A HISTORY OF HERTFORDSHIRE Europe, being a very decided physical as well as pateontological one. In both Cretaceous and Eocene strata the most abundant fossils are Mollusca, but the Protozoa, Rhizopoda, Crustacea, and Polyzoa which abounded in the Cretaceous seas were very sparsely represented in Eocene times ; on the other hand there were but few Cretaceous plants and no Cretaceous mammals, while plants, and especially Dicotyledons, are fairly well represented in Eocene strata, and the Tertiary era has been termed the Age of Mammals. The break however is not so much in the classes of plants or animals represented as in the fact that not a single species passes from Secondary to Tertiary rocks, indicating an enormous lapse of time, with perhaps a complete change of conditions. Physically the difference in the strata consists in the fact that hard, distinctly-bedded rocks, and especially those of a calcareous nature, cease with but few exceptions at the close of the Secondary period, giving place in the Tertiary to clays, sands, and gravels. This great break is very well marked in Hertfordshire, for we have neither the highest beds of the Chalk nor the lowest of the Eocenes. There are higher beds of the Chalk, though not the highest known, and lower Eocene beds, south of London than we have here. As the Cretaceous sea must have been continuous north and south of London, the inference is that our Chalk must have suffered a greater amount of denudation than that of Surrey, Sussex, Hants, and Kent. That much erosive action has taken place is proved by the great irregularity of the surface of the Chalk in this county and by the enormous quantity of flints and therefore great thickness of strata which must have been re- moved to form the sands and pebble-beds of the Reading Series. The Woolwich and Reading Beds are of two types : in the one, best represented in the Woolwich district, loamy beds with many fossils prevail ; in the other, or Reading type, the beds are more pebbly and sandy, with but few fossils, and it is a significant fact that a greater denudation of the Chalk has taken place where the Reading type is present, as in Hertford- shire, than where the beds are of the Woolwich type, as in Kent. In our area the term ' Woolwich ' is dropped because we have no beds of that type. Although such a long interval elapsed of which we have no record, it does not appear that any earth-movements except subsidence then took place within our area, the eroded surface of the Chalk, although uneven, having been approximately horizontal when the earliest Eocene beds were deposited upon it. When the Chalk is covered by a bed of sand through which water can percolate, there is on its surface a layer of unworn green-coated flints usually considered to form the base of the Thanet Sands, but it should rather be regarded as a reconstructed Cretaceous bed, for the formation is not due to the deposition of sedi- ment, the layer of flints being merely the insoluble residue of the Chalk, and its formation being a process probably continuous from or even before the upheaval of the Chalk to the present time. Why this layer seems to form the base of the Thanet Sands is due to the nature rather 12
