26 ROCK-METAMORPHISM. 



But, having in view the mineral changes which chrysotile and 

 the associated serpentine undergo in the process of developing the 

 separated aciculse and arborescent configurations of ' ( Eozoon," 

 we are more especially concerned with the fact that these 

 changes have been accompanied by the substitution of calcite 

 for a hydrous silicate of magnesia, unassociated with calcium 

 in any form of combination. 



A very simple modus operandi in this case would effect the 

 substitution, nothing more being required than for water, 

 holding carbonate of lime in solution, to penetrate into the 

 linear openings of the chrysotile and the cracks of the serpen- 

 tine : a portion of the silica and magnesia of these minerals 

 would be removed in the soluble state, the carbonate of lime 

 taking their place. According to this view, the calcite between 

 the acicnke in a layer of pectinated chrysotile, also that in which 

 the configurations are imbedded, has been conveyed thereto in 

 solution from an independent or extraneous source that is, from 

 the calcium silicate common to minerals of metamorphic rocks. 



Feldspars, hornblendes, augites, and several other minerals, 

 it is well known, are pseudomorphosed by the carbacidizing of 

 their calcium silicate. Crystals of hornblende occur filled with 

 calcite (Bischof, op. cit. vol. ii. p. 315); labradorite has been 

 found having some of its own constituents replaced by the same 

 mineral ; augite is frequently more or less calcitized ; it is the 

 same with garnet, epidote, wollastonite, and others. As in most 

 of these cases the pseudomorphs contain more lime than conld 

 be yielded by the calcium silicate of the original mineral, it 

 may be assumed that the solvent by which the latter were 

 affected supplied the additional quantity. 



Even the minerals forming syenite, diorite, and other rocks, 

 in situ, on becoming subject to carbacidizing processes are re- 

 placed by calcite. 



We have diorite from Jersey: amidst its component horn- 

 blende &c. there frequently occur patches of calcite associated 

 with epidote and serpentine, Little or no doubt can be enter- 

 tained as to the hornblende in this case having supplied the 

 magnesia for the serpentine. It is therefore highly probable 

 that the calcium silicate of the hornblende has been transmuted, 

 through the introduction of carbonic acid, into the associated 

 calcite, especially as there are no limestones nor other cal- 

 careous rocks in the island. In one place near Galway, the 



