The member of the oolitic series which' has supported the action of the London atmosphere the most successfully, after the Portland stone, is the material known as the Kelton atone, obtained from the midland counties ; and, like the Painswick variety of the same formation which also stands tolerably well In London, it appears to owe this property to the more decided character of the crystallisation of the molecules of the cementing materials of the oolites. The Bath stone, when very carefully selected, and used with all necessary precautions as to bedding, and removal from excessive moisture, resists tolerably well ; but the general character of the deposit is too irregular, and the habits of London masons are too careless in everything which regards the use of building stones, to allow either the Bath stone, or the analogous member of the great oolite obtained from Caen in Normandy, to be employed for external masonry in the metropolis. The Barnack stone, formerly used to a great extent In Lincolnshire, Cambridgeshire, and the Isle of Ely, has been exhausted, and its place is now locally supplied by the. Casterton stone; these local varieties of the °elite are characterised by an abundance of minute fossils, corals, and foramenifera, and when well selected they seem to be tolerably durable. But it may be observed with respect to them, and to the Ancaster stone (a very beautiful looking material, said to be used successfully in the midland counties), that until they have been employed for a long tithe in the peculiar atmosphere of London, It would be dangerous to pass an opinion upon their merits.
There are several descriptions of sandstones used in London, and hi various parts of England, for walling or external masonry, such as the Dundee and Arbroath stones, those from Cragleith, Darley Dale, Park Spring, and the neighbourhood of Leeds and Halifax ; but neither on the score of their colour, or of their durability, can they be considered to be superior to the Portland stone. Per some uses, as for steps, door sills, paving, hearths, &c., the Yorkshire and Arbroath stones present advantages on the score of their great strength, hardness, and ease of conversion in the direction of the bed, and also on account of their power of absorption of moisture, which renders their use desi rable for all masonry works in the interior of dwelling houses. These sandstones when used in elevation are, however, exposed to decay "between wind and water," as the masons designate'the zones of walls exposed to alternations of dryness or humidity. Unfortunately the magnesian limestones, lately introduced for the construction of the Houses of Parliament, have decayed iu these positions even more rapidly than the worst sandstones formerly employed ; and it is no exaggeration to say that the actual state of the masonry of that building has assumed the proportions of a national misfortune. The Kentish rag, so much used for the rubble filling between the dressed quoins of medieval architecture, in the modern revival of that style, appears to resist the action Of the atmosphere very well ; but it is unfortunate in these buildings the material used for the quoins is generally • selected from the softest and moat perishable atones. The conditions
under which the siliceous conglomerates, such as the Bromley Fall, or the trap, basaltic, and granitic rocks decay, and which therefore must regulate their application for purposes of masonry, have already been discussed. (ATMOSIIIERIC INFLUENCE.] It follows from these observations upon the decay of stones that they require to be used with precaution, oven when of good quality. Care should be taken to lay them in true bedding; the softer and more absorbent stones should not be used in the lower parts of a wall, or in situations where. they would be likely to take up moisture from the ground, or from any projections of the ornamental masonry, by capillary action ; and the use of limes, cements, or plasters, able to give rise to an efflorescence of the salts in the stones, must be avoided. there have lately been introduced some processes for the induration and preservation of building stones, the best of which seems at present to be Itansome's process, notwithstanding the adverse opinion of Faraday ; but in fact every method for obviating the decay of a bad stone can be only a palliation, more or less perfect, for an original defect; and the common sense of this matter is to use originally a stone which should be able from its own constitution to resist the con ditions to which it is likely to be exposed, rather than to attempt to remedy its defects. Highly ornamental masonry executed in soft atone may be, however, protected by the application of such processes as those above named ; but they should only be regarded as hazardous and temporary methods of obviating present dangers, especially as their success must essentially depend on the care and skill with which they are applied.
In the execution • of masonry, such as we generally see in the south of England, where the face is usually executed in ashlar of a less thickness than the wall, the ashlar is backed up with brickwork, which should be set in cement, in order to prevent any inconvenience from the unequal compression of the respective materials. Great care is also required in the execution of mixed masonry to obviate danger from the same cause; because the vertical chains of stone-work do not compress at the name rate as the filling-in brick-work, unless the latter be executed with a cementing material able to solidify in a very short space of time. The longitudinal courses of mixed masonry, as a general rule, run through the wall ; or if this should not be the case the outer ranges of stones are connected with the mass of the work by means of bond stones, a system of construction equally applied to ashlar work backed up with brick. In other respects the rules for the execution of masonry are identical with those of brickwork ; that is to say, the vertical joints in elevation should never be placed immediately over one another, nor should the facing work be distinct from the hacking, in such manner at least as to allow any separation between the two.