23 
Chalk Cliffs near Dover. 
consisted of lime, in case any should be found. The two first 
consisted wholly of siliceous matter; the last of 86 per cent, 
of silica and 14 of carbonate of lime. All the fragments were of a 
granular texture, and sufficiently hard to cut glass ; each also left a 
whitish, streak on the finger when rubbed with considerable pressure 
upon it ; the last in the greatest degree. 
Large fragments of striped flint of a grey colour are often dis- 
coverable among those 'which have been taken from the inferior 
beds in the, upper chalk, but they frequently contain a nucleus of 
black flint, from which the grey stripes diverge as from a common 
centre. 
Such flints as are interspersed in the chalk of the stratum with 
numerous flints, have usually some organic appearance. They are 
occasionally found in pear-shaped masses resembling the head of 
the alcyonium ; sometimes in the form of nearly perfect spheres, 
which are solid, and do not commonly exceed half an inch in dia- 
meter, but are often much less. Others of no particular external 
form, have internal cellular or ramifying cavities which seem to 
indicate the, same origin. Others again seem so decidedly to have 
been formed around sponges, that the flint has entered all the ramifi- 
cations, the forms of which therefore remain. 
It is not uncommon to find flints inclosing many of the shells 
observable in the chalk, and impressions of the few varieties of 
echinus common to that of Dover, the shells having been replaced 
by carbonate of lime, or the space they once occupied being left 
vacant; so that the internal cast of the shell, which is of flint, is 
in some cases connected with the surrounding mass by fine filaments 
of siliceous matter, arranged precisely in the order of the small 
perforations commonly visible in the shell, which therefore must 
have been formed while the shell was yet entire. Shells enclosed 
