The ore deposits in many Mississippi Valley-type districts occur in brecciated carbonate host rocks. In some districts the pattern of mineralization and the spatial relationship of ore to earlier karst breccias indicate that the ore solutions followed and enlarged preexisting caves. In such cases dissolution of favored layers, mainly limestone, has allowed collapse and fragmentation of overyling beds, usually dolostone. The subsidence is long continued and much of it probably occurs while the ore solution is circulating.In addition to gravity-induced collapse, there is evidence that tectonic compression and chemical brecciation working in concert with host-rock dissolution may have contributed to the fragmentation.Other types of breccias found in some of the ore deposits are caused by penecontempo-raneous sliding and by wave action breaking forcefully against algal reefs and producing talus. Economically these types of breccia are of much less importance.