A summary of the geology of the Bull Valley district, southwestern Utah, and a description of the occurrence of the magnetite and hematite deposits is given, as well as brief descriptions of the ore pits at Iron Mountain and Desert Mound 20 miles to the northeast, in which deposits formed under similar geologic conditions have been developed. Neither the ores nor the country rock show the features characteristic of contact-metamorphic deposits. The following explanation of the origin of the deposits is offered. During the intrusion of monzonite porphyry stocks, the hoods and overlying rock were fractured, causing a sudden release of pressure and liberating FeCl 3 and H 2 O gas from the stocks which were still at high temperature. These gases found ready egress to the surface along tension fissures without appreciably heating the country rock. They deposited magnetite and hematite in the fissures and replaced the contiguous fractured limestone. The period of gas emanation was brief and the hydrothermal stage probably evolved during the later magmatic history never reached this area. The deposits were formed under a cover of less than 5,000 feet.