In the course of thin-section and heavy-mineral studies of fluorite breccias from Jamestown, Colo., the mineral thorite, variety uranothorite, was identified on the basis of chemical, spectrographic, optical, and X-ray data. This uranothorite is compared with uranothorite from South Westland, New Zealand, described by Hutton (1950), and with similar material from an Alaskan placer sample studied by the writers. These uranothorites are in turn contrasted with uranothorites described in the older literature all of which represent hydrated material. Thorium halos in fluorite are described, probably for the first time. The uranothorite associated with fluorite breccias is clearly of hydrothermal origin--unlike all previously described thorites and uranothorites with one very doubtful exception.