﻿Vol. 6 1.] MICKOSCOPIC STRUCTURE OF SERPENTINE. 691 



Anglesey and of the Vosges, including the noted liauenthal, and 

 placed her collection, numbering over 270 specimens, 1 at the 

 disposal of her collaborator. Under these circumstances Prof. 

 Bonney is directly responsible for most of the microscopic work 

 (though certain parts have been independently studied by Miss 

 Raisin), while she is responsible for the field-work in the Vosges 

 and the Brenner district- 

 Serpentine, whether this name cover one or more minerals, all 

 minute, is formed by the alteration of certain magnesian or ferro- 

 magnesian silicates, — olivine and the non-aluminous pyroxenes and 

 amphiboles. We purpose to describe, as briefly as possible, the 

 changes in each one of these minerals, in order to discuss more 

 particularly the genesis and significance of that variety of serpentine 

 which is designated antigorite. 



. II. Serpentine from Olivine. 



Most of the typical serpentine-rocks are formed from peridotites 

 by alteration of the olivine, which is their essential and dominant 

 constituent. Thus, as the latter mineral was the first recognized 

 parent of the former, the passage from the one to the other has been 

 so often described that for the greater part of it a very brief 

 recapitulation will suffice. We must, however, remember that the 

 ratio of the two protoxide-constituents in olivine varies greatly, 

 the name covering a series at one end of which is forsterite, where 

 almost invariably the magnesia exceeds 50 per cent, and the ferrous 

 oxide is well under 4 per cent. ; and at the other fayalite, which 

 contains little or no magnesia. The latter, however, obviously cannot 

 be the parent of serpentine, and this mineral, so far as we have 

 observed, is seldom produced from olivines rich in iron. 



An ordinary olivine, in which the ferrous oxide is approximately 

 from 6 to 12 per cent., passes through the following changes. 

 W r ater makes its way along cleavage-planes or other cracks, forming 

 serpentine from the adjacent parts and throwing down the rejected 

 iron as minutely-granular magnetite in the dark, or haematite in the 

 red, varieties. Thus the former constituent appears like a network, 

 in which sometimes one set of strings runs rudely parallel for 

 a time, and is slightly stouter than the other. These strings 

 occasionally consist of fibres of serpentine, twisted almost as in a 

 cord, but more commonly of flakelets, inclining to be fibrous, which 

 are developed roughly at right angles to the surface of the crack. 

 Some amount of freedom, we may remark, seems favourable to the 

 latter mode of growth, for it can often be seen occupying minute 

 cracks in a rock-slice, and the most conspicuously-fibrous forms, 

 such as picrolite and chrysotile, usually occur in those of larger 

 size. We leave for the present further description of the mineral, 

 merely remarking that the iron-oxide is either deposited in the 



1 It contains, among others : from zVnglesey 174 specimens, from the Vosges 

 70 specimens, and from the Brenner district 27 specimens. 



