﻿712 PROP. T. G. BONNEY AND MISS C. RAISIN ON THE [Nov. IQ05? 



Several analyses of Alpine serpentines are tabulated by Dr. 

 M. E. Wadsworth, 1 and some by Miss E. Aston will be found in this 

 Journal. 2 One of the latter contained an unusually- high percentage 

 of nickel, but in other respects did not differ from the normal. The 

 differences in these analyses (including the Sprechenstein specimens) 

 seem insufficient as a basis for distinctions, and they all lie 

 within the limits of variation due to the presence of this or that 

 magnesian silicate or the conversion of them into serpentine. 



XII. Serpentines prom Japan. 



Through tho kind offices of Prof. John Milne, F.E.S., one of us 

 received, nearly twenty years ago, from Prof. Koto a number of 

 specimens of serpentine (dull green) from Japan. As all were rather 

 seriously affected by pressure, and he was then desirous of studying 

 unaltered types, none of them were sliced until he began to put 

 together the notes for this paper. Each of the eight specimens 

 examined under the microscope shows signs of pressure, but this 

 appears, as a rule, to have acted less definitely in one direction than 

 it has done in the Alps. Two or three have contained cnstatite • 

 one, much olivine with some augite; two others were probably 

 peridotites; in one or two augite may have dominated, although 

 this is not quite certain ; in two the chlorite-like mineral, with 

 dull-white polarization-tints, as already mentioned, is present, 

 forming in one a rather distinct band. The serpentine generally 

 occurs in fibrous flakes with low polarization-tints, but we find 

 occasionally typical antigorite, and in part of one slice the rather 

 bright-tinted variety with an oblique extinction at low angles. 



XIII. Chemical Changes. 



The chemical changes in the conversion of olivine and one or two 

 ferromagnesian silicates into serpentines were discussed by J. Roth 

 and have been explained by J. J. H. Teall. 3 If we take (for sim- 

 plicity) the n on -ferriferous forms, their composition is as follows : — 



Forsterite 2MgO + Si0 2 . 



Enstatite MgO 4- Si0 2 . 



Augite (diopside) (MgCa)O + Si0 o . 



Serpentine 3MgO + 2Si0 2 + 2H 2 0. 



Talc 3MgO + 4Si0 2 + H 2 0. 



Thus the conversion of two molecules of forsterite (4MgO + 

 2Si0 2 ) into one of serpentine requires the removal of one molecule 

 of magnesia and the addition of two of water — a slight increase of 

 bulk. But ordinary olivine contains an appreciable quantity of iron 

 which remains as an oxide, so the removal will be less and the 



1 ' Lithological Studies' Mem. Mus. Comp. Zool. Harv. vol. xi (1884) 

 pp. xxiv-xxviii. 



2 Vol. lii (1896) pp. 454-56. 



3 'British Petrography ' 1888, chapt. vi, p. 104. 



