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 A HISTORY OF SURREY shower of calcareous organic particles descended upon the sea floor ; and the duration of this shower was so long that the great Chalk forma- tion was built up by it. Then we found a wide gap in the records; and when we crossed this gap to the Eocene strata, we learnt once more of a shallow sea, inhabited by animals all different from those of the older time, and this sea had already made great inroads upon the consolidated sediment of the preceding period ; the climate was warm, perhaps sub-tropical, and not far off there was a land clothed with rich vegetation ; and as the sea grew shallower the estuary of a large river invaded our tract, but was soon driven back by a renewed sinking of the land. Then, as the Eocene Period drew to its close, we observed how re-elevation set in, with the renewal of shallow-water conditions ; and with this stage the building up of our county was concluded. Regarding subsequent events our evidence has been scanty, but we have been able to gather that the strata were disturbed and uplifted into dry land ; and that owing to inequalities of the uplift this land sloped to the north, so that the rivers which flowed from it took a northerly course, which they have since maintained. And we have traced the work of these rivers and their tributaries through a period of gradually increasing cold, until conditions of arctic severity ruled in the land ; and finally through a period of gradual amelioration, which has continued up to time recent ; and meanwhile great piles of strata have melted away, and the remnants have been carved into hill and dale under the persistent sapping of the agencies of erosion. 28