6 Prof. Nicholson — Progress in Palceontology. 



careous, such as Brachiopods, liamellibranchs, Corals, etc., are often 

 found in some particular deposit to have been entirely and perfectly 

 converted into flint. Not only is this silicification of calcareous 

 organisms a very common phenomenon, but there is not the least 

 difficulty in understanding how it occurs, since carbonate of lime is 

 a very soluble substance and flint is a very insoluble one. Hence, 

 any calcareous fossil, if subjected to percolation by water holding 

 silica in solution, would be liable to have its lime dissolved away, 

 and replaced by the more intractable flint. It is asserted, however, 

 that in the case of the fossil sponges of which I have been speaking, 

 as well as of some other fossils, the reverse change sometimes takes 

 place, and that a structure originally siliceous becomes converted into 

 carbonate of lime, or, rather, becomes replaced by this substance. 

 It must be admitted that it is at present very difficult to comprehend 

 how a skeleton of comparatively insoluble silica should be dis- 

 solved away, and should be replaced by the very readily soluble 

 calcite, but the researches of Zittel in particular seem to leave no 

 doubt as to the fact that siliceous sponges have occasionally under- 

 gone this change. The bearings of this discovery upon our investi- 

 gations of the lower Invertebrate fossils are very wide indeed, and I 

 can only point out here that in the absence of direct evidence, we 

 must rely, in doubtful cases, upon the condition of fossilization ex- 

 hibited by associated fossils, the original nature and constitution of 

 which is beyond doubt. If we take, for example, the singular and 

 problematical fossils grouped together at present under the name of 

 Stromatopora, which I have had especial occasion to study, we find 

 that some specimens exhibit a skeleton of flint, whilst others show 

 one of lime ; and the question, therefore, arises, whether the siliceous 

 or the calcareous skeleton is really the original one ? We find, how- 

 ever, — at least this is my experience, — that the siliceous specimens of 

 Stromatopora occur in deposits in which the Brachiopods and Corals 

 and other organisms which were unquestionably originally calcareous, 

 also usually present themselves in a silicified condition. On the 

 other hand, in those deposits ia which such calcareous fossils as the 

 Mollusca and Corals retain their original constitution, we find that 

 the StromatoporcB are also calcareous. Without, therefore, questioning 

 Prof. Zittel's conclusions as to the mode of preservation of the fossil 

 Hexactinellids, it remains certain that the question as to the originally 

 siliceous or calcareous constitution of any particular class of fossils 

 will have to be settled by an appeal to the rocks in which these 

 fossils occur, and cannot be decided simply by an examination of 

 isolated specimens in the cabinet of a collector. Moreover, without 

 at all doubting that Zittel has proved his ease, as regards the parti- 

 cular fossils which he has had under examination, we cannot avoid 

 the conclusion that the replacement of silica by carbonate of lime 

 must, in the nature of things, be an unusual phenomenon, and that, 

 so long as we are ignorant of the precise method lander which it is 

 effected, we are bound to suppose that all fossils which are found to 

 be sometimes siliceous and sometimes calcareous are really composed 

 of carbonate of lime, unless distinct and incontrovertible evidence to 

 the contrary can be adduced. 



