60 BRITISH FOSSIL SPONGES. 
Siliceous Sponges in Flint and Chert—Mention has already been made of the 
fact that some of these Sponges occur as solid cores, enclosed either partially or 
completely in a casing of flint, and also that in some the structures in flint are 
replaced by iron peroxide. 
In other instances the outer form of the Sponge is shown by the flint, but all 
traces both of the canal and spicular structures have disappeared entirely, and the 
interior of the Sponge is a compact mass of homogeneous flint. Very frequently, 
however, whilst the spicular structure of the Sponge has been completely obliterated 
in the interior of the flint, the canal-system is very clearly and definitely shown in 
a solid form, owing to the canals having been filled with flint of a different tint to 
that of the matrix. Such Sponges are common in the Chalk of the vicinity of 
Brighton, and they are generally known under the term Choanites, though most of 
them really belong to the genus Siphonia. In many of the Silurian Sponges from the 
Baltic basin and from Canada also, which are now compact masses of cherty silica, 
the minute spicular structure has been destroyed, whilst the canals are shown in 
the same manner as in those in the Upper-Chalk flints. In some cases also only 
the lower portion of a Sponge is enclosed in flint, whilst its upper portion has been 
changed into iron peroxide, and is enclosed in a matrix of soft chalk. The flint- 
enclosed basal portion of these Sponges very frequently occurs in gravels formed 
from chalk flints. 
Not only are entire Sponges thus enclosed in various ways in flints, but also 
detached spicules are preserved in great numbers in the cavities of Upper Chalk 
flints, and in irregular hollows in chert, from the Carboniferous and Upper Green- 
sand strata. They are usually loosely mingled in a fine siliceous powder with other 
organic remains. In some instances the silica of these spicules is amorphous, but 
more frequently it is either chalcedonic or crystalline. Detached spicules also 
occur in arenaceous strata, as in the Lower Greensand of Haslemere, Surrey, and 
in the Upper Greensand of Devonshire and in the Hocene Tertiary near Brussels. In 
some beds they occur in what may be regarded as an unaltered condition, that is, 
loosely intermingled in the sandy strata; in others some of the spicules appear to 
have been dissolved, and the sandy matrix and the spicules are partially cemented 
together by silica to form pumice-like, porous accretionary masses of cherty rock ; 
whilst in a further stage the spicules are for the most part dissolved, and beds of 
massive chert result, in which only hollow casts and shadowy outlines of the 
original spicules are occasionally visible. 
Though the larger skeletal spicules of siliceous Sponges are present in great 
numbers, and in a fairly perfect state of preservation, the smaller or so-called 
flesh-spicules are very seldom met with in the fossil state. A few of them have 
been figured by Zittel from the Senonian strata of Westphalia, and they also occur 
in Tertiary deposits, mingled with diatoms and radiolarians, from Hungary and 
