Any student of geology who inspects a sample of galena or fluorite from one of the lead-zinc mines of the Mississippi Valley Ore district will be immediately struck by the hydrologic processes that were required to concentrate the requisite far-flung chemical components into a relatively thin layer of limestone. What was the nature and scale of the flow system that formed this ore body? How long did it last? Was it unlike modern flow systems? Until recently, these questions have been the sole domain of economic geologists.