— 14 — 
of Chalcedonic silica is very irregular, the one form apparently 
gradually merges into the other. It would seem that the 
silica in these examples, to a certain extent retained the cha- 
racters of the originally colloidal silica of the spicules, whilst 
in the majority of the spicules in this material the metamor- 
phosis of the silica had been extended further, until, with 
the exception of a few minute spots, the whole of the spicule 
had become changed into chalcedony. I may remark here, 
that so distinctly are the sponge spicules in this deposit shown 
by polarized light, that, in a small quantity of the flint-meal 
mounted in balsam, the minutest particle of spicule at once 
reveals itself by the prismatic tints which it exhibits, and I 
availed myself of this fact to determine the characters of some 
delicate lamina, which otherwise had the appearance of small 
portions of shell. 
Not only has chalcedonic silica thus replaced the original 
colloidal silica in these spicules, but it has also, in the major- 
ity of instances, completely filled their interior canals, so that 
the spicules now appear solid throughout. Here and there 
however, spicules are met with, in which the canals are either 
wholly or in part preserved, but even in these instances they 
do not present the form of regular even tubes as in recent 
sponge spicules, but are abnormally and irregularly enlarged. 
Sometimes the canal yet extends throughout the length of the 
spicule, in other instances only a small part in the upper and 
thicker part of the spicule is to be seen. The canal, where 
retained , appears to be hollow, or infiltrated more or 
less with ochreous particles, and an example of this latter 
in which the canal is shown as a thin black tube extending 
along the central axis of the spicule is given on Plate V. 
fig. 30. 
The present condition of the sponge spicules in the interior 
of this Horstead flint, thus shows a great alteration from their 
original condition. The smooth surfaces have become rough 
and eroded, the delicate spines have been dissolved away, and 
