DR. G. J. HINDE ON BEDS OF SPONGE-REMAINS IN THE 
426 
into consideration the mineral characters of the deposits in which the sponge-remains 
are imbedded, which in many cases are derived from the spicular remains themselves. 
The detached spicules of the sponge-beds are now in the following mineral con¬ 
ditions :— 
I. Silica; either in the amorphous or colloid state; or crypto-crystalline like 
chalcedony; or crystalline. 
II. Glauconite and other silicates. 
III. Crystalline calcite. 
The beds or matrix inclosing the spicules, in addition to such mechanically 
derived constituents as quartz-sand, mica, and grains of glauconite, also 
contain silica in the same forms as in the spicules, and calcite, either granular 
or crystalline. The calcite is probably of organic origin, and derived from the 
remains of foraminifera and other organisms. 
IV. The spicules also occur in the negative form, in which the silica has been 
entirely removed, and the casts are either empty, or merely contain the solidi¬ 
fied canal of the spicule. 
A.s the mineral characters of the spicular remains, and of the beds in which 
they are inclosed are, to a great extent, interdependent, it is more convenient 
to consider them in connection wdth each other. 
1. Spicules and matrix of colloid silica .—The spicules which yet retain the silica in 
the amorphous condition are, as a rule, more perfectly preserved than those in which 
the silica is partially crystalline ; their surfaces are smooth and even, and their canals 
are distinctly shown. They are of a milky-white or opal tint by reflected light, and, 
when mounted in Canada balsam are nearly transparent. Between crossed nicols, 
the spicular walls are entirely neutral, though the canals, which are frequently infilled 
with chalcedonic silica, are doubly refracting. Viewed by transmitted light the 
spicular walls can be seen to be traversed in all directions by very minute curved lines 
(Plate 40, fig. 8), which, under high powers, are resolved into incomplete elliptical rings, 
with smooth even rims (Plate 40, fig. 9). Besides this tendency to circular aggregation, 
the silica of the spicules frequently exists as very minute granules of irregular forms, 
showing by transmitted light a decided pinkish tint. In no instance does the spicular 
canal exist as a hollow tube, but it has been filled either with silica, glauconite, or 
other silicate, or, rarely, with a ferrous compound. The infilling material is generally of 
a more resistant nature than the walls of the spicules, so that the walls ai’e frequently 
entirely dissolved, whilst the materials which have infilled the canal remain intact and 
retain the form of the spicule, and might readily be mistaken for the entire spicule 
(Plate 45, figs. 14, 14«). 
By treating the spicules of colloid silica with heated caustic potash the greater 
portion of their substance is dissolved, but their infilled canals remain, and not 
infrequently the form of the spicule is retained by a greenish-white mineral, which, 
