LOWER AND UPPER GREENSAND OP THE SOUTH OF ENGLAND. 
429 
pressure, but in the harder nodular portions they are inclosed by transparent chalce- 
donic silica, and seem to be gradually passing into the crypto-crystalline condition. 
They also appear at times to be invested with a surface layer of minute crystals of 
chalcedony. 
In some cases the silica seems to have assumed the globular form within the sponge- 
spicules, but, as a general rule, the silica of the spicules must have been dissolved and 
then re-deposited as minute globules. I am not prepared to explain the causes which 
have produced this singular method of deposition ; the chemical and physical forces 
which have brought it about have not been affected by extraordinary influences of 
heat or pressure, since the rocks are normal sedimentary deposits, and show no traces 
of alteration from these causes. There is no apparent reason why, under similar con¬ 
ditions of deposition and fossilisation, the sponge-remains of the siliceous and siliceo- 
calcareous deposits of the malm and firestone should have retained the original colloid 
condition of the silica, and the silica of the spicules when dissolved, should have 
assumed the globular form without passing from its hydrated condition, whilst in 
similar remains in most of the other greensand sponge-beds, the silica of the spicules 
has become altered to the more stable condition of chalcedony, and even to crystalline 
quartz, and, when the spicules have been dissolved, the silica has been deposited in 
layers and nodules of chalcedonic chert. The change of the originally colloid silica 
of the sponge-remains into the crypto-crystalline and crystalline condition is, indeed, 
that which nominally takes place in the course of fossilisation, and has been noticed 
in connexion with fossil sponges from silurian to upper cretaceous strata, and it is 
only under the particular circumstances, of which we are at present ignorant, that the 
colloidal state of the silica has been retained. 
Sedimentary deposits consisting in part of colloidal silica in a globular form seem to 
be of rare occurrence ; at all events, I cannot find any # previous notice of this 
character. But colloidal silica in a globular form has been recorded in rocks of 
volcanic origin. Thus Vogelsang! describes and figures, from a quartz-trachyte from 
near Schemnitz, and also from Borsva, spherulitic bodies, the so-called Globuliten, of 
isotropic silica, and of approximately similar dimensions to the greensand globules. 
Michel Levy| has also discovered silica in a globular form and isotropic condition in 
eurites from Les Settons. It is worthy of notice, also, that there is a remarkable 
similarity, both of form and size, between the colloid siliceous globules of the greensand 
and the calcareous bodies artificially produced by the reaction of ammonia carbonate 
* Professor Zittel mentions the fact that after treating with acid the now calcified, or partially 
calcified, siliceous sponges from jurassio strata the residue contains numerous rounded, rough, deeply 
indented siliceous discs. “ Studien fiber fossile Spongien,” I. Hexactinellidae. ‘Abhandl. der konigl. bay. 
Akad. der Wiss.,’ II. Cl., XIII. Bd., I. Ab., p. 14. I have not seen these bodies, but I should judge from 
the description of them that they are much larger, and that they further differ from those under con¬ 
sideration in being either crystalline or crypto-crystalline. 
t Die Krystalliten,’ pp. 139, 142, taf. xv., figs. 1, 2. 
f Bulletin de la Societe Geologique de France,’ 3 me ser., t. v., 1877, p. 140. 
MDCCCLXXXV. 3 K 
