The Lower London Tertiaries were defined by Prestwich, as consisting of three divisions. Two of these were relatively homogeneous in composition; but the third was made up of very varied materials. The extreme care and accuracy with which these divisions were traced out over the whole of the Eocene area in England, and subsequently correlated with those of the French area, led to their speedy and universal recognition. No modifications in this classification were even suggested until 1866, when Mr. Whitaker, while unreservedly adopting Prestwich's divisions of “Thanet Beds” and “Woolwich and Reading Beds,” making, indeed, copious use of his observations, separated portions of his “basement-bed” of the London clay, where this was assigned any considerable thickness, together with a small portion of his Woolwieh and Reading beds, as “Oldhaven Beds,” and thus almost restricted the “basement-bed” to the inconsiderable thickness of coarser material which nearly everywhere forms the base of the London Clay. Almost the only criticism that can be urged against Prestwich's classification is that he places the “basement-bed” of the London Clay in a different group of the Eocenes from the London Clay itself, his nomenclature implying a closer relationship than he admits. But the Survey, on the other hand, have unfortunately adopted a name ( “Oldhaven ”) which not only does not exist on the maps, but is scarcely known at the locality, an inquiry for “Oldhaven Gap,” where the beds are typically developed, being useless even at the gap