Two minerals commonly met with in earths used for brickmaking are pyrite and marcasite, both of which are of the same chemical composition, namely, iron disulphide. We may first consider them separately, for they are of great importance to the brickmaker. Iron pyrite occurs as regular cubic crystals, or irregular streaks, or as nodules or lumps; in clay, the last-mentioned is its commonest form. It is a good petrifying medium, so that it is frequently associated with organic remains, as is exemplified in almost any yard where stiff clay is being worked. The nodules, on being broken open, ordinarily exhibit a radiating structure of brassy lustre and extremely beautiful appearance, though often marred by brown iron stains due to decomposition of the mineral. In the refuse of slates, now so largely used in several parts of the world for brickmaking, pyrite is most frequently found as fine cubic crystals of a durable nature. Marcasite, on the other hand, crystallizes in a different manner (in the rhombic system of mineralogists), but is chiefly found in fibrous masses or dirty-brown nodules, the last-mentioned being common in clays. When bright it is paler in tint than pyrite, though this is not a constant character. It occurs abundantly in almost all sedimentary rocks diffused as minute particles, but sometimes in irregular layers. Sir Archibald Geikie states[5] that this form of the sulphide is especially characteristic of stratified rocks, and more particularly of those of Secondary and Tertiary age. That it is not abundant in Primary rocks is not to be wondered at when we consider its liability to rapid decomposition; indeed, for it to be preserved at all it must be well shielded from atmospheric agents by Nature. Exposure even for a short time to the air causes it to become brown, free sulphuric acid is produced, which may attack surrounding minerals, sometimes at once forming sulphates, at other times decomposing aluminous silicates and dissolving them in considerable quantity. It plays even a larger part than pyrite as a petrifying medium, at any rate in the younger rocks. Both pyrite and marcasite are abundant in many other rocks than those of special interest to the brickmaker; the former, in fact, is almost universal in its occurrence.