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XLYII.— ON THE PHYSICAL CHARACTERS OE CALCAEEOTJS 

 AND SILICEOUS SPONGE-SPICULES AND OTHER 

 STRUCTURES. By PROEESSOR W. J. SOLLAS, M.A., 

 D.Sc, E.R.S.E. (Plate XV.) 



[Eead, January 19, 1885.] 



Siliceous Skeletons. — Although in the great majority of existing 

 sponges the skeleton is of a siliceous nature, it was supposed, until 

 quite recently, by palaeontologists, that in nearly all fossil sponges 

 it was originally, as it usually is now, calcareous in composition ; 

 but since in structure it seemed to offer in no case any point of re- 

 semblance to that of existing calcisponges, it became necessary to 

 refer the fossil sponges to an extinct order, related to the existing 

 calcareous sponges simply by the chemical composition of the skele- 

 ton, and not in any way by its structure. In 1877, however. Pro- 

 fessor Zittel and myself showed that the majority of fossil sponges 

 are characterized by skeletal structures which resemble in the closest 

 manner those of existing siliceous sponges, and we accordingly 

 inferred that, originally siliceous in composition, they had since 

 become calcareous. Such a substitution as this involved appeared 

 to most geologists improbable, and some chemists went so far as to 

 assert that it was impossible. It has now, however, become one of 

 the accepted facts of science. 



The difficulty which was felt in admitting such a substitution 

 was no doubt due to an ambiguity of terms. " Siliceous " and con- 

 sisting of silica were regarded as synonymous expressions, and we 

 cannot wonder that a chemist would refuse to believe in the possi- 

 bility of calcite replacing quartz. But in my Paper on Pharetro- 

 spongia I pointed out that the siliceous material of sponge-skeletons 

 differs very widely from quartz in its physical characters. " The 

 latter is to all intents perfectly insoluble in carbonated waters, and 

 most other solvents ; it is crystalline, with a comparatively high 

 refraction-index, and is composed solely of oxygen and silicon. 

 The former, on the other hand, is colloidal, with a comparatively 

 low refraction-index, is soluble in caustic potash and carbonated 



