530 



And considering that the layer is immediately adjacent to the so-called 

 ''intermediate skeleton," composed of carbonate of lime, there is not 

 mnch difficulty in understanding that this substance might, in 

 numerous instances, infiltrate itself into the thin inter- acicular sepa- 

 rations. 



But the cases with which we are at present especially concerned do 

 not admit of so simple an explanation; as we have to account for the 

 presence, not of thin films of carbonate of lime, but of much thicker 

 intercalated portions of this substance, equalling the diameter of one, 

 two, or more aciculi. Guided by the changes undergone by the ser- 

 pentine and malacolite in the Neibiggen, Connemara, and other speci- 

 mens, we are strongly inclined to refer the present cases to Blum's class 

 of "replacement pseudomorphs." 



The mineral serpentine, although belonging to a group of difficultly 

 reducible silicates, is rendered, when in the condition of chrysotile, or 

 flocculite, comparatively easy of decomposition under proper conditions. 

 Besides, considering the difference between these allomorphs, one being 

 fibrous, and the other granular, — that they are often intermixed, — and 

 that the asbestiform variety is not structurally uniform, being incipiently 

 fibrous here, and perfectly fibrous there,— it must be admitted, that not 

 only do the divisional structures referred to afford facilities for 

 pseudomorphic action, but they are eminently favourable to the de- 

 velopment of that "infinite variety of detail," as noticed by Dr. Car- 

 penter, presented by the separated and juxtaposed aciculi of the 

 " nummuline layer" seen in decalcified specimens. From what is known 

 of the numerous examples of replacement pseudomorphs, described by 

 Blum, Bischof, Breithaupt, Delesse, Muller, and others, there is no 

 difficulty in assuming that the crowded and infinitesimally small fibres 

 forming this "layer," also the loosely aggregated particles composing 

 the granular flocculite — both kinds composed of a hydro-magnesian 

 silicate, the most soluble of its class — might be replaced by calcite or 

 dolomite, if the rock containing them were furnished with carbonate 

 of lime (as is the case with ophite), and had been subject at any time 

 to deep-seated hydrothermal action : or, a similar change is admissible, 

 supposing serpentine alone to be present, and allowing the rock to have 

 been permeated by heated water, holding a calcareous carbonate in 

 solution. The silicate composing the fibres or aciculi might in the latter 

 case be substituted by calcite, or dolomite. 



Such is our hypothesis, modifications non-essential to its principle 

 being allowed, to account for the origin of the calcite, where it separates 

 the fibres of the " nummuline layer." We also offer it to explain how 

 the " definite shapes" have been formed out of plates, prisms, and other 

 solids of serpentine, viz., by the erosion, or incompleted waste, of the 

 latter, and the replacement of the removed substance by calcite, — the 

 "definite shapes" being the residual portions of the solids that have 

 not completely disappeared. And we hold, in accordance with this 

 view, that the calcite or replacing carbonate, enclosing the re- 

 sidual portions, and which forms the "intermediate skeleton," is like- 



