It has long been recognized that, in early Eocene times, while the “Calcaire de Mons” was being deposited in the Belgian area, large tracts of the Upper Chalk were removed by denudation from the south-east of England. The true extent of the unconformity between the Chalk and our oldest Eocene strata is now to be seen only where the Eocene cover yet remains. Even where the Chalk is so covered, the unconformity must have been accentuated to some extent in consequence of the solvent action of percolating carbonated waters, the evidence of which is familiar to us as “pipes” in the Chalk filled with Eocene strata, and the persistent bed of unworn, green-coated flints (“Bull-head” Bed) which everywhere occurs between our lowest Eocenes and the Chalk; but such secondary action is likely to have been more or less uniform throughout the whole area beneath the Tertiary cover.