MODE OF PRESERVATION. 31 



substances in solution, the result being the formation of a thin layer of crystals of 

 carbonate of lime or of silica in the interior of the chambers ; and then at a later 

 stage all the remaining cavities have been filled up with transparent silica (Plate I, 

 fig. 2). 



In a third group of specimens, of comparatively rare occurrence, a still more 

 remarkable series of changes has taken place. The specimens in question are pre- 

 served in limestones or in argillaceous deposits, and the first change to which they 

 were subjected consisted in the complete infiltration of the porous skeleton, not 

 with transparent calcite as in the first group of cases, but with fine calcareous mud 

 or minutely levigated argillaceous sediment. When in this condition, we must 

 suppose that the calcareous skeleton was more readily soluble in percolating water 

 than the calcareous or argillaceous mud filling the interstices of the fossil — this 

 greater solubility being due either to the fact that the skeleton consisted of arra- 

 gonite, or perhaps merely to its being impregnated with organic matter. The next 

 step in the process, therefore, consisted in the gradual dissolution of the skeleton 

 and its replacement by crystalline carbonate of lime, the infilling of the chambers 

 remaining in the meanwhile unaltered. Hence, thin sections of such specimens 

 show a precisely reversed condition of matters to what we observe in ordinary non- 

 silicified examples. Instead of seeing the dark-coloured skeletal framework filled 

 in with transparent calcite, we now see the entire skeleton represented by clear 

 calcite, while the chambers, pores, and canal-system of the fossil are represented 

 by comparatively opaque calcareous mud or fine argillaceous sediment. 



It is only comparatively recently that I have been led to recognise this very 

 peculiar mode of preservation as occurring among the Stromatoporoids, and that I 

 have been able to interpret the very puzzling phenomena to which it gives rise. It 

 occurs, among British specimens, most commonly in certain of the Stromatoporoids 

 of the Devonian Limestones of Devonshire, and especially in a form (8. dartiugto- 

 nensis, Carter) with exceedingly large astrorhizae. 1 I have figured (Plate IV, fig. 1) 

 a portion of a tangential section of this form, in this state of preservation, showing 

 the appearances which it presents when the canal-system and chambers are in this 

 way filled up with comparatively opaque calcareous mud. For comparison with 

 this, I have also figured the same section, as it would appear supposing it to have 

 been preserved in the ordinary manner, viz. with the skeleton comparative!} 7 

 opaque and the canal-system and chambers filled in with transparent calcite (see 

 Plate IV, fig. \,a). 



A still more easily recognisable case of the same mode of preservation is 

 presented by a specimen (in my collection) of an apparently undescribed species of 

 Labechia from the Cincinnati group of Ohio, which I may provisionally term L. 



1 The form in question has been spoken of by Mr. Carter as Slromatopora elegans, Rosen, but it is 

 really only a peculiar condition of 8. JDartingtonensis, Carter, and is quite distinct from Rosen's species. 



