Prof. T. G. Bonney—Picrite in Sark. EEL 
occasional elongated enclosures, shuttle-shaped in section, like those 
of calcite figured by Prof. Zirkel.1 These often appear to be 
composed of a fibrous or lamellar minera] rather similar to the micro- 
lithic actinolite already mentioned. The larger grains of the mica 
inclose small grains of olivine, and grains or crystals of this horn- 
blendic mineral. In some cases needles of actinolite, in length up 
to about :04”, pierce it obliquely, like pins, making an angle of about 
' 35° with the cleavage planes. The penetrating power of acicular 
hornblende has often been noticed,? so I regard these as probably 
secondary. In other cases they lie, as above described, between the 
cleavage planes. 
4, Grains of iron oxide (in addition to much scattered opacite 
and small interstitial plates or rods, especially in the mica). These 
are no doubt original constituents, but I think that from their 
peculiar outline they have in some cases received secondary 
augmentation. Most of these grains are perfectly black and 
opaque, but occasionally a deep brown tint is just discernible at 
the edges, and two grains are in parts fairly translucent and of a 
distinct dull green colour. Hence it is probable that the mineral is 
chromiferous, and varies from magnetite or chromite to picotite.® 
One would not be surprised to find some variety of rhombic 
pyroxene in the rock, but I have failed to identify it, and am 
disposed to refer all the cleared and colourless mineral to mica, 
rather than to bastite or bronzite. But this group of rocks is so 
variable that another investigator may find something which I have 
not noticed. 
Some of the grains, I may observe, which I take for altered mica 
or a chlorite, exhibit a remarkable twin structure—singularly like 
the lamellar twinning of plagioclase—one set of bands extinguishing 
parallel or nearly parallel with the cleavage planes, the other at an 
angle with it which in one case is actually 18°, though generally it 
is less than 10°. Beyond this it has no resemblance whatever to a 
felspar. It might possibly be an enstatite, but I think the reference 
to the mica is more probably correct. I have seen similar twinning, 
mimicking both the ‘albite’ and ‘pericline’ types in an undoubted 
altered pyroxenic mineral—probably enstatite—in a serpentine from 
Carn Sparnack (Lizard), and in other cases. Probably we shall not 
be wrong in adopting Prof. Judd’s explanation, and referring the 
structure to molecular strains, which might well be set up in the 
changes which this mass has undergone. 
It is evident from the microscopic examination that this rock is 
very closely allied to the scyelite (altered mica-hornblende picrite) 
described by Prof. Judd. The published analysis of the latter, as 
1 Microscopic Petrography of 40th Parallel, pl. v. fig. 1. 
* Quart. Journ. Geol. Soc. vol. xxxiii. pp. 910, 913; Teall, British Petrography, 
- 162. 
3 Wadsworth, Lithological Studies, p. 170, etc. 
* One of my slides from the Rill (Lizard) is full of small crystals of a mineral 
which I formerly took to be enstatite (altered), but I now see has a very close resem- 
blance to the bleached mica. 
° Loc. cit. pp. 402, 406. 
