Sketch of the suggested hydrological and thermal arrangements in the Seine River valley during a cold period in the Quaternary. (1) Limestone reservoir groundwater flowing through (2) cold ground interface in presence of gypsum leads to silica precipitation and minor amounts of calcite. If it encounters gypsum + dolomite layers (3) it brings about precipitation of silica + large amounts of calcite. 1, 2 and 3 refer to the equilibrium geochemistry in Figure 15b.