in both is wholly or almost wholly enstatite, though often a little altered into a serpentinous mineral.
Mr. Allport has lent me two specimens from the Balk, collected from a lower level than my own: one shows the banded structure very well, and contains many augitic crystals, with one which I think is certainly enstatite; the other has some olivine still unchanged, and a good deal of enstatite partially converted into serpentine.
A Specimen from the Quarry, south of Kynance (no. 1).—The dull claret-coloured rock with many greenish grey strings, can be now identified as an olivine rock, though very highly altered, and much more difficult to recognize than the others. The metamorphic process has been carried very far; the grains between the strings are stained a very pale greyish brown; and there is a faint concentric banding like that of agate. Some of the iron remains as black dust; the rest is the red peroxide. The enstatite or diallage (in small crystals always) is almost replaced by an opaque brown mineral deposited between its cleavage-planes, probably some oxide of iron. Polarized light, as usual, shows the strings to be doubly refracting, the grains isotropic.
I possess four other slides, cut from specimens from the east coast, two from the Balk and two from near Poltesco; of the precise localities I am less certain than in the case of the above. It is, however, needless to describe them, as they would add nothing to the above evidence, being more highly altered than the majority. Still it is clear they have been olivine rocks; and the glistening mineral also appears to be enstatite[1].
That the Lizard serpentine is an intrusive rock I have already shown in the earlier part of this paper; the microscopic examination confirms the idea, which both a priori chemical considerations and the general aspect of the rock suggest, that it is an altered olivine rock. The process, as is shown by what we have seen above and what I have described in my paper on the "Lherzolite of the Ariége," apparently consists in the gradual decomposition of the olivine by the action of slowly infiltrated water, during which the hydrous compound serpentine is formed, and the iron thus liberated is thrown down as either Fe2O3 or Fe3 O4, commonly the latter. Mr. Macpherson[2] considers the reaction as a " loss of one fourth part of its (magnesia) base .... replaced by two molecules of water," a reaction which can be expressed by the formula
(4MgO 2SiO2) + 2HO = 3MgO 2SiO2 + 2HO + MgO.
The magnesia he conceives to have been removed. He does not,
- ↑ It may be interesting to mention that I have had a fragment sliced from that very dark serpentine with calcareous veins called Genoa marble, which has been used in the decoration of the Hall of the Fitzwilliam Museum, Cambridge. It, too, is a highly altered olivine rock, with small crystals of rather altered enstatite, and veins of dolomite (?).
- ↑ "On the Origin of the Serpentine of the Rhonda Mountains," Ann. Soc. Espan. Hist. Nat. vol. iv. pt. 1.
