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 paleontological 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.

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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

stones: the Metamorphic, its crystalline marbles; the Silurian, its coralline and shelly beds; the Old Red, its cornstones; the Devonian, its coralline and shelly marbles; the Carboniferous, its coralline, encrinal, shelly, and fresh-water beds; the Permian, its dolomites; the Trias, its muschelkalks and gypsums; the Jurassic, its oolites; the Wealden, its shelly bands; the Cretaceous, its chalks; the Tertiary, its gypseous and nummulitic strata; and the Post-Tertiary, its lacustrine marls. In Britain, the most of these are abundantly developed; and for its area few countries can boast of such a varied and available supply. As mixed rocks they vary, of course, in composition, some being almost pure carbonates, some dolomitic or magnesian, and others sulphates or gypsums; while these varieties may again be less or more silicious, argillaceous, ferruginous, or bituminous.

Whatever the varieties, or in whatever formations they may occur, the most of these limestones come to the surface in long stretches of outcrop, and are consequently quarried in open workings; hence the numerous openings, great and small, on the chalks, oolites, magnesian limestones, and mountain limestones of England, and the mountain limestones of Ireland. England and Ireland are magnificently supplied with limestones; Scotland but scantily so, and hence the more frequent recourse to mining of it in that country, as well as to its importation from the north of England and Antrim. In treating of these limestones in the present chapter, they may be conveniently considered under two main sections,Ist, those suitable for ordinary building and plastering purposes; and 2dly, those which are hydraulic, or harden and set under water.

Mortar Limestones.

The limestones best adapted for common or air-setting mortars, are the carbonates which are free from silica, alumina, and iron. These, as already stated, are very abundant; and, after being quarried and broken into moderate-sized pieces, are calcined, either in temporary or in continual kilns,—that is, in open kilns which are blown out till the calcined charge has been removed, or in draw-kilns, where the removal and charging proceed continuously. To avoid carriage, it is desirable to have the kilns as central as possible to the face of the quarries; and the longer the stone has been exposed to the air, the less fuel will it require to drive off the inherent moisture or quarry-water. The fuel employed in calcination is ordinary pit-coal (1 ton to 4 or 5 tons of limestone), and in remote

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districts peat and brushwood; but for some sorts of limestone, impure or shaly coals (while also much cheaper) are better adapted than the pure coals, as burning the stone more slowly and equally, as well as keeping it open and preventing slagging and sintering. More kiln-dust may be produced by the use of these slaty coals, but fewer cores and slags will be found among the lime.

When properly burnt-that is, when not slagged or covered with a silicious glaze by too sudden ignition-the limestone loses its carbonic acid, and is converted into caustic or quick lime (protoxide of calcium)-100 parts of the raw stone yielding 56 of burnt lime. This caustic lime is next slaked with water (1 volume of water to 3 of lime), when it falls down, with violent evolution of heat, into a greyish, bluish, or brownish powder, according to the original colour of the limestone. By the application of more water it is converted into a pulp or paste; and this pulp, thoroughly incorporated with three or four times its own volume of clean sharp sand, constitutes the common or air mortar of the builder. The proportion of sand should vary, of course, with the richness or "fatness" of the lime-2 or 22 parts being sufficient for some poor limes, while some fat varieties will stand as much as 4 or 5 parts, and be improved by that proportion. The purer the carbonate the fatter the slaked lime.

A great deal has been said and written by builders about the properties and admixtures of sand and lime, which would be out of place to repeat in a work on Economic Geology; but this much may be observed as essential to a good hard-setting mortar: first, the more rapidly and completely it falls to powder on being slaked, the better the lime; and secondly, the sharper and cleaner, the better the sand. A lime that falls slowly and unequally is never satisfactory, either for mortar or for plaster,-preventing cohesion in the one case, and causing blistering in the other. A sand containing earthy impurities interferes with the chemical union of the lime and silica; sea or shore sand impregnated with salt is likely to cause deliquescence or efflorescence; and it is only clean pit or river sand that can produce the finest mortar. The old-fashioned plan of covering slaked lime with turf, and allowing it to lie a twelvemonth, was more effective in reducing it to powder than the modern hasty method of crushing and pugging; and the older mode of using large gritty sand and stony fragments was infinitely superior to the mixture of earthy rubbish which, under the misnomer of "sand," is now used to the extent of four, five, or even six parts of the admixture. In many modern

buildings the mortar and stones never cohere; in taking down old baronial halls and ecclesiastical structures the blocks often give way before the mortar. So cohesive, indeed, are some of these olden walls, that explosives have to be employed in their demolition and severance. The ancient builder aimed at substantiality and permanence; the modern mason seems contented with mere face-up and temporary effect. We are aware that some modern architects prefer freshly-slaked lime—and, no doubt, when of prime quality and thoroughly incorporated with good sand, it may set well and solidify; but we have ample proofs of the efficacy of the olden system even when, as Pliny mentions, "the specifications provided that no slaked lime less than three years old should be used by the contractors." The olden plan of grouting-viz., rendering the mortar sufficiently liquid to penetrate every pore and crevice of the interior packing, and then pouring it in at intervals as the walls advanced-also greatly assisted to strengthen and solidify the edifices of our ancestors, and might well be revived at the present day.

Along with these air-setting mortars may be noticed the ordinary plasters used for smoothing and finishing interior walls and ceilings. These plasters are generally prepared from the finest limes and sands, extra care being taken in the sifting and incorporating of the materials. As several coatings are usually applied, the finer are laid on last, the ultimate surface often consisting of a "floating" of creamy consistence. The extra manipulation of plaster-lime, its admixture with tanyard hair to give it greater consistence, and the like, are matters that belong to the artificer and not to the geologist.

Recently, there has been brought into notice a new composition under the name of Selenitic Cement, which is in fact an intimate admixture of hydraulic lime, gypsum, and silicious sand. Any hydraulic limestone of fair quality is calcined and mixed with five per cent or thereby of dehydrated gyp

the two are thoroughly incorporated by being ground to a fine powder, and then worked up (by hand or machinery) with five or six parts of clean sharp sand into a cement. When laid on as plaster, it sets, dries, and hardens in the space of twenty-four hours, and can be finished off with a fine "floating" in course of the following day. It thus presents great advantages over the slow-drying ordinary plaster and its successive coatings, and can be applied to brick and stone walls without any admixture of hair. It can also be used, accord