An important group of iron-ore deposits has resulted from mineral replacement along the contact of sedimentary rocks with igneous intrusives. These ores usually occur in limestones not far from intrusive masses of granite, monzonite, syenite or diorite, but they may be found within the igneous rocks themselves, near the contact. They are rarely associated with the more basic igneous rocks. These ores are known as igneous contact ores, and their origin is ascribed to iron-bearing solutions that accompanied or followed the intrusion of the igneous rocks with which the ores are associated. Such ores are extremely widespread, occurring in practically every continent. Locally, extensive deposits exist, as in the Cornwall district of Pennsylvania, in the western United States and British Columbia, in Chile, and in China and Japan. Igneous contact ores have furnished only a relatively small percentage of the world’s total production of iron ore, however. There are also widespread replacement deposits in sedimentary rocks that are not associated with igneous rocks. These are believed to be formed by ordinary meteoric waters which dissolve disseminated iron minerals from certain beds or masses of rock, and redeposit the mineral elsewhere in a more concentrated form. Such ore deposits may be roughly tabular and resemble bedded deposits, or they may be very irregular. Most deposits of this type consist of siderite which has replaced limestone, but hematite and limonite deposits formed by replacement also exist. Among the important deposits of this group are the siderite ores of Bilbao, Spain, largely altered to limonite near the surface; the siderite ore of Eisenerz, Styria; and the hematite deposits near Hartville, Wyoming. Small deposits of siderite, hematite, and limonite of this type are found in many parts of the world.