﻿Vol. 6 1.] MICROSCOPIC STRUCTURE OF SERPENTINE. 693 



in the olivine nor found a minute fibrous hornblende in his own 

 specimens so often as that excellent observer appears to have done. 

 They contain, however, the two fibrous microti ths which the latter 

 refers to bastite/ though the dull-blue one is rare, and we doubt 

 whether it is more than a local staining. The olivine alters into a 

 minutely-fibrous serpentine, generally rudely perpendicular to, but 

 sometimes parallel with, the outer surface, which forms a sort 

 of thin shell, the interior being converted (some olivine often 

 remains) into colourless fibres or even narrow blades sometimes 

 about one twenty-fifth of an inch long (1 mm.). Their polarization- 

 tints are generally the dull bluish-whites of the first order, but rise 

 occasionally to the clear- or the orange-yellow (both tints may occur 

 in the same slice), the extinction being generally straight, but 

 occasionally oblique up to about 20°. In some grains these flakes 

 form divergent tufts, which may develop longitudinally until the 

 group roughly resembles a feather, the pinnules on opposite sides 

 making angles of 22° to 24° (PI. XLV, fig. 1). In one or two 

 cases we find broad irregular flakes giving low polarization- tints, 

 and notice that, in elongated grains, the flakes of the interior tend 

 to lie parallel with the longer sides. We may, we think, conclude 

 that the olivines in this rock are generally poor in iron, and 

 may perhaps attribute the differences in structural change between 

 them and the olivines in peridotites to their occurrence as isolated 

 grains in a rather permeable material. 



III. Amphibole. 2 



The less ferriferous members of this group, as is well known, 

 change, though not so readily as olivine, into serpentine. In fact, 

 as might be expected, the ferromagnesian silicates with lime alter 

 less rapidly than those without it. As this particular meta- 

 morphism has been rather fully discussed, we may pass briefly over 

 it. The best-known instance is the serpentine of the Eauenthal, 

 which in 1875 was asserted to have originated in great masses 

 from an amphibolite, and this to exhibit a passage into a normal 

 gneiss. One of us, in 1887, when publicity was given to that 

 statement in an important English work, showed the former part 

 of it to be only so far correct that the original rock (like some at 

 the Lizard) had been a hornblende-peridotite.* This was confirmed, 

 and the second part of it disproved, by the other of us in 1897. l 

 The hornblende in the Rauenthal serpentine has been a slightly- 

 ferriferous variety. Between its prismatic cleavages strings or 

 blades of mineral serpentine (best examined in longitudinal sections) 



1 ' The Genesis & Matrix of the Diamond ' 1897, pp. 14, 21. 



2 We have not been able to study the alteration of the rhombic ampbibole, 

 antbopbyllite. One of us has been allowed to examine the specimens in the 

 British Museum cabinet of slices, but these are practically unaltered, and a 

 slice in his own collection (given to him by the late Mr. Rutley) is not helpful 

 for this purpose. 



3 T. G. Bonney, Geol. Mag. 1887, p. 65. 



4 0. A. Raisin, Quart, Journ. Geol. Soc. vol. liii (1897) p. 246. 



