OPHITES AND RELATED ROCKS. 17 



for this reputed fossil. The margins of the layers of serpentine, 

 0, are each unmistakably converted at the borders into chryso- 

 tile. This allomorph, in some of the layers, is characterized 

 with bright colours ; but in others, where a calcareous layer is 

 in immediate contact, the fibres are more or less separated, and 

 thus converted into a " uummuline wall," as shown by the dark- 

 coloured lines between them. It will also be observed that the 

 fibres have been abruptly bent where they are acicular, which, 

 though evidently due to pressure, causes them to appear as if 

 they were structures distinct from the chrysotile fibres : this 

 abrupt bending, however, has not always taken place; for it 

 often happens (as occurs in other specimens we have figured *) 

 that such fibres pass into the aciculse without losing their con- 

 tinuity of direction. In some places the fibres are undulating or 

 curved, as has been observed in numerous instances that have 

 passed under our notice. The serpentine layer on the right- 

 hand side of the figure appears to have been in a fluidal condi- 

 tion, and on consolidation to have become cracked : the cracks 

 are brightly and variously coloured, indicating the presence of 

 another substance (peridote) hereafter to be noticed. 



Fig. 4, Plate II., represents a portion of a slide from Canada, 

 presented to one of us (T. H. R.) some years ago by Dr. 

 Carpenter. The portion is part of a thick layer of serpentine, 

 bounded on each side by a calcareous layer. The serpentine- 

 layer is transversely intersected by separated and parallel laminae 

 of chrysotile in its various modifications. Certain of the laminae 

 have become openly divided; and the resultant opening filled in 

 with calcite. In the instance represented at , the divided 

 lamina we hold to have originally consisted of chrysotile : strictly 

 parallel with the others, its fibres not only graduate into closely- 

 packed and separated aciculse, but they pass without break of 

 continuity into the latter variety. Moreover the overlying 

 lamina, b, is in the condition of incipient chrysotile, consisting 

 of separated lines so finely cut as to be scarcely visible except 

 by polarized light. Obviously the calcite which occupies the 

 opening in the lowest lamina had been introduced ; and it is also 

 inferable that the calcitic films between the separated aciculae are 

 nothing more than infiltrations of the same mineral carbonate. 



Although the last is an instance which maybe safely accepted 

 * Proc. Roy. Irish Acad. vol. x. pi, xli. tig. 2, d. 



C 



