Nodules of the Trimmingham Chalk. 439 



boniferous beds of Scotland we have the same association ; 

 and in those of North Wales pseudomorphs of Radiolaria in 

 calcite occur along with minute quartz crystals. The Lias of 

 South Wales contains beds of chert literally crammed with 

 sponge-spicules of large size ; and in some of the Lias lime- 

 stones dispersed spicules are abundantly present along with 

 minute quartz crystals and chalcedonized shells. In the Coral- 

 line Oolite of Yorkshire we find the calcitic pseudomorphs of 

 Geodites iSorbyanus, to the abundance of which Sorby testi- 

 fies ; and accompanying them are chalcedonized shells and 

 numerous granules of silica with a radiate crystalline struc- 

 ture. The sponges of the Yorkshire oolite, often of large size, 

 are known to have been siliceous solely by a study of their 

 form and structure ; for they now consist of carbonate of lime, 

 the silica which they once contained being, according to our 

 view, chiefly collected in radiating crystalline patches or 

 granules, which occur in association with them. In the fresh- 

 water Purbeck beds of Lulworth freshwater chert occurs, 

 in which Mr. John S. Young, F.G.S.*, has found numerous 

 spicules of Spongilla (S. purbeckensis, Young). In the 

 Cambridge Greensand we have a remarkable instance of the 

 association we are illustrating in the fossil Renierid sponge 

 Pharetrospongia Strcihani, the spicular fibres of which have 

 exchanged a siliceous for ■ a calcareous composition, while the 

 chalk surrounding them in the interstices of the sponge has 

 been converted into silex, with but slight alteration in mor- 

 phological character. 



I believe I may fairly claim to have substantiated the 

 statement with which I set out, and will now only add the 

 following passage, which I venture to extract from my paper 

 on Gatagma\ ; it indicates the same line of reasoning, though it 

 was used in a quite different connexion : — " As regards sili- 

 ceous sponges, many of these often exist now in a calcareous 

 state ; but it may be as well to note that whenever a siliceous 

 sponge becomes calcitized in fossilization the deposited silica 

 is generally to be found somewhere not far off, either in 

 patches in the sponge itself, or in granules or nodules such as 

 flints in the surrounding matrix, or as chalcedony silicifying 

 associated calcareous shells, ex. gr. in the Lias of the South- 

 Welsh coast, or in minute dispersed crystals of quartz, ex. gr. 

 in the Devonian and Carboniferous limestone. In compact 

 strata, such as chalk or limestone, it may be taken as an 

 almost invariable rule that the replacement of organic silica 

 by calcite is always accompanied by a subsequent deposition 



* Geol. Mag. new ser. dec. ii. vol. v. p. 220 (1878). 

 t Ann. & Mag. Nat. Hist. 1878, ser. 5, vol. ii. p. 361. 



