INTEODirCTION. I 



discovering their true characters is rendered very difficult. But even in many of the 

 sponges inclosed in solid flints the silica has been replaced by peroxide of iron ; 

 and the former presence of the sponge is indicated by reddish markings in the flint. 

 These are particularly noticeable in the case of sponges belonging to the genus 

 Plocoscyi^Ma, which are characterized by thin anastomosing walls. These walls 

 appear in section on the surface of broken flints as delicate labyrinthine lines of a 

 reddish tint. In a few cases where the spicular structure is of an open character it 

 has been replaced by peroxide of iron in such a manner that the form of the 

 spicules is perfectly preserved. 



Siliceous Spicular Structure replaced hy Calcite. — The fact of calcite taking the 

 place of silica in fossil sponges has been greatly contested on account of the supposed 

 more stable character of silica in comparison with calcite. But the frequency of the 

 occurrence of undoubted siliceous sponges with skeletons in the condition of calcite 

 is a convincing proof of the instability of the colloid silica of sponge-skeletons. The 

 calcite which has replaced the silica is in a crystalline condition, and it appears to 

 have infiltrated the moulds after the siliceous skeleton has been dissolved and 

 removed. In the case of relatively large spicules, and where the skeletal mesh is of 

 an open character, the replacement has been eff'ected without obliterating the cha- 

 racter of the structure ; but in those instances where the spicules are minute and 

 they are united closely together, the finer details have disappeai'ed, and only general, 

 ill-defined outlines of the original spicules and skeleton remain. As a rule the 

 spicular canals are obliterated where calcitic replacement has taken place; but occa- 

 sionally some other material has filled in the canals previous to the deposition of the 

 calcite, and in that case they can be distinguished. 



Instances of the replacement of siliceous sponges by calcite occur in Silurian, 

 Devonian, Jurassic, and Cretaceous strata. The replacement appears to be confined 

 to sponges imbedded in calcareous or marly deposits. I am not aware of a single 

 instance occurring in arenaceous beds. The majority of the siliceous sponges from 

 the Jurassic limestone in certain localities are now calcareous, whilst in the same 

 strata elsewhere the same species of sponge remains siliceous. Not unfrequently too 

 the same individual specimen will be partly siliceous and partly calcareous ; in the 

 siliceous portion the spicules will retain their canals and other finer details, whilst 

 in the calcareous portion the spicules will be solid and the outlines of the skeletal 

 mesh confused and indefinite. In the sponges from the Upper Green Sand (so-called) 

 of Cambridge and the Grey Chalk strata of Dover and Folkestone the silica has been 

 almost entirely replaced by calcite. In some of the Upper Chalk sponges from 

 Flamborough, calcite has partially replaced the silica ; but, as a rule, the skeletons 

 of the sponges from this locality remain siliceous. In only one instance have I found 

 detached spicules of siliceous sponges changed to calcite, and this occurs in the 

 spicules oi Astroeosponrjia from the Silurian strata of Gotland. 



