General Committee of the British Association; Member of the Geologists' Association; Hon. Member Hamp. F. C., etc. Author of "The Study of Geology," "Geology for All," "Mount Vesuvius," "Hampstead Hill," "The Inter-relations of the Field Naturalist's Knowledge," "The Cretaceous Rocks of England," "The Causes of Volcanic Action," "The Origin of Gold," etc.

LIME is so useful a substance for agricultural purposes, often so effective as a manure, and so much required for mortar, that the occurrence of a bed of limestone must add value to land, although it may not be suitable for use as building stone, and not sold and sent off the estate. The manufacture of hydraulic cement, however, the making of whiting, the employment of limestone as a flux, and the use of lime in other ways, in addition to its ordinary use for agricultural and building purposes, sometimes give to a bed of limestone very considerable value.

Some limestones form thick and massive rocks, rising into bold hills, others are rocks of moderate thickness, and in still other cases they are in thin beds. The limestones used as building stones having been treated of in another chapter, those used for other purposes will receive consideration here.

Pure limestone is altogether carbonate of lime from which pure lime is obtained by the common burning

process, which drives off the carbonic acid.

But nearly all limestones are more or less impure, and contain an admixture of other substances. In some there is free silica in the form of fine grains of sand, in some clay or silicate of alumina, in some oxide of iron, and in some all these substances with others in addition. In many cases a limestone derives a special value from its impurities, since the lime it produces is rendered thereby more suitable for the manufacture of certain cements, notably of that called hydraulic cement, from its power of setting under water. Thus, the Lower Lias limestone is in great demand, and extensively quarried for this purpose, and the very argillaceous limestones called Septaria, so abundant in the London clay, are in request for the manufacture of Roman cement.

Very low down in the series of sedimentary rocks, there are two limestones of great value for agricultural purposes in the localities in which they occur, because there is such an enormous area in these districts, to which lime-dressing is highly beneficial, and under which there is no limestone. These are the Bala and Hirnant limestones, members of the Lower Silurian series of rocks. The Bala limestone is about twenty-five feet thick, and the Hirnant limestone only ten feet thick, while they are separated by rocks devoid of lime of one thousand four hundred feet in thickness. In the north of England, the Bala is represented by the Coniston limestone. At Woolhope, in Herefordshire, and some other places in that district, there is a hard limestone of about thirty feet in thickness, suitable for burning for agricultural requirements in the Upper Silurian rocks. The Wenlock limestone is a much more important one, however, both in thickness and extension, since it sometimes thickens to three hundred feet, and forms a ridge of land

in the Wenlock district, of twenty miles in length. It is also well seen at Dudley, where it protrudes through the Coal Measures, and forms the Castle Hill and "Wren's Nest," a verdant island as it were in the dark sea of the "Black Country." In Shropshire, there is another Silurian limestone, called the Aymestry limestone, of great agricultural value. It is in thin beds, and is of an earthy and nodular character. The great and massive limestones of South Devon, are largely worked for building stones, and furnish the Devonshire marbles, now so much used, but they are also employed for paving stones, and other purposes.

In the Carboniferous, Mountain, Scaur, or Scar, limestone, we have one of the greatest developments of limestone rock in the British Islands. In the range of hills called the Pennine Chain, extending from Scotland to Derbyshire, this limestone attains the enormous thickness of four thousand feet, but it thins southwards, so that near Bristol it is reduced to two thousand feet, and in South Wales to five hundred feet thick. It is largely quarried in many localities both for flux, for iron furnaces, and for burning for lime.

The limestone is very extensively worked, especially in Warwickshire and Leicestershire, for the production of hydraulic lime, and even the masses that fall on to the sea shore by the action of the sea on the cliffs near Lyme Regis, in Dorsetshire, are collected for that purpose. The rough nodular "Marlstone" of the Middle Lias is quarried chiefly for agricultural lime, but the Oolitic limestones are mainly building stones.

The Chalk is another great, and, in the south-east of England, a very conspicuous limestone, forming as it does, the North and South Downs, Marlborough Downs, the Chiltern Hills, and large areas in the Eastern Coun

ties. It likewise forms the Wolds of Lincolnshire and terminates northwards in Flamborough Head. In all these localities it is the great source of lime, and along the banks of the Lower Thames is extensively employed for the making of "whiting." The upper, or white chalk, known by its bands of flint nodules, is almost pure carbonate of lime, giving only about two per cent. of clayey matter and oxide of iron. It is composed of the shell-matter of a microscopic deep sea organism still living in the Atlantic Ocean.

There are limestones of value in the Tertiary rocks of the Isle of Wight, and some others of minor importance, but those that have been mentioned are our chief lime-producing rocks.

J. LOGAN LOBley.