28ti Profs. W. King and T. H. Ilowney on the 



Similarly demonstrative is another fact to be mentioned. The 

 mineral coloured red in our Plate, which we provisionally con- 

 sider to be a variety of tremolite *, consists, in the specimen 

 represented in fig. 6, of a mass of divergent fibres that origi- 

 nated, in the outer layer, a ; the mass is closed in by serpentine, 

 saponite, and calcite, the latter being in immediate contact with 

 its right side. It will be observed that there is an absence of 

 the outer layer on this side of the tremolite, and yet the cha- 

 racteristic fibres are present in the adjacent portion of the 

 mass. What have these fibres originated from? Clearly 

 from the same layer as all the others. Then can we do otherwise 

 than amplify our answer by stating that the layer in which 

 they had their origin has disappeared ? And are we not equally 

 warranted in unreservedly maintaining that the layer has been 

 simultaneously replaced by the calcite occupying its place? 



Serpentine rocks occurring in other places have furnished 

 us with numerous evidences of the same kind resulting from 

 chemical changes, but in other minerals, as serpentine and 

 malacolite. Examples of chrysotile (an allomorph of serpen- 

 tine) changing into calcite have also occurred to us in ophites 

 from Connemara and Italy f : in the latter instances the change 

 had not destroyed the fibrous structure of the replaced mineral. 

 Veins of fibrous dolomite are not uncommon in the Cornish 

 serpentine ; but we have not succeeded in procuring any which 

 we can affirm to be pseudomorphic after chrysotile, though a 

 strong suspicion may be entertained that the veins referred to 

 are of this nature. As somewhat countenancing such a sus- 

 picion, we have the fact that these veins occur intersecting sapo- 

 nite, and that, while some of them are indefinitely fibrous like 

 chrysotile here, they are decidedly columnar or prismatic 

 there, and without any structure of either kind in another 

 part : in the latter case the mineral is more or less affected 

 with rhombohedral cleavage. Fig. 7 represents a thin vein in 

 these three conditions, as displayed in specimens collected near 

 Kenwick Cove. 



But -though admitting that the last cases are not directly to 

 the purpose in hand, we must take a different view of some 

 yet to be briefly noticed. Fig. 8 represents two cylindrical 

 rods composed of saponite ; fig. 9, other four of serpentine: both 



* This mineral is usually of a reddish colour, white where it is gra- 

 nular. Amorphous green serpentine is occasionally intermixed "with it. 

 A qualitative analysis (the only one Tve have succeeded in obtaining) 

 afforded silica, alumina, magnesia, and lime, which, with a divergent 

 fibrous structure and the absence of water, leads us to suggest the identi- 

 fication given in the text. 



t Proceedings of the Royal Irish Academy, vol. x. pi. 42. fig. 6 ; pi. 44. 

 fig. 9. 



