314 Dr. S. B. Holl — On Fossil Sponges. 



attached to the surface of the Warminster sponges are frequently, 

 like the sponge tissue, in the condition of siliceous casts. As shown 

 by Liebig,^ silica when long in contact with lime in alkaline solu- 

 tions becomes soluble, and hence it is that we so often find the sponges 

 of the Chalk encased with silex, or with the interstices filled with a 

 more or less porous mass of the same material, which is altogether 

 adventitious to the sponge tissue. In a similar manner the mould 

 may be refilled with carbonate of lime. On the other hand, an 

 originally calcareous sponge may become conA^erted into a siliceous 

 fossil, as we see in the case of the mollusca of the Great Orme's 

 Head and elsewhere, and that an interchange of this kind has actually 

 taken place seems necessary to explain the fact, that siliceous and 

 calcareous sponges are not usually found associated in the same spot. 



That the calcareous sponges, those of the English Oolite for 

 instance, are merely casts of the original structure, may be shown in 

 another way. If then sections of the fibres be made sufficiently 

 translucent for the employment of a quarter-inch power of the 

 microscope, it will be seen that the fibre has often the asbestiform 

 structure, radiating at right angles from a central axis, peculiarly a 

 mineral arrangement, and especially of carbonate of lime. The 

 structure of true sponge fibre on the contrary is concentric. The 

 preservation of the sponge, therefore, in its fossil state, depends 

 very much on the nature of the sediment in which it is embedded, 

 and on the mode of its entombment. Hence they are met with 

 but rarely in stiff argillaceous deposits ; and although abundant in 

 Mesozoic times, they are absent from all the clayey members of 

 the series, such as the clays of the Lias, and the Oxford and 

 Kimmeridge Clays ; yet they occur in continental beds of cor- 

 responding ages, but differing in lithological character. At the 

 same time it is possible that the muddy waters of the seas in which 

 these deposits were thrown down may have been ill adapted to the 

 well-being of the sponge, nevertheless they are met with in the 

 Lingula Flags, and in the Upper Silurian Shales. 



The condition of the sponges of the gravel pits of Farringdon is 

 remarkable, and may perhaps tend to throw some light on the mode 

 in which fossilization of the sponge takes place in calcareous and 

 such sandy deposits as contain lime ; and help to explain, in some 

 manner, the absence of compression in the fossil. Every twig of 

 the sponge presenting a free surface throughout its entire thickness, 

 is invested by a thin coating of minute dog-tooth crystals of car- 

 bonate of lime, forming a complete crust over the fibre, much in the 

 same manner as moss, etc., is encrusted by a calcareous spring. 

 These crystals seldom exceed yJ-^. of an inch in height, the average 

 being about y-^-q-^ *^^ ^^ inch, or even less ; and on their surface they 

 are slightly tinged by peroxide of iron. All the other fossils of the 

 same locality are similarly coated with these minute crystals, even 

 to the interior of the cells of the Polyzoa. When slightly acted 

 upon by dilute acids, the crystalline layer is removed, and the cast 

 of the sponge-fibre exposed ; but if the action of the acid be con- 

 ^ "Lectures on Chemistry," p. 491. 



