258 Dr. F. H. Hatch—A Peridotite from Kilimanjaro. 
Freshly fractured surfaces of this rock present numerous glistening 
facets, which with the aid of the pocket-lens are seen to be the cleavage- 
faces of a yellowish-green, a dark-green, and a garnet-red mineral. 
These minerals are respectively olivine, hornblende, and hypersthene 
in nearly equal proportions, the hornblende being perhaps slightly 
predominant. They form a holocrystalline granular aggregate, in 
which the grains are of nearly equal size and without crystalline 
contours; in other words, the structure is allotriomorphic-granular. 
Subordinate both in size and quantity to the three principal 
constituents is magnetic iron-ore. 
A rude foliation or banding, which is only just perceptible in the 
hand-specimen, is rendered apparent under the microscope by a 
tendency to uniform orientation of the constituents, more especially 
of the hornblende-grains. This seems to indicate that the rock, like 
many other peridotites, constitutes, in sti, an integral part of a banded 
crystalline series—an assumption which by no means excludes the 
possibility of igneous origin.’ 
Under the microscope a brilliant picture is produced by the 
contrast between the bright grass-green of the hornblende and the 
beautiful salmon colour of the hypersthene, the olivine-grains being 
colourless. On rotating the section in polarized light (without the 
upper Nicol) the striking pleochroism of the hypersthene becomes 
visible. In order to determine the colour of the rays vibrating 
parallel to the different axes of elasticity, sections were sought in 
which the cleavage-lines intersected at an angle of about 90°. Such 
sections present, in convergent polarized light, the emergence of an 
axis of least elasticity (vy). In parallel light the colours of the rays 
vibrating parallel to a and f can be determined. They are :— 
a=rich salmon-red. 
B=very pale yellow (almost colourless). 
Sections which have only one set of cleavage-lines extinguish 
straight, and give for rays vibrating in this direction (y) the 
characteristic pale sea-green colour. 
Thus we have a > y > £. 
’ The hornblende, which is of a rich green colour, is chiefly 
remarkable for the imperfection of its cleavage-cracks, these being 
not nearly so sharply defined as is usual in this mineral. Its 
relation both to the olivine and to the hypersthene shows that the 
hornblende was the last product of consolidation. It is found, for 
instance, enclosing grains of both hypersthene and olivine—a 
structure which, when more largely developed, produces the ‘ lustre- 
mottling’ described by Williams and others. The pleochroism, 
which is marked, is as follows :— 
a=pale yellow. 
B=yellowish green. 
y=bluish green. 
Mh Bore se 
1 Thomson (‘¢ Through Massai-Land,’’ London, 1885) repeatedly mentions the 
occurrence of metamorphic rocks in this neighbourhood. ‘They are seen, according to 
