1. Equivalent freshwater head and dissolved-solids concentration of water in rocks of Cambrian, Ordovician, and Mississippian age in the northern midcontinent, U.S.A. Yale Public content 1986. 2002-06-01T04:00:00Z. 2002-06-01T04:00:00Z. Geological Survey (U.S.). The Survey ; For sale by Branch of Distribution. English. Public. Middle West. Cambrian., Ordovician., and Mississippian.