parent granular-crystalline variety of gypsum, or sulphate of lime, of various colours, but most esteemed when of a pure snow-white, and usually compact enough to stand the turninglathe; the latter is a carbonate of lime (oriental alabaster), usually white or yellowish-white, and found as a stalactite or stalagmite. The gypseous alabaster is a mineral of common occurrence in secondary and tertiary formations (Cheshire, Worcester, Germany, Switzerland, the Tyrol, Montmartre near Paris, Volterra in Tuscany, &c.), and, being soft and readily turned by the lathe, is manufactured into vases, statuettes, cups, and other domestic ornaments. Very few of the British alabasters are sufficiently pure and transparent for the finer ornaments; but masses of pale-pink and lighter shades are found— as at Grantham, Newark-on-Trent, and other places-which are used as adjuncts in pulpits, tombs, screens, and similar sculptured works. The headquarters of sculpturing in alabaster are in Florence, Volterra, Pisa, and other towns of central Italy. This beautiful and semi-transparent variety of gypsum must not be confounded with the common massive sulphate of lime (plaster of Paris or stucco-rock), which is not used with us as an ornamental stone, though in ancient times the purer and more compact sorts were employed in Nineveh, Egypt, Tuscany, &c., for sculptured wall-slabs, sarcophagi, cinereal urns, and other kindred purposes. The modern uses of common gypsum are noticed under the head of mortars and cements (Chap. VI.), and also under that of mineral manures (Chap. III.), to which, for further information, the reader is referred.

Another beautiful variety of gypsum is the fibrous, known also as satin-spar, from its fine, glossy, and glistening lustre when cut and polished. It is found in thin veins and layers traversing beds of common gypsum, and is pretty largely manufactured into minor ornaments, such as cups, vases, necklaces, bracelets, and the like. Some very pure bands occur in the gypsum of Chellaston Hill, near Derby, where, as well as at Bakewell, Matlock, and Buxton, it is fashioned into the above-named articles.

Fluor-spar, which occurs in various colours-blue, purple, green, and yellow-is another calcareous mineral employed in the fabrication of minor ornaments. It is found both crystallised and crypto-crystalline in masses, not as an independent rock, but in veins and drusy cavities, in several formations, and in many countries. The variety most usually employed is "Blue John" or "Derbyshire spar;" and in several towns of

that county it is fashioned, with rather pretty effect, into cups, vases, jars, obelisks, and various minor ornaments. Like alabaster, it is occasionally employed as an adjunct in sculptured work; but, like alabaster and satin-spar, it is easily tarnished and far from durable. The gypsums rarely exceed 2 in the scale of hardness, and the fluors are about 5; and all, especially the pure white alabasters, require to be kept under glass shades to preserve their colour, lustre, and brilliancy.

Rock-crystal, Agate, Jasper, &c.

The rock-crystals, agates, jaspers, and other silicious stones, though frequently employed in inlaid work and fashioned into minor ornaments, come more appropriately under the head of Precious Stones (Chap. XVI.), where ample details will be found respecting their geological nature and occurrence. They seldom appear in masses of any magnitude, but are found in crystals, geodes, stalagmitic incrustations, and concretionary. nodules. From this circumstance they are only fitted for smaller articles-cups, boxes, vases, caskets, toilet-trays, knifehandles, and the like; or, when sliced and polished, for several varieties of inlaid work, for which their hardness, variegated colours, and fine polish, render them extremely suitable. Though some beautiful specimens of cairngorm, agate, and jasper are found in Britain, Banffshire, Kincardineshire, Forfarshire, and Ayrshire,* the finer and larger silicious stones are obtained from Brazil, Quito, Northern United States, Canada, Siberia, India and Ceylon, Egypt, Italy and Switzerland. Their hardness renders their preparation expensive; hence the comparative rarity and greater value of objects fashioned from rock-crystal, agate, carnelian, chalcedony, and jasper, compared with those from marble, alabaster, fluor, or other soft and less durable material.

Under this head we might also notice jade, eclogite, garnetrock, and other silicio-magnesian and silicio-aluminous rocks, which are frequently employed in ornamentation; but these, perhaps, will better be deferred till we come to treat of the precious stones in another chapter. Occasionally the septaria or bettle-stones (argillo-ferruginous nodules) of the coal and other formations, when of large size and sufficient solidity, are sliced for ornamental table-tops, and produce a fair effect. So also are the large silicio-calcareous fossil-trunks of Cold

The yellow mottled jaspers of Ayrshire, and the red, banded, and mottled varieties found along the shores of Forfar, Kincardine, and Banff, make fair inlayings when well polished and assorted, though the nodules rarely exceed eight or ten inches in diameter.

stream, with their concentric rings, rays, and mottlings; and the rolled flint conglomerate or puddingstone of Herefordshire; and not unfrequently blocks of cannel-coal, like that of Wigan and Wemyss, are cut, polished, and fashioned into tables, seats, vases, and other articles of fancy furniture. Such substances as these, however, have a local rather than a general interest.

Here we may likewise advert to the Malachites or green carbonates of copper, which fall to be considered more fully as Precious Stones in a succeeding chapter (XVI.) When found in large and solid masses, as in the Urals, Northern States of America, and Burra Burra in Australia, they are successfully worked, by inlaying, into vases, tables, caskets, timepiecestands, fireplaces, and other objects of internal decoration and ornament. The irregularly concentric layers of different shades of green occurring in the concretions produce, when skilfully sliced and united, a very rich effect, as was witnessed on a large scale in the Russian department of the Industrial Exhibition of 1851. From her possession of the Urals, Russia is still the headquarters of malachite manufactures.

Such is an outline—and it is merely a sketch in outline-of the stony substances employed in architecture and architectural ornaments. To have done justice to the subject from a builder's point of view would have required a volume; but enough, perhaps, has been given to show how intimate and important are the relations which subsist between geology and the art of the architect. Though deficient in some of the ornamental stones, we possess within these islands abundance of building materials, at once beautiful and durable, and fitted for the requirements of every structure-temple, tower, or palace; suburban villa, rural cottage, or country mansion; street-front, warehouse, or factory; fortress, sea-pier, or dockwall. And if it be that many of our public buildings are a reproach to us, and not to be compared, either in elegance, dimensions, or durability, with those of ancient Greece and Rome, it is not that we are wanting in materials or in skill to construct them, but because we live in an age of makeshifts, which seeks nothing beyond the necessities of present requirements. Our granites, sandstones, limestones, and calcareous freestones give ample choice for every variety of structure, while our roofing-slates and flagstones furnish adjuncts unexcelled by those of any other country. It is true we are deficient in some of the finer stones for internal decoration; but even in this respect much more might be made of the granites,

porphyries, serpentines, and marbles we possess, if sufficient time and labour-or what is the same thing, sufficient outlay— were spent on their preparation.

So far as Geology is concerned, it has as yet but slenderly discharged its duty to the builder and architect. It has busied itself, and properly enough, with mapping out formations, making sections, and defining palæontological zones; but it has done comparatively little in the way of pointing out the economic materials in these formations, or of indicating their relative values and appropriateness for special industrial purposes. It is one thing to determine the position, strike, dip, and thickness of a limestone, for example; but it is another thing, and one of paramount importance, to indicate its special mineral character, so that some reliable inference can be drawn as to its fitness for building, for mortar, for flux, for hydraulic cement, or for other industrial applications. Until geological surveys supply this desideratum in a regular and systematic manner, they are only partially fulfilling their function. It is surely as important to direct attention to rocks and minerals that may bear on the industrial purposes of civilised life, as it is to describe and dwell upon the remains of a life that has passed away. Both have their importance; but the one need not be exclusively studied to the detriment of the other. Above all, it is the duty of the economic geologist to note these things—ever acting under the impression that much as may have been utilised, there are still many substances in the earth's crust which can be turned to account in the increasing requirements of modern civilisation.

Works which may be consulted.

Hull's Treatise on the Building and Ornamental Stones of Great Britain and Foreign Countries;' Gwilt's Encyclopedia of Architecture, Historical, Theoretical, and Practical'-Papworth's Edition; Report of Commissioners on Building-Stones for the New Houses of Parliament, 1839 and 1845.

VI.

GEOLOGY AND ARCHITECTURE.

PART II.-MORTARS, CONCRETES, AND CEMENTS.

THE invention and preparation of mortars and cements form an essential department of architecture. It is not enough that we select stones of pleasing tints and durable texture; we must have some material capable of binding them together in one compact and substantial structure. Mere tooling and squaring may do for cyclopean walls; well-worked clay may give a certain amount of solidity to lowly erections; and bitumen, where obtainable, may give coherence to a pile but what is specially needed is a substance easily applied, and which, in course of time, will undergo such a mineral change as to bind together with stony consistence. Such, in general, are the limes, mortars, and cements of the builder-mineral pastes, if we may so speak-which, when well prepared and tempered, become often tougher and harder than the blocks they are employed to cement. These preparations, though very numerous, and many of them patented, may be conveniently arranged for description into Mortars, common and hydraulic; Cements, water and oil; and Concretes or Artificial Stones. Their ingredients are all, of course, obtained from the mineral kingdom-the great secret of their efficiency depending on the treatment of the raw materials, and the proportions of their admixture.

I. LIMES AND MORTARS.

The limestones which lie at the foundation of all these preparations are abundantly diffused through the stratified formations, there being scarcely a system which does not present one or more horizons of calcareous deposits. Indeed, every system from the oldest to the most recent has its lime