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DR. G. J. HINDE ON BEDS OF SPONGE-REMAINS IN THE 
The siliceo-calcareous rock of which the sponge-beds are largely composed is of a 
whitish appearance, and consists of silica, both amorphous and as chalcedony, calcite, 
a small quantity of mica, and grains of glauconite and quartz. It is filled with 
spicules of relatively large size ; they are now of a porcelanic white by reflected light, 
and transparent in Canada balsam by transmitted light; they are usually perfect, and 
their canals are well-preserved. These spicules are of amorphous silica and are 
entirely negative between crossed nicols, though their canals are usually infilled 
with clialcedonic silica or with glauconite, and the transparent siliceous matrix in 
which they are imbedded is also chalcedonic. The spicules are in such profusion in 
some of the beds that they form distinctly-marked thin white parallel layers with 
intermediate layers, which, from the larger proportion of glauconite grains in them, 
are of a greenish tint. These parallel layers must have been thus arranged at the 
tine the beds were forming, and they indicate the disintegration of the sponges and 
the mingling of their spicules before they were buried under fresh deposits. The 
perfect condition of the spicules likewise indicates that they could not have been 
drifted to any extent over the sea-bottom (Plate 40, fig. 1). 
The white siliceous rock filled with spicules occasionally passes into a brown chert; 
in this rock the spicules are to a large extent dissolved away, and only their infilled 
canals and shadowy casts remain. It is evident that the cherty portions were 
originally as filled with perfect spicules as the siliceous rock into which they gradually 
pass, and the difference results from the solution of the amorphous silica of the walls 
of the spicules, and its re-deposition in the chalcedonic condition to form the chert. 
In some of the sponge-beds there are cavities or pockets in the chert filled with 
a loose, powdery, unconsolidated material, of the same character as that already 
mentioned in the chert at Yentnor. The spicules in this material are, however, for 
the most part, in a perfect condition, and can be washed perfectly free from the 
matrix; but others have undergone a similar alteration to that of the Yentnor 
spicules, and they have been partially replaced by a nearly transparent mineral, and 
the silica is in the globular form (Plate 45, figs. 15, 16). Much of the silica in the 
powder of these cavities is in the form of minute globules or discs, and the silica 
is amorphous (Plate 45, figs. 19-19e). 
Penzlewood, Somerset .—On the plateau formed by the summit beds of the upper 
greensand at this place, and at the adjoining villages of Stourton and Zeals, in 
Wiltshire, there are exposed beds of chert and siliceous rock, which are estimated 
by Mr. Horace B. Woodward* to be from 20 to 30 feet in thickness. The beds 
are largely worked for road material, and they have been formerly extensively 
employed for building and also for the manufacture of whetstones t for sharpening 
* “Geology of East Somerset.” ‘Memoirs of the Geological Survey,’ p. 138. 
t My attention was first directed to these beds, by my friend Mr. Alebed Gillett, of Street, who 
showed me one of the whetstones made at this place. A fractured surface at ouce showed that the 
hard, porous siliceous rock, of which it was made, was a portion of a sponge-bed, since it was filled 
throughout with the empty casts of siliceous spicules. 
