529 



It must next be borne in mind that the cleavage partings above 

 noticed had an infilling of carbonate of lime before they were decalci- 

 fied. A precisely similar case, it will be recollected, we pointed out in 

 a specimen of ophite from Connemara.* How will a mineralogist explain 

 this phenomenon ? The cleavage partings in the serpentine of the one 

 case, and in the malacolite of the other, he knows full well were originally 

 closed ; and in that state each parting had its two walls in perfect con- 

 tact. There is no other explanation open to him than that the substance 

 of these mineral silicates has been abstracted from the cleavage divisions, 

 and replaced by calcareous matter. In the Neibiggen specimen, the 

 partings are observed to be gradually getting wider : at first they were 

 divisional lines of the finest character ; next, slightly open separations ; 

 afterwards, well-marked chinks or wide fissures ; finally, indefinite 

 irregular passages. It would be unphilosophical to assume that the 

 process of abstraction stopped at the last stage. 



The cases lately brought forward have an important bearing on 

 a question discussed in our former Paper, and which has already 

 been briefly alluded to in the present one : we refer to the pre- 

 sence of carbonate of lime between the aciculi of the " proper wall," 

 where they are separated. No doubt whatever rests on our mind 

 that the presence of this substance in the cleavage partings of the 

 Neibiggen and other specimens, also in the acicular interspaces of the 

 nummuline layer, is due to one and the same canse ; but how it got into 

 these openings is a point on which we can still offer no more than a 

 hypothetical explanation. 



Our hypothesis is based on pseudomorphism, as understood by 

 mineralogists. Blum, who has laboured most assiduously at this de- 

 partment of science, separates the phenomena, embraced by it, into two 

 classes — one comprising " alteration pseudomorphs," and the other, 

 " replacement pseudomorphs. "f The first class includes examples 

 of minerals in which certain of their essential chemical constituents 

 have been removed, and replaced by others, as in cuprite (CuO) con- 

 verted into malachite (CuO, C0 3 +HO), leucite into oligoclase, &o. 

 The second class includes those minerals in which all their original 

 constituents, being completely eliminated, have been replaced by others, 

 as chlorite after magnetite, chalcedony after fluor, cassiterite after 

 orthoclase, hematite after calcite, &c, &c.J 



Reverting to the acicular layer, and assuming it to consist of a 

 hydro-magnesian silicate, there can be no doubt that in the cases on 

 which we are engaged the aciculi are separated by carbonate of lime. 



* See ante, p. 519. 



f Pseudomorphic phenomena have been investigated, with more or less success, by 

 a number of mineralogists and chemists. Other divisions have been proposed ; but we 

 adopt the one given by Blum as being the simplest for our purpose. 



X The Mineralogical collection in the British Museum contains several very interest- 

 ing specimens of pseudomorphs, which we have been allowed to examine by Professor 

 Maskeyline, and his assistant, Mr. Davis. 



