XVI INTRODUCTION. 



Over these foundations of the present south-eastern 

 part of our Island the ocean continued to roll, but 

 under influences of heat and light favourable to the de- 

 velopment of corals and microscopic shells, during a period 

 of time which has permitted the successive accumula- 

 tion of layers of these skeletons, in a more or less de- 

 composed state, with probable additions from submarine 

 calcareous and siliceous springs, to the height of one 

 thousand feet. But although amongst the remains of 

 higher organized animals that have become enveloped 

 in the cretaceous deposits, there have been recognised 

 Birds, Pterodactyles, and a land Lizard, probably washed 

 down from some neighbouring shore, no trace of a Mam- 

 malian quadruped has yet been discovered in them. 



The surface of the chalk, after it had become con- 

 solidated, was long exposed to the eroding action of waves 

 and currents. Into deep indentations so formed have 

 been rolled fragments of chalk and flint, with much sand. 

 The perforations of marine animals on that surface have 

 been filled with fine sand ; and there are many other 

 proofs of the lapse of a long interval of time between the 

 completion of the chalk deposits of Britain and the com- 

 mencement of the next or tertiary era. Of this era our 

 present Island gives the first indication in traces of mighty 

 rivers, which defiled the fair surface of the rising chalk by 

 pouring over it the debris of the great continent which they 

 drained, a continent which has again sunk, and probably 

 now lies beneath the Atlantic. 



The masses of clay and sand that have been thus 

 deposited upon the chalk are accumulated chiefly in two 

 tracts, called the London and Hampshire Basins, which 

 seem to have been two estuaries or mouths of the great river : 

 the one extends from Cambridgeshire through Hertfordshire 



