MINERAL CHARACTERS. 57 
Siliceous Skeleton dissolved, leaving empty casts.—The spicular skeleton of many 
fossil Sponges, after being enclosed in the rock, has been dissolved, leaving hollow 
impressions of its minute structure in the matrix. The outer form in these 
Sponges is usually retained by the matrix, which may be either of chalcedonic or 
crystalline silica, limestone, or chalk. The impressions of the minute spicular 
structures are usually most distinct when the matrix is of silica, which now forms 
a solid mass, in which the minute hollow moulds of the skeleton are contained ; but 
even when the matrix is of such a soft material as the Upper Chalk of the South 
of England, the hollow casts of the spicular skeleton are retained very perfectly; 
and even such minute structural details as the hollow lantern-nodes of the hexac- 
tinellid spicules are clearly shown. Not only entire Sponges occur as empty casts, 
but even the moulds of detached spicules are also present in the bands of chert and 
siliceous rock of the Upper Greensand, and frequently to such an extent that the 
rock is porous and light on account of the numerous minute cavities in it formed 
by the dissolution of the spicules. In some of these minute fusiform hollows there 
is an axial filiform siliceous rod, which is the solid cast of the spicular canal, 
remaining after the walls of the spicule itself have been dissolved away. 
Another modification of the process by which the cast of the skeletal structures 
is retained after the solution of the skeleton itself, is shown in some hexactinellid 
Sponges from the Upper Greensand, in which the spicular mesh is invested with a 
delicate pellicle of silica, and then subsequently dissolved, leaving, as it were, an 
outer shell of its form. A similar alteration has also been recorded by Manzoni' in 
hexactinellid Sponges from the Miocene strata of Bologna, but in these the spicular 
mesh is in part retained after its investment by the siliceous pellicle. 
Examples of the removal, by solution, of the spicular skeleton of siliceous Sponges 
occur in almost every geological horizon in which Sponges are present. Thus in 
the Silurian genera Astylospongia and Hindia the skeleton in many of the specimens 
forms negative casts in a siliceous matrix; in many of the hexactinellid Sponges 
from the Upper Chalk of the South of England the hollow moulds of the skeleton 
are preserved in the soft chalky matrix, and a similar dissolution of the skeleton 
has also taken place in the siliceous Tertiary Sponges of Bologna. 
These undoubted instances of the solution and removal of the silica of Sponge 
skeletons completely negative the idea, held to within a comparatively recent date, 
that silica is sufficiently stable to resist the ordinary influences of fossilization. It 
may be accepted as proved that silica in the colloid state, in which it occurs in the 
skeleton of recent siliceous Sponges, and also in the original condition of fossil 
Sponges, is extremely lable to chemical changes, and that it is only when in the 
condition of chalcedony or crystalline that it can be regarded as stable. The 
changes in the siliceous skeletons of fossil Sponges, mentioned above, show the 
1 © Spugne silicee del Miocene medio,’ p. 17, pl. 3, figs. 15, 16, 1882. 
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