LOWER AND UPPER GREENSAND OF THE SOUTH OF ENGLAND 433 
living sponge. Professor T. Rupert Jones * * * § maintains the view that the silica of 
chert generally is derived directly from sea-water, and similar opinions as to the origin 
of the chert bands in the upper carboniferous limestones of Ireland have been put 
forward by Messrs. Hull and Hardman t and by M. Renard| with respect to the 
phthanites in rocks of the same age in Belgium. 
It is a significant fact, however, in connexion with the chert-beds of the Irish 
upper carboniferous strata that some have been discovered filled with sponge-spicules 
like the chert of the English greensand, and this indicates a similar origin for the 
silica, and negatives the supposition of Professor Hull§ that it was deposited “ from 
warm shallow water charged with silica in solution, in which chemical reactions would 
be at once set up, favoured and promoted by tidal and other currents.” 
III.— The Nature of the Sponges present in the Sponge-Beds. 
As the skeletons of entire sponges, or even fragments of them, are nearly wholly 
absent in the sponge-beds, a determination of the character of the sponges of which 
they are formed must be based upon a study of the detached microscopic spicules 
which are indiscriminately mingled in the deposits. In the majority of cases these 
spicules are combined with the mechanically-derived materials of the beds, and with 
the silica produced from their own dissolution, into hard rock-masses, in which the 
form and proportion of the individual spicules are but very indistinctly and imper¬ 
fectly shown on fractured or weathered surfaces or in thin microscopic sections. 
Under these conditions the knowledge to be obtained of the character of the spicules 
is very limited. In a few cases, however, the microscopic spicules occur loosely 
mingled together in incoherent beds of sand, or filling pockets or cavities in beds of 
chert, and it is then possible to obtain them perfectly free from the matrix, and to 
examine their characters under the microscope, in precisely the same manner as those 
of recent siliceous sponges. In one respect, indeed, these detached spicules some¬ 
times present greater advantages for studying their individual forms than those of 
recent sponges, since, through gentle natural influences, they have become separated 
* “ Quartz and other Forms of Silica.” ‘ Proceedings of the Geologists Association,’ vol. iv., p. 447. 
t “ On the Nature and Origin of the Beds of Chert in the upper Carboniferous Limestone of Ireland.” 
By Professor E. Hull, and “On the chemical Composition of Chert and the Chemistry of the Process 
by which it is formed.” By E. T. Hardman. ‘ Scientific Transactions of the Royal Dublin Society,’ 
vol. i., new series, 1878, p. 71. 
+ “ Recherches lithologiques sur les Phthanites du Calcaire carbonifere de Belgique.” 1 Bulletin de 
l’Academie Royale de Belgique,’ 2 me s., t. xlvi., p. 471. The author expressly states that there is nothing 
to show that the infiltrated silica of these deposits has been derived from the decomposition of sponge- 
spicules or of diatoms. There are shown, however, in one of the figures (fig. 2) accompanying the 
paper, circular sections which more nearly resemble those of sponge-spicules tban of crinoid-stems, to 
which they are assigned. 
§ Op. cit., p. 83. 
