110 Prof. T. G. Bonney—Picrite in Sark. 
penninite. The specific gravity of the rock is 2°88.' Examined 
with the microscope the rock is seen to be composed mainly of the 
following minerals :— 
1. Olivine, i in process of conversion into serpentine. The grains, 
in diameter from about -07” downwards, exhibit the characteristic 
network structure produced by ‘strings’ of serpentine, often 
blackened, as usual, with opacite, but give brilliant chromatic 
polarization in the interstices occupied by the unaltered olivine. 
2. A pyroxenic mineral, often in larger grains, most of which 
is either colourless or of a very pale greenish-grey tint. 
Evidently more than one variety or species is present. Occasionally 
the cleavage of a hornblende may be recognized; but much, 
especially of the greener variety, has an acicular habit, though the 
needles lie parallel ; thus, though sections be taken at right angles, 
the characteristic cleavage of hornblende is much more rarely seen 
than one would expect. These acicular or possibly platy groups 
often have their divisional planes spotted with opacite and are 
traversed by thin strings of serpentine. They extinguish simul- 
taneously and usually at angles below 20°, but occasionally range 
up to 24°. Moreover, here and there grains occur with augite 
cleavage or augite extinction, colourless or a very light brown, some 
of which, with crossing Nicols, have a curious pallid ‘moribund’ 
aspect. These are commonly surrounded by a felted mass of a 
fibrous mineral, which gives brilliant tints with the polarizing 
apparatus. It occurs elsewhere in the slide, and sometimes almost 
resembles a ground-mass. I regard it as a form of actinolite. On 
the whole I believe that the original mineral of the rock was augite, 
perhaps a little variable in composition, and that, as has often been 
noted in picrites, we now find it in different stages of alteration into 
varieties of hornblende. 
3. A mica-like mineral. Some of this exhibits a distinct though 
rather weak dichroism, changing from colourless with vibrations 
perpendicular to the basal plane to pale brown with vibrations 
parallel with it. Occasionally there is a slight ‘ edging’ of pale dull 
green. This mineral, when rotated from the positions of extinction 
(parallel with the cleavage planes), exhibits rather brilliant colours, 
and the peculiar mottled or ‘shot’ look so common ina mica. The 
remainder (and greater part) of the mineral is perfectly clear and 
colourless, and between the crossed Nicols only varies from black to 
a dull pallid grey or white. Rods or plates of opacite are frequently 
present between the cleavage planes. I have no doubt that the 
former is a magnesia-biotite. probably similar to that described by 
Prof. Judd? in the scyelite of Caithness, and the latter represents 
an alteration product of the same. I find a rather similar mineral 
in some of my slides of chlorite schists, and Prof. Judd’s analysis 
does not differ much from that of a penninite, where the percentage 
of magnesia is rather low. Certainly a rather large plate in one of 
my hand-specimens much resembles that mineral. There are also 
1 Kindly determined for me by Mr. J. H. Holland in Prof. Judd’s laboratory. 
2 Quart. Journ. Geol. Soc., 1888, vol. xli. pp. 401-407. 
