22 STRUCTURAL AND FIELD GEOLOGY 
yellowish-green or olive-green, has a glassy lustre, and breaks 
with a conchoidal fracture. As a rock-former it sometimes 
constitutes the whole mass or the larger proportion of a rock, 
as in dunites (peridotites). It is present also in many other 
igneous rocks—more especially in those of basic and inter- 
mediate composition, as certain gabbros, basalts, and fels- 
pathoid rocks. It is readily recognised in such rocks by the 
naked eye as granules or blebs, usually of a greenish tint 
with a glassy lustre, and showing its conchoidal fracture. 
Now and again it occurs in basalts as large granular agegre- 
gates resembling nodules, some of which may measure 5 or 
6 inches across, but they are generally smaller. forsterite, 
a light-coloured variety, is met with as a “contact mineral” in 
metamorphosed limestones. In nature, olivine alters readily to 
serpentine; probably, indeed, most serpentines have originated 
from the alteration of olivine-rocks. The finely coloured 
(yellow or green) transparent varieties of olivine are used in 
jewellery, and are known as Chrysolite and Peridote. 
In thin rock-slices olivine is usually almost colourless, but may show 
pale yellowish-green or yellowish-brown tints. In basic eruptive rocks 
it appears sometimes in good crystal forms, with lozenge-shaped or long 
rectangular outlines (see Plate V. 1, 2), but the outlines are frequently 
rounded as if from magmatic corrosion. It shows high relief, the out- 
lines of the mineral and the cracks traversing it being strongly 
pronounced. It is not pleochroic, but polarises rather brilliantly. 
THE CHLORITE GROUP 
Under this head are included certain greenish coloured 
minerals which are composed essentially of hydrated silicate 
of magnesium and aluminium, usually with some iron. Asa 
rock-former Chlorite occurs in the form of pseudo-hexagonal 
non-elastic plates, but most frequently as bent and irregularly 
bounded scales, tufts, and fibres, or as scaly or earthy 
aggregates. Often it somewhat resembles mica. The hard- 
ness is 2 to 3, and the specific gravity 2.6 to 2-8. The only 
rock largely composed of this mineral is chlorite-schist. It 
occurs frequently, however, in eruptive rocks as a secondary 
product, from the alteration of such minerals as hornblende, 
augite, biotite, etc. Many igneous rocks, indeed, owe their 
