MINUTE STRUCTUEE OP STROMATOPORA AND ITS ALLIES. 199 



some of the true siliceous Hexactinellid Sponges have their ske- 

 leton replaced by carbonate of lime, but that in these cases the 

 replacing material has the form of crystalline calcite. This, there- 

 fore, we should be justified in expecting to find also the case if 

 the Stromatoporoids had been primitively siliceous and had been 

 replaced by lime ; but in reality the calcareous skeleton is under 

 no circumstances, so far as we have observed, crystalline. On 

 the other hand, the granular appearance of the carbonate of lime 

 is a very strong proof that it constituted the primitive skeleton 

 of the Stromatoporoids. 



As regards the actual condition of preservation in which speci- 

 mens occur, we find that there are five diff'erent states in which 

 the Stromatoporoids are found : — 



{a) In the first place, many specimens not only have the skele- 

 ton entirely calcareous, but have the sarcode-chambers further 

 filled with calcareous matter in the form of transparent calcite. 

 This is the case with all specimens from beds where the fossils 

 generally are unaltered, as in the Dudley Limestone (Wenlock) 

 and the corresponding limestones of Grotland, as well as in the 

 equivalent Niagara Limestone of North America. It is also the 

 case, essentially,- with the Stromatoporoids of the Guelph Lime- 

 stones and of the Clinton formation of North America, though in 

 these the sarcode-chambers are filled with a crystalline aggregate 

 of dolomite instead of calcite. 



(b) In the second place, many specimens are found in which 

 the skeleton of the fossil is more or less completely calcareous, 

 whilst the sarcode-chambers are infiltrated more or less completely 

 with silica. "When this is the case, decalcification, whether 

 natural or artificial, and whether applied to thin sections, to 

 polished surfaces, or to small fragments, results in the production 

 of a siliceous structure representing the casts of the sarcode- 

 chambers and interlaminar spaces, together, in some cases, with 

 the canals connecting these. Transparent sections of similar 

 specimens show, as a rule, however, that the infiltration with 

 silica has not been absolutely perfect. On the contrary, the 

 porous skeleton of the fossil seems to have been first infiltrated 

 with water holding carbonate of lime in solution, resulting in the 

 deposition of a thin layer of small crystals of calcite throughout 

 the whole of the sarcode-chambers, and the space still left empty 

 was subsequently filled with silica. This is beautifully shown by 

 the polariscope in specimens of this nature, the actual calcareous 



