32. _Basalt_ is a dark or almost black compact homogeneous rock,
composed of felspar and augite with magnetic iron. An olive-green
mineral called _olivine_ is very frequently present. The coarser-grained
basalts are called _dolerite_. The columnar structure is not peculiarly
characteristic of basalt. Many basalts are not columnar, and not a few
columnar rocks are not basalts.
_Greenstone_ or _diorite_ is usually a dull greenish rock, sometimes
gray, however, speckled with green. It is composed of soda-felspar and
hornblende. The fine-grained compact greenstones are called _aphanite_.
_Syenite_, like granite, is recognised as an igneous as well as a
metamorphic rock. There are several other rocks which come into the
basic group, but those mentioned are the more common and typical
species.
33. _Fragmental Igneous Rocks._--All the igneous rocks briefly described
above are more or less distinctly crystalline in texture. There is a
class of igneous rocks, however, which do not present this character,
but when fine-grained are dull and earthy in texture, and frequently
consist merely of a rude agglomeration of rough angular fragments of
various rocks. These form the FRAGMENTAL group of igneous rocks. The
ejectamenta of loose materials which are thrown out during a volcanic
eruption, consist in chief measure of fragments of lava, &c. of all
sizes, from mere dust, sand, and grit, up to blocks of more than a ton
in weight. These materials, as we shall afterwards see, are scattered
round the orifice of eruption in more or less irregular beds. The terms
applied to the varieties of ejectamenta found among modern volcanic
accumulations, will be given and explained when we come to consider the
nature of geological agencies. In the British Islands, and many other
non-volcanic regions, we find besides crystalline igneous rocks,
abundant traces of loose ejectamenta, which clearly prove the former
presence of volcanoes. These materials are sometimes quite
amorphous--that is to say, they shew no trace of water action--they have
not been spread out in layers, but consist of rude tumultuous
accumulations of angular and subangular fragments of igneous rocks. Such
masses are termed _trappean agglomerate_ and _trappean breccia_. At
other times, however, the ejectamenta give evidence of having been
arranged by the action of water, the materials having been sifted and
spread out in more or less regular layers. What were formerly rude
breccias and agglomerates of angular stones now become _trappean
conglomerates_--the stones having been rounded and water-worn--while the
fine ingredients, the grit, and sand, and mud, form the rock called
_trap tuff_. Fragmental rocks are often quite indurated--the matrix
being as hard as the included stones. But as a rule they are less hard
than crystalline igneous rocks, and in many cases are loose and
crumbling. When a fragmental rock is composed chiefly of rocks belonging
to the acidic group, we say it is _felspathic_. When augitic and
hornblendic materials predominate, then other terms are used; as, for
example, _dolerite tuff_, _greenstone tuff_.