152 On Crystallography, 



Amon^ the, different explanations which have been given 

 of this pYviienomcnon, that which seems \o be nio:^t gene- 

 rally admitted, although not exempt from objections, con- 

 sists in supposing that the stony matter is substituted for 

 the vegetable in proportion as the latter is decomposed; 

 and because the substitution take* place succcasivcly, and 

 as it were molecule by molecule, the stoin' panicles, in 

 arranging themselves in the places rendered empty by the 

 disappearance of the ligneous particles, and by moulding 

 themselves into the same cavities, take the impression of the 

 vegetable organization, and :;opy the traits of it precisely. 



The mineral kingdom also has its pseudomorphoses. 

 We find some substances of this kingdom under crystal- 

 line forms, which are only borrowed ; and it is probable 

 that, in some cases at least, the new substance has been 

 substituted gradually for that which has ceded its place 

 to it, as we suppose takes place with respect to petrified 

 wood. 



The various pseudomorphic bodies imprint their form 

 on the matter which surrounds them, and frequently also 

 the impression serves as a cell for an organic substance which 

 is simply in a fossil state, or which has received a certain 

 degree oF alteration only. This takes place in particular 

 with respect to the ferns and other plants of the same family, 

 the form of which is moulded on a schistous matter, as we 

 shall afterwards more fully detail. 



We generally denominate petrifactums all the variously 

 modified substances which we have mentioned, even those 

 which' only present impressions of animal or vegetable 

 productions. Daubenton applies this term only to bodies 

 which, in their natural state, being partly stony and partly 

 cartilaginous, such as shells, have become entirely stony. 



As we merely purpose to mention a few examples of the 

 ipiodifications in question, and not to unite then) methodi- 

 cally under one aqd the same point of view as several authors 

 have done, we shall confine, ourselves to the enunciation 

 of some of them in speaking of the substances which have 

 formed their secondary matter, and shall adopt the no- 

 menclature to this method of classifying. 



We ought not to omit that there are also pseudomor- 

 phoscs, which, arise from the substitution oF a metal in the 

 room of an organic body. Sulphurated iron presents se« 

 vera! examples of this kind of metallization. 



By referrmg to all that has preceded, we may define in 

 the followiiig manner the different concretions of which 

 wc have given the description : — • 



The 



