There are over 70 occurrences of manganese oxides throughout Scotland; of these, only 13 are sufficiently well developed to be worthy of description. The deposits are categorized on the basis of the formation processes; eight are supergene ground-water or soil deposits, three formed by weathering, and two are hydrothermal in origin. The chemistry of marine sedimentary exhalative deposits can be related to the paleospreading rate of the once-active ridge and, under favorable conditions of stratigraphic preservation, be used to estimate the halfwidth of the ancient ocean. This approach yields estimates of 5 cm yr (super -1) and about 1,000 km for the spreading rate and maximum half-width of the Ordovician Iapetus ocean, respectively: figures which compare favorably with earlier estimates from faunal migration studies.Several of the Scottish manganese outcrops are associated with Old Red Sandstone (Devonian) sediments--a continental fluvial-lacustrine facies in northern Scotland. Diagenetic remobilization, with subsequent oxidation and deposition, of the manganese within the lacustrine sediments led to the formation of a manganese-rich horizon in the uppermost sections of the Old Red Sandstone sedimentary pile. Previously immobile in the pre-Devonian inorganic reaction-dominant regime, manganese mobility increased dramatically with the introduction of large quantities of organic acids following terrestrial colonization by plants. Climate change from arid to tropical with increased vegetation development promoted the formation of thick, organic-rich soils with the concomitant increase in available manganese-chelating ligands. Organomanganese reactions fundamentally changed the pattern of manganese mobility in the postcolonization surficial environment, making the Devonian a stratigraphic geochemical boundary. These changes in weathering processes would also affect the dispersal patterns of other metals which form organo complexes, gold being an obvious example.