Are all composed of silex nearly pure; they break in general with a conchoidal fracture, and to divide them into plates it is necessary to resort to the lapidary process. They may be slit with emery, but it is far more economical to employ diamond powder, as the time then required is only one third of that called for when emery is used; these stones are always ground with emery, and polished with rotten-stone, at will be explained in the third volume.
* Four beautifully polished granite columns may be seen in the King's Library, British Museum; and in the Museum of Economic Geology, there are also many ornamental polished works in granites, elvans, marbles, serpentines, etc, and likewise six inch cubes of a great variety of the same, and of numerous other building stones, collected under the "Commission appointed to visit the Quarries, and to inquire into the qualities of the stone to be used in building the New Houses of Parliament." The various materials are most elaborately detailed in the Parliamentary Report of the Commission, together with every information respecting cost, supply, and transport; the dates of various buildings in which the stones have been used, and remarks on their present states of preservation: followed by the chemical and physical examination of the stones, and other points which concern the civil engineer, architect, and builder, to whom the report would appear to be almost invaluable. Ultimately four varieties of magnesian limestone were selected for the erection of the national edifice.
The Museum of Economic Geology also contains a most interesting series of specimens of all the metallic ores found in Great Britain and Ireland; and in many instances there are associated with them geological maps and models of the mines, working models of the machinery for mines, and also specimens explanatory of the various processes of manufacture from the ore to the marketable metal. For an infant museum it is most rich, interesting and instructive.
The Institution of Civil Engineers also has a collection of granites.
Agate is used as the bearing planes for the knife edges of delicate balances, for pestles and mortars, burnishers for gilders, and book hinders, and also for some other purposes in the mechanical arts; the whole of the stones in this group are largely employed for the purposes of jewellery, the handles of knives, snuff-boxes, and a variety of ornaments.