INTEODUCTION. 5 



cent glassy silica, which is shown to be crystalline by giving brilliant prismatic tints 

 under the polariscope ; whilst the spicules, on the other hand, are either neutral or 

 only give faint prismatic tints, thus indicating that the silica of which they are com- 

 posed is in a crypto-crystalline condition. 



In the siliceous sponges from the Jurassic limestones of Franconia, which yet retain 

 a siliceous structure, the silica, seen by reflected light, has a white snowy aspect, and 

 is distinctly crystalline in character. In this condition the spicular elements are 

 nearly invisible when mounted in Canada balsam, but in glycerine or water show 

 perfect definition. The mineral change in these sponges, however, does not appear 

 to have afli"ected the finer structural details of the skeleton, which are usually well 

 preserved. 



Most of the siliceous spicules from the Lower and Upper Green Sand and from tlie 

 interior of chalk flints exhibit an exterior surface-aspect like that of ground-glass, 

 which is seen under the microscope to be owing to the minutely eroded condition of 

 their surfaces. The silica composing them is now crystalline, and frequently appears 

 like delicate fibres radiating from different centres. In these spicules the canals are 

 usually infiltrated with silica ; in a few instances, however, they are filled with iron 

 peroxide. 



Another modification of the silica is presented by sponges from the Upper Chalk 

 of Flamborough, in Yorkshire, which are so distinct in their mineral characters that 

 they can be at once recognized by this feature alone. On removing the chalky matrix 

 by acid the sponge itself is usually perfect in outer form, and the fibrous mesh has a 

 rough surface-aspect, as if composed of minute granular particles of a dull earthy 

 greyish aspect. Thin sections of the fibre show that the silica is distinctly crystal- 

 line. In these sponges the spicular components of the skeleton, save in a few examples 

 where they possess relatively large dimensions, are not distinguishable ; they appear 

 to have been, as it were, fused together, so that the individual forms are entirely 

 obliterated. 



Siliceous Sponges inclosed in Chalk Flints. — The sponges met with in the interior 

 of chalk flints present very various conditions of preservation. In some examples the 

 flint is hollow and the interior cavity is partially or wholly filled with a fine powdery 

 material, largely made up of detached Tetractinellid sponge-spicules, which ai'C now 

 composed of crystalline silica. In other cases the hollow flint incloses the perfect 

 form of a sponge, whose walls are either in immediate contact with the interior 

 surface of the flint, or separated therefrom by a small space usually filled with a white 

 siliceous powder. The sponges thus inclosed frequently retain the spicular structure 

 of the outer surface in perfect preservation, particularly those species with relatively 

 large spicules ; and even in some instances the minute spicules of the dermal layer 

 of the sponge yet remain intact and in their relative positions. This favourable state 

 of preservation, however, is limited to the outer surface of the sponge, for the spicular 



