_Limonite_ or brown iron ore. This ore is frequently found deposited on the upper surface of clay layers traversing sandy strata, the clay having arrested the carbonate solution and thus given time to the air to effect the change. Sometimes such deposits form great masses in rock-caves, fissure-veins, or crevices; and like siderite, it is an important iron ore, though frequently quite impure, as in the case of _bog ore_, which is formed in ill-drained subsoils. It is also sometimes found as the residue from the weathering of rocks rich in hornblende or pyroxene, and in this, as well as in other cases, is pulverulent, constituting _yellow ochre_. It makes a rust-colored streak on biscuit porcelain or unglazed queensware. _It is the coloring material of all yellow or “red” soils and clays_, as well as of brown sandstones, which are cemented by it. As is well known, such clays and sandstones become dark red by heating or “burning,” as in the case of common brick clays; the brown or yellow ferric hydrate losing its water and becoming red ferric oxid. The latter sometimes occurs in nature in the impure, pulverulent condition, constituting “red ochre”; but more commonly and abundantly it is found in the form of _Hematite_ or red iron ore, which is sometimes formed in nature by limonite losing its water, but more commonly in different ways. It is but rarely found in soils and is of no special interest in that connection. A fourth form of iron ore, quite common in the soils of some regions, is _Magnetite_ or magnetic iron ore, also known as lodestone. This mineral, the oxygen-compound of iron corresponding to “blacksmith’s scale,” also occurs in large masses and is an important and usually a very pure iron ore. It occurs very commonly disseminated through certain rocks, and in their weathering it remains unattacked and thus passes unchanged into the soils and sands, constituting the “black sand” so well known to gold miners and almost universally present in the alluvial soils of the Pacific coast. These black grains are of course attracted by the magnet and can thus be easily recognized and extracted. In soils they are simply inert, like quartz sand. But while the ore is of little interest to the farmer, it is quite otherwise with the compound of this oxid with water, the ferroso-ferric hydrate; intermediate in composition between the white ferrous and the brown ferric hydrates. As mentioned above, the black silicate minerals, such as hornblende and pyroxene, are bottle-green when seen in thin sections. Nearly the same color, with modifications running toward blue and bright green, is often seen in natural clays and rocks, and is almost always caused by the ferroso-ferric hydrate. Such materials always become red or reddish when heated by the formation of red ferric oxid; while when exposed to damp air, they assume the rust color of ferric hydrate.