§ I. We have seen in the last Chapter how, in converting the wall into the square or cylindrical shaft, we parted at every change of form with some quantity of material. In proportion to the quantity thus surrendered, is the necessity that what we retain should be good of its kind, and well set together, since everything now depends on it. It is clear also that the best material, and the closest concentration, is that of the natural crystalline rocks; and that, by having reduced our wall into the shape of shafts, we may be enabled to avail ourselves of this better material, and to exchange cemented bricks for crystallised blocks of stone. Therefore, the general idea of a perfect shaft is that of a single stone hewn into a form more or less elongated and cylindrical. Under this form, or at least under the ruder one of a long stone set upright, the conception of true shafts appears first to have occurred to the human mind; for the reader must note this carefully, once for all, it does not in the least follow that the order of architectural features which is most reasonable in their arrangement, is most probable in their invention. I have theoretically deduced shafts from walls, but shafts were never so reasoned out in architectural practice. The man who first propped a thatched roof with poles was the discoverer of their principle; and he who first hewed a long stone into a cylinder, the perfecter of their practice. § II. It is clearly necessary that shafts of this kind (we will call them, for convenience, _block_ shafts) should be composed of stone not liable to flaws or fissures; and therefore that we must no longer continue our argument as if it were always possible to do what is to be done in the best way; for the style of a national architecture may evidently depend, in great measure, upon the nature of the rocks of the country. Our own English rocks, which supply excellent building stone from their thin and easily divisible beds, are for the most part entirely incapable of being worked into shafts of any size, except only the granites and whinstones, whose hardness renders them intractable for ordinary purposes;--and English architecture therefore supplies no instances of the block shaft applied on an extensive scale; while the facility of obtaining large masses of marble has in Greece and Italy been partly the cause of the adoption of certain noble types of architectural form peculiar to those countries, or, when occurring elsewhere, derived from them. We have not, however, in reducing our walls to shafts, calculated on the probabilities of our obtaining better materials than those of which the walls were built; and we shall therefore first consider the form of shaft which will be best when we have the best materials; and then consider how far we can imitate, or how far it will be wise to imitate, this form with any materials we can obtain.