﻿702 PEOF. T. G. BONNET AND MISS C. EAISIN ON THE [Nov. T905, 



massive and jointed rock, extending (partly within a fence) for 

 about 100 yards. 



A slice cut from the edge of the serpentine, adjacent to the vein- 

 stuff, proves to be mainly composed of a mica-like mineral in slightly- 

 irregular flakes, from about -01 inch downwards, all but colourless 

 and without perceptible pleochroism, giving low polarization- tints 

 (bluish-whites of the first order) and straight extinction; little 

 streaks or patches being occasionally replaced by a carbonate allied 

 to dolomite. Here and there are a few fibres with higher tints, 

 and extinguishing obliquely, probably actinolite. The mica-like 

 minerals sometimes exhibit an approach to foliation, sometimes to 

 rectangular arrangement ; but closer examination shows this, where 

 it is not illusory, to affect the whole slice and to have no relation to 

 any original mineral structure (PI. XLV, fig. 3). 1 Iron-oxide occurs 

 in clustered or trailing granules, and this occasionally includes or is 

 pierced by the mica-like mineral, giving a minute ' ophitic ' structure 

 (fig. 1). The rock has unquestionably been modified by pressure. 



Pig. 1. — Antif/orite, sliowing i opliitic'' structure in magnetite 

 (from a lliffelliorn specimen). 



[Magnified 30 diameters.] 



Two slices have been cut from specimens taken farther in the 

 schistose zone. In these the flakes are sometimes very slightly 



1 Hereafter, for brevity, we shall refer to this as ' thorn-structure,' a name 

 which we gave it for purposes of reference. I first came across it in a serpen- 

 tine to the east of Andermatt, in 1878, and we found it occasionally in Anglesey. 

 [T. G. B.] 



