CHAP. x. | CONTINUITY OF THE CHALK. 48] 
of land and in the path of slight currents, whilst the 
extreme purity of the white chalk of Sussex would 
seem to indicate that it had been laid down in deep 
still water far from land. A considerable proportion 
of the silica of the chalk-mud, however, consists of 
the spicules of sponges, of the spicules and shields 
of radiolarians, and of the frustules of diatoms; 
and this organic silica is uniformly distributed 
through the whole mass. Taken in connection with 
the absence of diffused silica in the white chalk, 
we have the singular fact of the presence of regular 
layers of flinty masses of nearly pure silica, pre- 
senting frequently the external form of more or 
less regularly-shaped sponges, and frequently filling 
up the cavities of sea-urchins or bivalve shells. If 
we take the simple instance of pure grey flint filling 
up entirely the cavity of an urchin, such as Ga/le- 
rites albo-galerus, or Ananchytes ovatus, and. showing 
at the oral opening of the shell a little projecting 
knob, lke a bullet-mould filled with lead, we have 
no escape from the conclusion that after the death 
of the urchin the silica has percolated into the shell 
in solution or in a gelatinous condition, and the silica 
must have previously existed in some other form, 
either in the chalk or elsewhere. In the chalk which 
contains not a trace of silica we often find the moulds 
and outlines of organisms which we know to have been 
silicious, from which the whole of the silica has been 
removed; and I have more than once seen cases in 
which a portion of the delicate tracery of a silicious 
sponge has been preserved entire in a flint, while the 
remainder of the vase which projected beyond the 
outline of the flint appeared in the chalk as a trellis- 
se 
