FKOM THE IXFEEIOK OOLITE. 



543 



nellid structure, and so resembles the minute secondary rete which 

 so commonly proceeds from the main fibres of many Dictyonine 

 Hexactinellids. Its occurrence, however, in places where a sponge- 

 skeleton certainly was never present, the wide variation and ex- 

 cessive minuteness of the size of the meshes, and their frequently 

 quite irregular form, all show that we have here to do with a purely 

 inorganic structure • and its explanation is probably to be sought 

 in the segregation of the fine granules of the original calcareous 

 mud about innumerable centres, by which pathways freer from 

 sediment were left, forming a network to be subsequently filled in 

 with calcite. In some places small nodules of white opaque porous 

 silica occur in the matrix. The porosity of this silica is so great 

 that on adding balsam it becomes almost instantly transparent ; one 

 can then discern in it a few hollow spaces, lined with a thin film of 

 red oxide of iron, representing casts of the Dictyonine network of 

 the sponge ; while more abundantly present are crystals of colour- 

 less calcite, similar in character to those which project into it on all 

 sides from the surrounding matrix. One may hence infer that the 

 siliceous nodules are of later growth than the calcitic infilling of the 

 sponge. In connexion with the occurrence of silica it is interesting 

 to note that, corresponding with the replacement of the originally 

 siliceous sponges of Dundry Hill by calcite, we find the opposite 

 replacement to have taken place in the corals of the same deposit, 

 which now sometimes exist as siliceous pseudomorphs : silica is also 

 not uncommon as an infilling material of the associated Porami- 

 nifera. 



The smaller canals of the sponge, both excurrent and incurrent, 

 are generally traversed axially by a thin thread of calcite, the rest 

 of the canal being filled up with consolidated calcareous mud *. In 

 describing the Siphonice of the Blackdown Greensand I mentioned 

 a similar thread (Q,. J. G. S. vol. xxxiii. p. 814), but in this case con- 

 sisting of silica, and separated by an empty space from the adjoining 

 skeleton. It was conjectured that this thread had been produced 

 from colloidal silica which originally filled the whole of the canal 

 from side to side, but subsequently, on undergoing solidification, 

 shrank to its present dimensions. Since, however, in other sponges, 

 we now find precisely similar threads, but consisting of calcite, 

 which, unlike silica, is not known to originate from a colloidal 

 state, recourse must be had to a different mode of explanation, and 

 the following may be suggested. In some sponges — e. g. Thenea 

 Wallichii — the walls of the canals in the living state are formed by 

 a considerable thickness of gelatinous connective tissue, from which 

 the body-spicules are absent, these occurring immediately about the 

 exterior of this gelatinous tissue. If such a sponge should, soon 

 after death, become covered up by fine calcareous mud, it is quite 

 conceivable that the entrance of solid particles of foreign material 

 might be prevented for a time by the presence of an investing skin 



* Quenstcclt describes similar threads in the superficial canals, now exposed 

 as shallow grooves, in S. radiata {v. Petrefact. Deutschlands, part i. Bd. v. p. 251, 

 pi. exxvi. fig. 60 z). 



