The mineral known as brown ironstone consists of ferric hydroxide, and usually forms compact masses, no decided crystals having, so far, been observed. The lumps have an irregular or earthy fracture, a hardness of 5–5·5, and a sp. gr. between 3·40 and 3·95. The colour ranges, in the different varieties, from yellowish (rusty) brown, through cinnamon to blackish-brown. The chemical composition of the pure lumps may be expressed by the symbol 2Fe{2}O{3} + 3H{2}O; but a little manganese oxide and silica is generally present even in the pure kinds. The chief varieties of this mineral are:-- (_a_) Fibrous brown iron ore, or brown hematite, mostly forming reniform or stalactitic masses. (_b_) Compact brown ironstone, usually in dense masses, and not infrequently also appearing in pseudo-morphs of other minerals. (_c_) Ochreous brown ironstone. This variety is the most important to the colour-maker, for whose purposes it is preferably used. It nearly always forms very loose, earthy masses, yellow or brown in colour. (_d_) Clay ironstone. This consists of a mixture of the above-mentioned varieties with variable proportions of other minerals, clay being the most common ingredient. Nodular iron ore, oölitic, bog and siliceous ore belong to this class, as also the minette ores that are found in great abundance in Alsace-Lorraine, Belgium and Luxemburg, and are classed with the oölitic brown ironstones. In most cases, the varieties enumerated are found together, and are used for the production of iron. The ochre constituting the most interesting member to the colour-maker often occurs as deposits embedded in dense masses of brown ironstone, though in many places it is found by itself. CHEMICAL COMPOSITION OF VARIOUS BROWN IRONSTONES The following analyses of brown ironstone from different deposits will give an idea of the composition of these minerals. _Ordinary Brown Ironstone_