161 



23. — The Eocenes of England and theie Extension in 

 Heetfoedshiee. 



By J. Logan Lobley, F.G.S., F.E.G.S. 



[A Lecture delivered 14th December, 187G.] 



When I previously had the honour of addressing the members 

 of the Watford Natural History Society, a brief description of the 

 Cretaceous group of British rocks was attempted. This series of 

 strata includes the Chalk — that great limestone formation which 

 forms as it were the foundation-stone of Hertfordshire. Above 

 the Chalk lie superimposed a series of beds, some of which only, 

 however, are to be found in this county, which have been de- 

 posited since the Cretaceous epoch ; and it is the lower portion of 

 this series, that lying immediately on the Chalk, to which I wish 

 to ask your attention this evening. 



As you are aware, the entii'e series of sedimentary rocks is 

 divided into three great divisions : — the Primary or Palaeozoic, the 

 Secondary or Mesozoic, and the Tertiary or Cainozoic. The Chalk 

 is the highest of the British Secondary rocks. The beds, therefore, 

 lying upon the Chalk in regular sequence, form the lowermost 

 division of the Tertiary. To this division the name Eocene- has 

 been given, the precise meaning of which name may be briefly 

 explained. From investigations made by M. Deshayes upwards of 

 forty years ago, it appeared possible to Sir Charles Lyell to name 

 the three divisions of the Tertiaiy deposits in accordance with 

 the proportion of recent species found among the fossil shells of 

 each. To the lower beds, in which a small proportion of recent 

 shells were found, the name Eocene, from eos, dawn, and cainos, 

 recent, was accordingly given. The middle beds, in which less 

 than half of the fossil shells were recent, were named Miocene, 

 from melon, less ; and the upper beds, containing an assemblage of 

 shells of which considerably more than half were recent species, 

 were called the Pliocene, from pleion, more. 



The Eocene, or the dawn of the recent, is, therefore, the division 

 which includes the beds formed during that period of the earth's 

 history when the fauna of the present day began to make its 

 appearance, and though our higher foims of life were then un- 

 represented, yet many genera and a few species of our existing 

 MoUusca were at that period inhabitants of the waters, if not of 

 the lands of the globe. 



The Eocene beds of England are found occupying two areas 

 distinctly separated by a large and elevated district. One of 

 these areas is triangular in shape, and stretches from Hunger- 

 ford in Berkshire to the Geiman Ocean, along the course of the 

 Thames, but chiefly on its northern side ; and to this extension 

 of Lower Tertiary beds the name London Basin has been 

 given. The second area, including as it dees a large portion of 



VOL. I.— PT, VII. 13 



