XV1 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 marme 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 
