CHAPTER XVII. 



EOCENE PERIOD. 



AFTER a long interval of time the great chalk deposit, partly 

 raised from the sea-bed, so as with the older oolites to shape out 

 in some degree the boundaries of a broad sea-loch, where now flows 

 the Thames, was covered by sediments of the cainozoic period, to 

 which Lyell gave the title of Eocene strata. Before this happened 

 the surface of the chalk was wasted, in the district near Reading 

 literally ground down in some places to a plane or undulated sur- 

 face, as it is this day on some parts of the Yorkshire coast ; and 

 from this worn surface to the depth of a foot, in various directions 

 the rock was bored, drilled we may say, by 'lithodomous' bivalves, 

 much as happens to the chalk below the cliffs of Flamborough. 

 The holes are filled by the dark eocene sands which were the 

 earliest deposits to follow. 



This curious phenomenon may be very well studied at Theale, 

 near Reading, where a small chalk pit occasionally allows the 

 upper layer to project and shew its under surface, so that 

 the holes may be examined both in the vertical and horizontal 

 aspects. 



For considerable spaces round Readme: the chalk is in this 



Jt O 



manner covered uniformly by sands and clays of different tints and 

 thicknesses ; in some cases they are not only undulated, but sink 

 into hollows, pits, or pipes in the chalk. These pits and hollows 

 are of subsequent date ; they are mostly the effect of atmospheric 

 water, which, by dissolving the chalk in particular lines more than 

 elsewhere and carrying it downwards, has caused a gradual sinking 

 of small spaces, in which the eocene beds form curves more or less 

 conformed to the walls of the cavity in the chalk. This is a 

 common phenomenon in the regions of the English chalk. 



