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58 THE CIVIL ENGINEER AND ARCHITECT’S JOURNAL. [February 1,1S6S. easily recognised at once as metamorphosed. Those stones, which are certainly and directly due to the action of intense heat suffi cient to produce fusion, are rarely valuable except for road material. They are always too hard and generally too brittle for building stones. They also present an inconvenient and irregular natural fracture, ill-adapted for reduction to definite forms. Varieties of Constructive Material. — The mineral composition of rocks is in some cases very simple and evident, in other cases moderately complicated but easily recognised, in others again, so complex as to need long experience or chemical analysis to determine its nature. As the enduring power of stones, when exposed to certain influences, depends in some measure on mineral composition, this subject is one to which the architect should direct his attention. A large proportion of the building material in common use is limestone, more or less pure, and the rest is chiefly sandstone. In the former case carbonate of lime, in the latter silica is the chief ingredient. Clays unaltered are only used when manu factured into bricks, and the sulphates of lime (gypsum and alabaster) are chiefly adapted for internal use or small orna ments. The granites and porphyries are limited to special uses, owing to their extreme hardness. The varities of marble may be regarded as varieties of limestone. Slates and slabs are very useful in architecture, and form a special class of rocks due to metamorphic action and derived from clay. Limestones are either simple carbonates of lime, mixed car bonates of lime and magnesia (magnesian limestones), silicates of lime and magnesia (serpentines), or sulphates of lime (gypsum and alabaster). According to their texture, to the cementing medium in the case of the bedded limestones, and in others to the extent of crystalline action or metamorphosis they have undergone, they are available for special purposes. They owe their colour to metallic oxides. When semi-crystallised and having a grain too fine to be recognised, they are marbles of the common kind, gypsum or serpentine, according to their chemical composition; when perfectly crystallised, they are statuary marbles or alabaster ; when the grain can be recognised, they are generally free-stones; when the grain of the stone is mixed with fragments of shell, they are rag-stones or shelly limestones; when the grain is formed by a multitude of round concentric particles like the roe of a fish, they are roe stones or oolites; when the stone is compact, the stratification evident, and the stones split readily, they are flags. To all these varieties local names are attached in different districts. The limestones in common use in England for architectural purposes are of several distinct kinds, the Portland stones, Bath stones, and many others belonging to the oolites ; semi-marbles of very compact grain derived chiefly from rocks of the carbon iferous system, tire mixed carbonates of lime and magnesia almost entirely from the Permian rocks on the borders of Derby shire, Nottinghamshire, and Yorkshire, known as magnesian limestones, and some hard varieties of chalk. Elsewhere, especially in the south of Europe, the cretaceous and tertiary limestones are developed into admirable free stones of exquisite colour, very compact and almost non-absorbent. In Italy especially, are found the limestones called tufa or more properly calcareous tufa* derived from the exposure of waters containing a large quantity of carbonate of lime. On the evaporation of the water the carbonate of lime is deposited in a form more or less compact, according to the material on which it is thrown down and the slowness of the deposit. Some very useful build ing materials are thus obtained, but they are limited to those localities where very large quantities of mineral waters run over the surface. In limestone caverns, and in veins in limestone rocks, the irregular drooping pendants or stalactites, the floor called stalagmite, and the lining walls of the veins are formed in the same way; but the floor is often very compact and may be used as a marble. Oriental alabaster is of this kind and it is very abundant in Egypt. As well from their chemical composition as from their physical peculiarities, there are few building stones anywhere that excel the finer qualities of Portland. Their great draw back is, that they are heavy and expensive. The ordinary oolites of which the variety is very great, but of which Bath and Caen 0 Another kind of tufa (volcanic tufa or tuff) is also common in Italy, but it is of no value for building purposes, except as replacing pozzolana. It is volcanic ash, cemented by carbonate of lime, carried in by rain-water. The material under which Pompeii was buried is of this kind. stones are the types, are sometimes round and good, wearing well and being obtained in very large blocks. Many of them on the other hand decay almost immediately. The colour and texture is generally excellent, and they cut with the greatest facility, so that for internal work they are unrivalled. Some of them are singularly uniform in their oolitic structure, some are crowded with fragments of shells with only a few oolitic por tions, and some parts are quite crystalline. According to their nature and composition they are more or less durable, and more or less affected by ordinary atmospheric influences. The magnesian limestones, generally more crystalline than the oolites are in proportion to their crystalline state, valuable building materials. They are heavy and hard, but work well. The great objection to their use arises from the extreme irregu larity of their composition and texture in the quarry. Sandstones are more simple in their composition than lime stones, as they vary only in the grain or magnitude of the particles of which they are composed, in the nature of the cementing medium, and in the extent to which they contain foreign impurities. There are no crystalline sandstones adapted for architectural purposes, and the metamorphosed sandstones are too hard to be treated as free stones. The colour of sand stones is due chiefly to oxide of iron, which renders them yellow or red according to circumstances. The material of all sandstones is silica. There are many admirable and more very indifferent sand stones used in the British Islands for architectural purposes. The red sandstones, abundant in the middle of England, and continuous geologically from Devonshire into Lancashire, are soft, cheap, and very convenient, but they are rarely durable. The hard grey and micaceous grits of the carboniferous series, both of Yorkshire and the South of Scotland, are far superior in all respects ; less absorbent, and more durable under all circumstances of exposure. Sandstone, however, is not very convenient for ornamentation, nor, indeed, are the bedded varieties well adapted for delicate sculpture, but for certain styles of composition they afford every advantage. The public buildings of Edinburgh and Dundee show good proof of this. Clay I do not propose to consider in detail on this occasion. They can only be used for constructive purposes when manufac tured into bricks. They are all silicates of alumina, varying in quality according to their mineral composition, but the quantity of alumina necessary to form a sound brick is exceedingly small, and the useful proportions are almost indefinite. Gypsum and alabaster are pure crystalline sulphates of lime in somewhat different mechanical conditions. They are neither of them in the state in which stone is originally deposited. They are found sometimes in veins, sometimes in bands of nodules of large size. Among very useful material in countries, where it is abundant I ought not to omit mention of the • soapstones (silicates of magnesia) which in their quality of resisting fire are excelled by none. America yields them very largely and at a moderate cost. They are found in Cornwall, but large blocks for build ing could not, I believe, be obtained of uniform quality and in sufficient quantity to be used extensively. The gritstones of the carboniferous system are the best stones that can be used for fire-proof buildings. Granites of all kinds belong to the class of metamorphic rocks. They were certainly not deposited as they now are, nor were they brought into their present condition by any action of mere heat. Originally, perhaps, of mechanical origin, they have been rendered crystalline by long continued chemical action, assisted by intense heat and enormous pressure while in the interior of the earth. In this way have been produced, not only the peculiar state of the mass of the rock, but the fissures and their varied contents, the veins of metalliferous and other matter that penetrate them, and all the changes they induce in the other rocks adjacent. Rocks of this kind are double silicates. Slates, once thought to be crystalline rocks, have been pro duced from clay by mechanical pressure. The fissile character can be given to wax, or to any other substance whose particles are sufficiently minute; and all the varieties of colour and texture, all degrees of fissile nature, all the peculiarities of hard ness and resistance may be distinctly traced to the mechanical cause of the phenomenon. It requires, however, a careful examination of slates in the quarry, and some habit of examin ing slate quarries, to appreciate the fact that they are so nearly