

50 STRUCTURAL AND FIELD GEOLOGY 
actinolite. The pyroxenes (diallage and hypersthene) almost invariably 
exhibit a characteristic pearly or submetallic lustre (=schzllerisation), 
due to the development of thin brown films along their cleavage-cracks. 
The gabbros are widely distributed, occurring as plutonic masses, sills, 
and dykes. 
Dolerite is for the most part holocrystalline, and varies 
in texture from medium-grained to coarsely granular. The 
chief constituents are plagioclase, augite, and iron-oxides. 
Occasionally, some interstitial glass may occur. Olivine, 
hypersthene, biotite, hornblende or quartz may be present, 
and thus give rise to varieties, as Olzvzne-dolerite, Hypersthene- 
dolerite, Mica-dolerite, etc. 
The plagioclase usually occurs as well-formed crystals and microlites, 
which penetrate or are enclosed as endomorphs in the ferromagnesian 
mineral (augite or olivine). This is known as off/ztic structure (see 
Plate VIII. 4). Quartz, when present as an original constituent, is 
usually devoid of crystallographic form, but is sometimes intergrown 
with the felspar = micropegmatitic or granophyric structure. 
The common accessory ingredients are magnetite, ilmenite, 
and apatite. Dolerite is a dark coloured rock, the medium 
grained types being almost black when fresh, while the 
coarser grained varieties are speckled dark green (or black) 
and white (or pale pink). Owing to decomposition of the 
ferromagnesian constituents, however, many dolerites have 
a dull, dark greenish colour. Rocks of this altered type are 
known as Dzabase. In these the plagioclase is often aitered 
into an aggregate of granules of epidote, calcite, kaolin, etc., 
while the ferromagnesian minerals are usually largely replaced 
by chlorite, serpentine, etc., and the ilmenite more or less 
changed into leucoxene (titanite). Dolerite (Diabase) occurs 
usually in bosses, sills, and dykes, and is very widely 
distributed. 
Basalt.—This is a greyish-black to black, heavy rock, 
so compact, as a rule, that the mineral constituents cannot 
be recognised by the naked eye. Frequently, however, small 
phenocrysts are present, some basalts being markedly por- 
phyritic with such minerals as plagioclase, augite, and olivine. 
Microscopic examination shows that the rock consists of an aggregate 
of small crystals and crystalline granules of plagioclase, augite, and 
usually olivine (see Plate IV.). Almost constant accessories are magnetite 
and ilmenite. Interstitial glass is frequently present, plentifully or 
