430 
DR. G. J. HIXDE OX BEDS OF SPONGE-REMAINS IX THE 
and calcium chloride, which have been figured by Vogelsang. # Similar calcareous 
globules or discs of natural origin have been discovered by BarroisI filling the cavities 
of shells and corals in the devonian limestones of Spain, which, though inorganic, he 
believes to be of the same nature as fossil and recent coccoliths. j 
There is at first sight a great similarity between the colloidal globules from the 
greensand and recent coccoliths from the Atlantic deep-sea mud, but the globules are 
round and not elliptical in figure, they do not occur in the sleeve-link form, and they 
coalesce together in a manner which has never been observed in coccoliths. There is 
therefore no ground for the supposition that they are silicified coccoliths. For the 
sake of comparison, I have represented on Plate 45, fig. 20, a coccolith of average 
size from the Atlantic ooze on the same scale as the globules. 
The sponge-remains which are now of chalcedonic and crystalline silica have a 
very different aspect to those in which the silica is colloidal. By reflected light they 
have somewhat the appearance of ground glass, and when mounted in Canada balsam 
they become nearly invisible ; in glycerine, however, their forms are very distinctly 
shown. The surface of the spicules in this condition is usually rough and eroded, so 
that it sometimes has a reticulated appearance ; at other times it seems covered with 
minute circular spots of about ‘006 mm. in diameter. These spots sometimes appear 
to be minute excavations in the surface of the spicule, in other cases they look like 
minute mammiform or botryoidal elevations. This surface character is very con¬ 
spicuous in the spicules from Haldon and Blackdown, and has already been noticed 
by Mr. Carter. § The surface of spicules which have been changed to crystalline 
silica is less eroded than the exterior of those in which the silica is in the condition 
of chalcedony. These crystalline spicules frequently exhibit a finely radiate structure 
(Plate 45, fig. 17). 
The canals of spicules in which the silica is either chalcedonic or crystalline are less 
regularly preserved than those of spicides of colloidal silica. In the majority of 
spicules no canal at all is visible, but this probably arises from the fact that the canal 
having been infilled with chalcedonic silica of the same character as that of the walls, 
cannot, either by ordinary or by polarised light, be distinguished therefrom. In other 
instances the canals are infilled with glauconite or with a light-brown or opaque 
mineral, not improbably iron peroxide. Not infrequently the solidified canal is dis¬ 
continuous and broken up, and no longer occupies the normal position in the axis of 
the spicule, but is deflected to near its outer surface (Plate 42, figs. 36, 6a). In many 
instances, even in these spicules of chalcedonic silica, the silica which has infilled the 
* ‘Die Krystalliten,’ p. 87, taf. si., fig. 1. 
t ‘ Recherches sur les Terrains Anciens des Asturies et de la Galice,’ p. 45, pi. si., fig. 4. 
J Dr. Gumbel also regards tliese minute, round, calcareous bodies, which he has discovered in lime¬ 
stones and marls of various geological epochs, as coccoliths, but their identity with the genuine coccoliths 
of the Atlantic, described by Huxley and by Wallich, is open to doubt. 
§ “Fossil Sponge-spicules of the Greensand compared with those of existing Species.” Annals and 
Mag. Xat. Hist., s. 4, vol. vii., p. 114. 
