Vol. 55.] CAEBONIFEROUS LIMESTONE OP DERBYSHIRE. 717 



be loosely termed the ' surface-tension ' of solids. An alternative 

 explanation rests on the undoubted fact that over these regions 

 molecular bombardment must attain a maximum ; but in the present 

 case this explanation is excluded, since at the surface where the 

 growing rhombohedron of calcite meets the solid substance of the 

 sponge-spicule such a bombardment will most probably be at a 

 minimum. It seems to me possible that the excavation of the 

 spicule stands in close connexion with the pressure exerted upon it 

 by the growing crystals of calcite, where these are in apposition. 



However this may be, the final result of the corrosion by calcite 

 is to replace entirely the opal of which the spicule originally con- 

 sisted, and to replace it by a congeries of minute crystals of calcite. 

 Since these crystals may, and generally do, commence their growth 

 outside the limits of the spicule, the resulting pseudomorph rarely 

 preserves the regular outlines which characterized its antecedent 

 state, but exchanges this for a rough and jagged surface, almost 

 precisely like that of a stick of sugar-candy. 



The spicules in the slice that I have examined from Mr. Arnold- 

 Bemrose's collection are in this state, and were it not for the fact that 

 the crystals of calcite are very minute, and frequently bounded by 

 impurities in the limestone, their individuality could only be traced 

 doubtfully; owing to these fortunate circumstances, however, they 

 are as clearly defined as corresponding structures from the Chalk. It 

 may be hoped, now that attention has been called to their occurrence, 

 that further examples of these spicules may be obtained which may 

 throw some light on their nearer affinities. In insoluble residues 

 of the limestone, after treatment with acid, the occurrence of either 

 remnants of sponge-spicules or minute crystals of quartz may be 

 predicted.^ 



A physical or chemical explanation of the replacement of the 

 comparatively insoluble opal of siliceous organisms by carbonate of 

 lime has not yet been suggested, and seems as remote as when Zittel 

 and myself first demonstrated its occurrence more than twenty years 

 ago. 



DiSCFSSIOIS". 



Dr. G. J. HiNDE said that some time since, with the permission 

 of Mr. Arnold-Bemrose, he examined the identical microscopic 

 section of limestone (No. 809) from the Tissington cutting referred 

 to by the Author, but failed to recognize any sponge-spicules in it ; 

 and even now he (the speaker) did not feel convinced that the 

 fragments of rods shown in the slide were sponge-spicules, while 

 the calcite-bodies with circular outlines appeared to belong to 

 Calcisphcera. The solitary siliceous fragment obtained by dissolving 

 some of the limestone was probably a portion of a spicule. 



Dr. Hicks also spoke, and the Author replied. 



' [Since this was written a small fragment of limestone, kindly given to me 

 by Mr. Arnold-Bemrose, has been treated with acid, and in the insoluble 

 residue several fragments of siliceous spicules have been found which agree in 

 form and dimensions with those in the thin slice.] 



