922 T. G. BONNET ON THE SERPENTINE AND 



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 un- 

 changed, and a good deal of enstatite partially converted into ser- 

 pentine. 



A Specimen from the Quarry , south of Ey nance (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 dej)ositcd 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*. 



That the Lizard serpentine is an intrusive rock I have already 

 shown in the earlier part of this paper; the microscopic exami- 

 nation 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 

 Ariege," 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 eithei\Fe 2 3 or Fe 3 4 , commonly the latter. Mr. 

 Macphersonf 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 2Si0 2 ) + 2HO = 3MgO 2Si0 2 + 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 a eins called Genoa marble, which 

 has been used in the decoration of the Hall of (he Fitzwilliam Museum, Cam- 

 bridge. It, too, is a highly altered olivine rock, with small crystals of rather 

 altered enstatite, and veins of dolomite (?). 



t " On the Origin of the Serpentine of the Rhonda Mountains," Ann. Soc. 

 Espan. Hist. Nat. vol. iv. pt. 1. 



