, and various types of twinning. It weathers to chlorite, uralite,
calcite, &c. The plagioclase felspar, if fresh, is transparent and appears
simple in ordinary light, but when polarized breaks up into a series of
bars of different colours owing to its complex twinned structure.
Practically all varieties of this mineral from anorthite to albite are
known to occur in basalt, but by far the commonest species are bytownite
and labradorite. Weathering destroys the limpid character of the fresh
mineral, producing turbid pseudomorphs containing epidote, calcite, white
micas, kaolin, &c. When these minerals occur as phenocrysts their
crystalline outlines may be very perfect (though, especially in the
olivine, corrosion and partial resorption may give rise to rounded or
irregular forms).

In the groundmass, or second generation of crystal, not only are the
ingredients smaller, but their crystals are less perfect; yet in many
basalts small lath-shaped felspars and minute prisms of augite, densely
crowded together, form the matrix. With these there may be a greater or
less amount of brown, isotropic glass. Olivine rarely occurs as an
ingredient of the groundmass. In the vitreous basalts sometimes very few
crystallized minerals are observable; the greater part of the rock is a
dark brown glassy material, almost opaque even in the thinnest sections,
and generally charged with black grains of magnetite, skeleton crystals of
augite or felspar, spherulites, perlitic cracks, or steam vesicles. In
other basaltic rocks no glassy material appears, but the whole mass is
thoroughly crystallized; rocks of this nature are generally known to
British petrologists as dolerites (_q.v._). Till recent years it was widely
believed by continental geologists that the pre-Tertiary basalts differed
so fundamentally from their Tertiary and recent representatives that they
were entitled to be regarded as a distinct class. For the older rocks the
names anamesite, diabase porphyrite, _diabas-mandel-stein_, or melaphyre
were used, and are still favoured by many writers, to indicate varieties
and states of more or less altered basalts and dolerites, though no longer
held to differ in any essential respects from the better preserved basalts.
Still older is the term _trap_, which is derived from a Swedish word
meaning "a stair," for in many places superposed sheets of basalt weather
with well-marked step-like or terraced features. This designation is still
used