Harveij B. Hnll—Oit Fo.<^Al Sponger. 



69 



ble. Ill the fossil, however, the horny fiber is rephiccd by silica lime, 

 or iron. That the tissue in the fossil is not iclentical Avith that of tin- 

 original sponge, may be inferred from the circumstance that we com- 

 monly find all the sponges from one locality, or one deposit, in the same 

 mineral condition. Thus, all those from the carboniferous limestone of 

 the Great Orme's Head are silicified ; but so also are the associate 

 Zoophytes, Conchifera and Gasteropods, etc. All the sponges fj'ora the 

 Farringdon green sand are calcareous, while those of Warminster are 

 all siliceous. The sponges of the English Oolite are all calcareous : 

 those of the chalk are either silicified or else in the state of moulds or 

 casts, the walls of which are stained with peroxide of iron. [Both 

 Lchadites and Phjlospongia occurred to Eichwald sometimes calcified, and 

 sometimes converted into bisulphuret of iron, more or less peroxidized. 

 His Manon deforine from GherikoflT, was silicified, while the examples of 

 the same species from the environs of Poulkowa were all calcified. 

 Lethcea Eossica, p. 339. Ischadifes Kcmicju occurs in our British Upper 

 Silurian rocks, both as a calcareous and a pyritized fossil.] 



It has been thought that the iron staining of the moulds and their 

 refilling with pyrites renders it probable that in the original sponge the 

 fiber was keratose. That in the horny sponges pyritous casts may be 

 more frequent than in the others is highly probable, but the amount of 

 sulphur in the keratose is far too small to enter into combination with 

 all the iron in the cast in accordance with the theory implied ; and as- 

 suming that the original skeleton of the sponge was keratose, there is 

 no reason to suppose that the .mould would not be refilled in harmony 

 v,ith a general law, /. e., the cast was siliceous when deposited from 

 Avater, holding in solution silica rendered soluble by the presence of 

 lime and alkalies ; calcareous from waters holding lime in solution by 

 the aid of an excess of carbonic acid ; iron in other cases, and even 

 bisulphuret of lead has been found replacing carbonaceous matter in 

 the plant remains of the Lias of Dunraven. The manner in which 

 the mould is refilled with silica was precisely similar to that by which 

 it is made to replace the carbonate of lime in the tests of the moun- 

 tain limestone mollusca of the Great Orme's Head, the Portland rocks 

 of Tisbury, or the green sand of Blackdown. The Ostreida? and Ser- 

 pulte, and other parasites attached to the surface of the Warminster 

 sponges are frequently like the sponge tissue, in the condition of sili- 

 ceous casts. As shown by Liebig, silica, when long in contact with 

 lime in alkaline solutions, becomes soluble, and hence it is that we so 

 often find the sponges of the chalk encased with silex, or with the in- 

 terstices filled with a more or less porous mass of the same material, 

 which is altogether adventitious to the sponge tissue. In a similar 



