30 BRITISH STROMATOPOROIDS. 



Stromatoporoids possessed a primitively siliceous skeleton, and that all calcareous 

 specimens owe their present constitution to the fact that the original silica of the 

 skeleton has been replaced by carbonate of lime. This conjecture lias been fully 

 discussed by Dr. Murie and myself (' Journ. Linn. Soc.,' vol. xiv, p. 197), and the 

 evidence against its correctness is so overwhelming that it is unnecessary to enter 

 again into the question here. It is sufficient to point out that adequate proof of 

 the fact that the skeleton of the Stromatoporoids was primitively calcareous in its 

 constitution is to be obtained from the following considerations : Firstly, in all 

 those Silurian and Devonian Limestones in which the Corals, Brachiopods, and other 

 fossils are normally calcareous or non-silicified, the Stromatoporoids are also 

 calcareous. Secondly, in all those deposits in which we meet commonly with 

 Stromatoporoids having a siliceous skeleton, we find *the Corals, Brachiopods, and 

 associated fossils to be mostly or wholly silicified. Thirdly, the skeleton of the 

 Stromatoporoids is composed, normally, of granular carbonate of lime, whereas if 

 it had been originally composed of silica and had been replaced by carbonate of 

 lime at some period subsequent to fossilisation, it ought to consist of crystalline 

 carbonate of lime. 



As I shall point out immediately, there are cases among the Stromatoporoids 

 where the original skeleton has been replaced by calcite ; but these lend no support 

 to the view that the skeleton was primitively siliceous, and seem really to point to 

 the fact that the skeleton was composed of arragonite, rather than of ordinary 

 carbonate of lime. 



There are, in fact, three principal modes of preservation under which specimens 

 of the Stromatoporoids present themselves. In the first group of specimens, com- 

 prising by far the larger number of ordinary examples, the actual skeleton has 

 been preserved more or less unchanged, and all the cavities of the skeleton have 

 been infiltrated with transparent calcite. In such specimens (Plate I, fig. 1) the 

 skeleton is readily distinguished from the calcareous infilling of the chambers, in 

 thin sections, by its brown colour and granular or cloudy, non-crystalline texture. 

 In certain cases, however, the skeleton has undergone a partial secondary crystal- 

 lisation, and is then only distinguishable from the calcite-matrix by its darker colour 

 and less complete crystallisation. Specimens of this kind occur more commonly 

 in dolomitic limestones than in ordinary limestones, though sometimes seen in the 

 latter. 



In a second group of specimens, more or less complete silicification has taken 

 place. In some examples, the actual skeleton has remained more or less completely 

 calcareous, while the cavities of the skeleton have been filled in with silica. In 

 other cases, not only is the infilling of the chambers siliceous, but the skeleton 

 itself has been replaced by silica. In other cases, again, the porous skeleton of the 

 Stromatoporoid has, to begin with, been infiltrated with water holding mineral 



