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February 1,186S.] THE CIVIL ENGINEER AND ARCHITECT’S JOURNAL. 59 allied to clay pits, and that so narrow a line separates materials apparently so very different. I say nothing here of the shales, which are transition rocks between clay and slate, nor of the limestone flags, such as Stonesfield slate, neither do I speak of such paving stones as the Yorkshire flags. These hardly belong to architecture, though occasionally used in construction, and though of different material they are formed in the same manner as slates. Mode of formation of various kinds of Stone.—It is not enough for the architect to be told that the various materials used for construction have been derived from other rocks, partly by mechanical disintegration, partly by chemical action, that they have been deposited at the bottom of the sea, and that they have been altered by heat and chemical action. All these are important facts, but the evidence on which they rest, and conclusions derived from a consideration of the whole matter, are also in the highest degree valuable to the practical man. The architect should know the nature and history of the forma tion of stones, if he would make the best use of such of them as come under his notice. Limestones, sandstones and clays of all kinds, have been at one time held in suspension or solution by water. They were for the most part deposited from moving water, though some times separated on evaporation, as in the case of calcareous tufa and cavern incrustations and stalactites. This is the case with the carbonates of lime very clearly and certainly, with the sulphates less clearly, and with the double silicates of lime and magnesia, or serpentines, least clearly of all. It is only after much doubt and discussion, that the latter have been distinctly recognised as metamorphic limestones. Even now the Italian geologists recognise, and perhaps with reason, certain erupted serpentines. In all cases the change or metamorphosis '.rill be found to be extensive and very com plete, and the ordinary characteristics of limestone entirely lost. Till very recently, fossils or remains of organic bodies, such as are almost always found in limestones, had not been observed in serpentines, and even now the steps of conversion or metamorphosis are rather guessed at by the chemist than traced by the geologist. So also the mechanical origin of sand stones of all kinds and of clays, is seen at once by a very slight examination of the stones, but the subsequent history of all these rocks is not so readily made out. After the deposit in the sea of mud or sand, brought down by rivers from mountains and various lands, and mixed with the detritus obtained from the disintegration of cliffs, this raw material of rocks remains exposed for a long time to the action of water, by which means a certain mechanical arrangement of the particles must take place, according to their relative specific gravity. During this time also a large quantity of remains of plants and animals cannot fail to have been introduced. In some places the whole mass may consist of corals or shells, of bones of fishes, of scales, or even of the exuviae of animals, in others fragments only arc retained, and in some there will be casts of such objects, the original having become decomposed, or markings and impressions sufficient to prove the existence of organisms, even when no actual fragment remains. The next step in the conversion of accumulated heaps of mud and sand into definite strata of stone, is that of consolida tion. There are two ways in which this is brought about, one mechanical, the other chemical. Mechanically, wet mud parts with superfluous water and is converted into a compact mass by mere continued pressure. There are always foreign sub stances present, which under favourable circumstances will act as a cement, and the solidification is usually effected by car bonate of lime, aluminous earth, or oxide of iron, held in solution in the water and discharged under chemical action. Occasionally silica, in a dissolved state, answers the same purpose. The work that is thus effected requires time, and during this time a process of rearrangement of particles generally takes place, so that in the beds or strata now formed, the impurities and foreign substances are separated from the mass, and accu mulated in nodules or layers; whilst the drying takes place there is always contraction, and thus many fissures and clefts are produced. They are often filled up by crystallised plates of the same material as the mass. In this way are produced certain varieties of limestone, but not those common in England. The English calcareous building materials belong to the oolites, in which the grains themselves, before being cemented together by carbonate of lime, have been curiously formed by numerous concentric coats of much smaller particles. Of these there is a great variety, some being white, compact and heavy, such as Portland, others cream-coloured, soft and light, as Bath, and many of intermediate character and of different value. No two stones from different quarries have precisely the same qualities, inasmuch as the conditions under which stones have been formed cannot have been exactly the same in two parts of a coast line. The oolitic grains are believed to have been formed in water very slightly agitated, enabling each grain to become a centre of solidification. The magnesian limestones (carbonates of lime and magnesia) are combinations of carbonate of lime and carbonate of mag nesia, more or less crystalline according to circumstances. When the two carbonates arc nearly in equal proportions, and both well crystallised, these stones are eminently compact and fine grained, but owing to their nature and mode of formation this condition is apt to be partial, and in this case the quality of the stone is irregular and uneven, and it weathers badly. The cause of the association of magnesia has not been very clearly made out, nor is it essential to the architect to enter upon this question. It is certainly the result of change after the original deposit, and in so far belongs to metamorphic action, but at what part of the history of the deposit the mag nesian element is introduced is still open to discussion. Marbles of all kinds are crystalline limestones. Many of the coloured varieties are so little changed that their mechanical origin is evident, and they are loaded with organic remains. This is not the case with statuary marble, nor with the white granular marbles, extensively distributed in certain localities. There are veins consisting of slowly crystallised material, filling up crevices or fissures made in limestone. They are perfect in proportion to the completeness of the metamorphosis, and the removal of all foreign substances. The higher qualities arc thus few in number of deposits and small in quantity. The inferior qualities are abundant and varied. The varieties of marble called serpentine are double silicates of limo and magnesia. They arc often extremely beautiful, but owing to the mode in which they have been produced, they arc crossed in every direction with threads and streaks, and are much cracked. It is very rare that large slabs can be obtained of uniform quality for decorative purposes, or that blocks of length enough arc found to serve for columns.* The mode of formation of these rocks is not easy to see, but they have been rendered what they arc by change long subsequent to deposit. All the varieties of granite, all the veins in granite, and all the numerous porphyries, must also be regarded as meta morphic. They were originally aqueous rocks, but they have been completely changed in all essential characteristics, in combination with water under great pressure, and exposed during a long period of time to high and uniform temperature. The peculiar construction of these rocks, as crystals embedded in a kind of glassy paste, does not appear to be the result of strict igneous fusion, as was once thought. It is rather the result of the action of chemical force assisted by time and high uniform temperature. Having been formed in this way, the result is a stone of extraordinary compactness and durability, but at the same time extremely difficult and costly to deal with. Porphyries arc comparatively little affected by weather, and are often of extreme beauty. But every largo mass of granite rock is interpenetrated by numerous dykes, represent ing fissures in the first formed mass subsequently filled up by similar accumulations of crystals embedded in a glassy paste, but as these have been formed under somewhat different con ditions, they arc always more or less compact, and have special properties. Thus a large mass of granite in the quarry varies greatly in different parts, sometimes being hard and tough, sometimes decomposing rapidly, and sometimes passing into a fine granular state quite different from the ordinary rock, and more fit for road metal than building material. Thus are produced infinite varieties of granite and porphyry, varying in hardness, in grain, in durability, and in facility of working, from the comparatively soft kinds quarried in Cornwall to the glorious specimens of Rhodonite that have lately been • I have lately seen in the island of Corsica, a material of this kind, hardly inferior in quality to the finest varieties of vends antico, and capable of supplying both slabs and columns of very largo size. Specimens of it have been used in the Emperor’s box at tlie new Opera House in Paris.
