Iron may occur in clays simply as a stain, when it is usually not in large quantity, or it may occur combined with some mineral or minerals present--as for instance certain felspars and micas. The brown, yellow, or blue appearance of the clay is due to it. In loam it may be found also as a species of ochreous earth, and in thin bedded loams (as the upper part of the Woolwich and Reading series of the London basin) each layer frequently varies in the proportion of iron present. In the more arenaceous parts of these loamy deposits, little grains of iron sometimes make their appearance, as also in certain sands employed in brickmaking; on careful examination, however, many of these grains are found to be other mineral substances coated with iron. Certain horizons in what are known as the Jurassic rocks contain great quantities of ferruginous matter in little pellets. Iron, in large proportion, acts as a flux to other constituents when the brick-earth is subjected to great heat in the kiln, and on that account must be carefully watched. But, to the average brickmaker, the ferruginous constituent is far more interesting as a colouring medium. At a later stage we shall have something to say concerning the colouring of bricks, &c., but it may now be remarked that red bricks, in practically all cases, owe their colour to the effects of firing on iron. It is a great mistake to imagine, however, that a large percentage of iron in a clay will necessarily produce a good red tint. In the first place, a great deal depends on the way the clay has been mixed or prepared; and in the second, the method of burning and the temperature employed, taken in conjunction with the general composition of the earth, are all important. This much may be said, however, that without the iron (or some mineral colouring matter possessing similar properties in the kiln) a red brick would not result. An even colour is the effect of thorough and homogeneous incorporation of the iron with the brick-earth; that may have been brought about by natural processes, but it is most frequently obtained in the careful preparation and mixing of the clays. A very essential point is that the earths must be of such a character as to withstand the requisite heat in the kiln without becoming vitreous, or twisting or warping. It must not be forgotten that a certain proportion of the iron, under great temperatures, may be carried away out of the kiln in union with other things, in the form of vapour. To successfully treat a raw earth, so that all these points may be taken into account, and to produce a thoroughly uniform red brick, that shall not vary in tint from kiln to kiln, is a matter requiring considerable skill and attention, though fairly good bricks of that character have been produced by sheer accident in burning natural earths fairly rich in thoroughly disseminated iron oxides.