284 Professors King and Rowney's 



distinctly show." Observations properly and patiently con- 

 ducted, with a true appreciation of all the collateral elements, 

 and discarding- partial simulations, instead of giving rise to 

 the idea that the openings on the " chamber " side should be 

 represented as bounded by a line and filled with calcite, would 

 have resolved them into portions of serpentine, remaining un- 

 converted into chrysotile or into the acicular condition. 



It will now, to some extent, be understood in what sense 

 we contend that the " nummuline wall " is not a chemically 

 differentiated part. In certain places, as shown on the left side 

 of fig. 4, it is largely made up of calcite (it may be dolomite, 

 or magnesite) ; and, as such, shutting out of view the fact that 

 it oftener consists of closely compacted aciculse, it might, allow- 

 ing some exaggeration, be called " a calcareous lamella" ; but 

 in these places the "wall" certainly cannot be regarded other- 

 wise than as having assumed an exceptional condition, it being, 

 according to its discoverer, " rarely well preserved " (the ex- 

 pression evidently refers to what is considered to be its "true" 

 or "typical" character) ; and, which is of far greater weight 

 with us, more especially when, as in the places already noted, 

 it completely and insensibly passes into the state of true 

 chrysotile*. 



Dr. Carpenter has brought forward an entirely new "pro- 

 bative fact," consisting of a fragment of the " nummuline 



* According to our theory, stated elsewhere (Quart. Journ. Geol. Soc. 

 vol. xxii. pi. xiv. fig. 2, p. 192 ; Proc. Roy. Irish Acad. vol. x. pi. xli. 

 fig. 2, p. 315 ; Geol. Mag. Jan. 1872), the presence of calcite in the 

 " nummuline wall " is the result of chemical action, effected by the 

 agency of carbonated solutions, similar to what has taken place in the 

 production of pseudomorphs consisting of calcite after a silacid mineral. 

 In the latter, the original mineral substance is often represented by a 

 siliceous skeleton, or it is entirely removed, nothing being left but its 

 crystalline form composed of calcite. With the exception that no ori- 

 ginal crystalline form is preserved (for a rock mass has been dealt with), 

 the "wall" displays similar changes; the calcite has partially, or wholly 

 replaced the serpentine — partially where the aciculre are imbedded in it, 

 and wholly where they are absent. The aciculse, when separated merely 

 by divisional spaces, manifest the first change of the chrysotile (which is 

 indefinitely fibrous) : when separated by calcitic interspaces, as in the 

 "true nummuline wall," they are no more than the remains of the latter 

 mineral, preserving in their " usual straight and parallel lie," and their 

 "often more or less curvedness," its characteristic Jibrosity. Similar 

 chemical action, or methylosis as we have called it, has converted amor- 

 phous serpentine into lobulated grains (" chamber-casts ") and arborescent 

 forms ("canal-system"), but mainly shaped by irregular conchoidal divi- 

 sional structure. In all cases the change terminates with the production 

 of the "intermediate skeleton," the result of the conversion of serpentine 

 into calcite. 



