The chloride (Cl) and bromide (Br) concentrations of ~1300 brine analyses from the Michigan, Illinois and Appalachian Basins essentially plot as ternary mixtures of meteoric water and two end-member brines. Concentrations of Cl and Br plotted on a scatterplot (Fig. 1) fall within fields of these ternary mixtures. The first end-member brine (B) evolved along the seawater evaporative trend to a point before halite precipitated (Cl/Br ~ 800). The second end-member (A) evolved well past halite precipitation (Cl/Br ~250). Brines that originally formed in Silurian saltpans of the Michigan and Appalachian basins probably had Cl/Br >250, whereas seawater in the Illinois Basin evaporated only to the C/Br ~800. Michigan basin brines now consist of mixtures of end-members A and B, reflecting the confluence of westward migration of Appalachian basin brines and northward migration of Illinois basin brines during the Allegheny Orogeny. Many brines plot on dilution trends from brine end-member concentrations towards the origin. This hypothesis is supported by the Cl/Br ratios of fluid inclusions in the Mississippi Valley-type deposits that were formed by brines migrating from the Allegheny Orogeny. Mixing models suggest that brines mostly were diluted by Pleistocene-age water, probably a combination of subglacial meltwater and incident precipitation.