has remained stationary at one level above the sea. The valley of the lower Thames was probably excavated in the Pleiocene age, and is proved by the large sheets of boulder clay and the marine shingle in Essex and Hertfordshire to have been submerged after the end of the early Pleistocene age. It was re-elevated while the late Pleistocene deposits were being formed in the area between Oxford and the mouth of the Thames. Here, then, as in the case of the submarine forest-bed of Norfolk, we cannot consider that the age is settled by the level. The lower brick-earths seem to me to be isolated patches of a series of fluviatile deposits, of which the higher and more exposed portions have been destroyed by the rain, rivers, snow, ice, and sea, and other agents ever at work in re-modelling the surface of the earth.
Mid Pleistocene Caverns.
It is a very singular and striking fact, that although caverns must have existed in all ages of the earth's history, and have been used for shelter by the animals, there are none older than the mid Pleistocene times. There is every reason to believe that they were haunted by the Eocene, Meiocene, and Pleiocene beasts of prey, and that the anoplotheres and palæotheres, the deino- theres and mastodons, the deer and the antelopes, were either dragged in by the carnivores, or swept in by the flow of water, after the same manner as the remains of the successive groups of animals have been introduced which have inhabited Europe from the Pleistocene age down to the present day. Why then do we not meet with ossiferous caverns of those times? The simple answer is to be found in the realisation of the enormous
