

18 STRUCTURAL AND FIELD GEOLOGY 
crystallise like them in monoclinic and orthorhombic forms. 
They differ amongst themselves as regards fusibility—those 
containing much iron being usually more fusible than 
the less ferriferous varieties. The monoclinic forms are 
divisible, like the corresponding amphiboles, into non- 
aluminous and aluminous types. The xon-aluminous pyroxenes 
are mostly light-coloured—white or, more commonly, some 
pale shade of green. They occur chiefly in crystalline schists 
and in crystalline limestones and marbles, but are not such 
important rock-formers as the corresponding light-coloured 
amphiboles. Their alteration-products; are usually tale or 
serpentine. Of the aluminous pyroxenes the most notable is 
Augite (see Plate IV. 1, 2); it crystallises in prismatic forms, 
which are often twinned. As rock-constituents the crystals 
frequently have their edges and angles rounded off. Augite is 
dark brown to black, but in thin sections may be almost colour- 
less or show various shades of brown or yellow, and sometimes 
of green. It is often altered into an aggregate of chlorite, 
scattered through which may be minute granules of epidote, 
calcite, and quartz; or it may be still further changed to a 
mixture of limonite, quartz, and carbonates. Sometimiesmt 
is replaced by biotite, epidote, calcite, ete. It is amvessemnial 
constituent of such basic rocks as basalt, dolerite, ete, but 
occurs aS an accessory ingredient of many other eruptive 
rocks. Diallage is a brownish, grey, or greenish variety of 
augite, which rarely assumes a crystalline form, and has a 
lamellar or foliated structure. Numerous platy inclusions 
occur along the cleavage-planes, so that the mineral exhibits a 
submetallic lustre on broken surfaces. It is an essential con- 
stituent of gabbro, and occurs also as an occasional ingredient 
of serpentine and olivine-rocks ; but appears never to be met 
with in effusive igneous rocks (lavas). 
Omphacite is a bright green pyroxene occurring in granular and lamellar 
aggregates, and associated with smaragdite and red garnet in the rock 
known as eclogite. 
The orthorhombic pyroxenes (hardness, 4 to 6; specific gravity, 3 to 
3:5) play a more important réle as rock-formers than the orthorhombic 
amphiboles. There are three types recognised—namely, zstatzte, 
Bronzite, and [ypersthene—but they seem to be varieties of one and the 
same species. Enstatite is a silicate of magnesium with a small per- 
centage of iron. It is greenish-white usually, but sometimes darker 
