﻿Vol. 6l.] MICROSCOPIC STRUCTURE OF SERPENTINE. 697 



brighter ' thorn.' The scattered grains exhibit various stages of 

 serpentinization, which most affects the thinner streaks. The fibres 

 or flakes occasionally tend to be rudely parallel, but rarely, if ever, 

 recall by their arrangement the almost rectangular cleavages so 

 characteristic of augite when cut parallel with a basal plane. As 

 we purpose to refer to the chemical significance of these changes, we 

 quote analyses by the late Dr. T. S. Hunt : (I) of the pyroxene, 

 and (II) of the serpentine from Grenville, where similar rocks occur 

 with the so-called Eozoon. 1 



I. II. 



Si0 o 54-90 12-85 



MgO 1676 41-68 



CaO 27'67 



FeO 0-67 



H 2 0-80 2 13-89 



10013 9909 



Augite, as Miss Raisin found during her visits to the Vosges, 3 is 

 not an unfrequent constituent of their serpentines. Certain of these, 

 southward of the Rauenthal in the Central Vosges, are more than 

 usually variable in composition. For instance, in one rather large 

 mass to the north-east of Bonhomme, cropping out in craglets over a 

 space of about 100 yards, the mineral structure varies from obscurely 

 parallel to distinctly banded. In the latter case the bands, some- 

 times made conspicuous by a peculiar weathering and jointing, are 

 often 1 or 2 inches thick. They are dislocated by a few slight 

 faults, but the rock affords no other sign of mechanical disturbance. 

 Microscopic examination shows the chief constituents to be picotite 

 (fairly abundant), olivine (almost wholly converted into serpentine), 1 

 enstatite (partly changed), and augite. The last mineral has some- 

 times assumed the diallage-cleavage, and is often rather ' dusty ' ; 

 cracks and cleavage-partings are occupied by very thin strings of 

 serpentine, which often seems as if it had been formed by percolation 

 from outside rather than by alteration of the adjacent mineral. 

 Sometimes, however, the outer part of an augite-grain in contact 

 with serpentine becomes rather fibrous. Curiously enough, the 

 ' interleaving ' with films of the latter mineral is least conspicuous 

 in those grains which exhibit the two nearly-rectangular 5 cleavages. 

 A slice (thin) from one of the bands (PI. XLV, fig. 2) shows 



1 Quart. Journ. Geol. Soc. vol. xxi (1865) p. 68. It is rather remarkable 

 that, as so much lime (to say nothing of the silica) has been removed, the change 

 from augite to serpentine has left no trace in the adjacent calcite (or dolomite) 

 of my St. Pierre specimens. [T. G. B.] 



* In this case called ' volatile matter.' 



3 Undertaken for the immediate purpose of studying the noted Rauenthal 

 serpentine. See this Journal, vol. liii (1897) p. 246. 



4 The structure is as described above, but the polarization-tints are rather 

 bright ; the two slices, however, are a little thick. 



5 In other words, those which are cut nearly parallel to the basal plane, and 

 have not assumed the diallage-habit. 



