6 INTRODUCTION. 



structure of the interior is completely obliterated, and is now a mass of botryoidal or 

 porous crypto-crystalline silica, in which even the course of the canals has disappeared. 

 The surface spicular structure is usually of a snowy white aspect, and the silica is 

 crystalline. 



In other examples, of somewhat rare occurrence, the hollow flint contains merely 

 the solid casts, in silica, of the central cloaca and canals of the sponge, whilst the 

 spicular skeleton itself has entirely disappeared. 



In all the instances above quoted the sponges are contained in holloio flints ; but 

 numerous examples occur in which the inclosing flint is quite solid. In some cases 

 the flint nodule has a rough conformity to the form of the sponge, in others no 

 relation is apparent. As a general rule the spicular structure in these solid flints is 

 completely obliterated, and the interior of the flint merely exhibits the cloacal cavity 

 and canals, which are distinguishable from the circumstance that the silica which has 

 infilled them is of a diff"erent tint from that of the rest of the nodule. 



Spicules of Tetractinellid and other sponges also occur scattered loosely in strata 

 of glauconitic sand and in places aggregated together into distinct beds. Sometimes 

 the individual spicules can be distinguished on the weathered surface of these sponge- 

 beds ; but in the interior of the beds the spicular forms are merged into glassy chert. 

 The silica of these spicules is usually crystalline. These spicules and spicular masses 

 occur in the Hils formation of Germany, in Neocomian sandstone at Haslemere in 

 Surrey, in the Kentish Rag quarries near Maidstone in Kent, at Folkestone, and in 

 the Upper Green Sand of Blackdown and the Haldon Hills in Devonshire. Spicules 

 of siliceous sponges in connexion with beds or nodules of chert have been met with 

 in Carboniferous strata in Ireland, in Lias strata from the Austrian Tyrol and 

 Glamorganshire, and in the Portland limestones of Upway in Dorsetshire. 



Siliceous Spicules replaced by Peroxide of Iron and Iron Pyrites. — Examples where 

 the original silica has been replaced by pyrites are not of very common occurrence. 

 A well-known instance is that of the earliest known sponge, Protospongia fenestrata, 

 Salter, from the Menevian of Wales, where the sponge is imbedded in black shale. 

 In a few sponges from the Jurassic limestones, the Grey Chalk, and the Upper 

 Chalk the same replacement occurs. 



The replacement of silica by peroxide of iron is of extremely frequent occurrence, 

 particularly in the sponges from the Upper Chalk of this country. The peroxide is 

 present as a light reddish-brown powdery material, usually incoherent, though in a 

 few instances sufiiciently firm to retain the spicular mesh intact after the removal of 

 the chalky matrix. As a general rule, however, the sponge-structure when replaced 

 by peroxide is extremely indistinct and merely represented by a thin rusty film, in 

 which the character of the spicules is generally obliterated. Unfortunately most of 

 the siliceous sponges from the Upper Chalk of the south of England, which have not 

 been inclosed in flints, are in this very unsatisfactory condition ; and the task of 



