30. _Subdivisions._--In their chemical and mineralogical composition,
igneous rocks offer great variety; but they all agree in having
felspar for their base. They may be roughly divided into two classes,
distinguished by the relative quantity of silica which they contain.
Those in which the silica ranges from about 50 to 70 or 80 per cent.
form what is termed the _acidic_ group; while those in which the
percentage of silica is less constitute the _basic_ group of igneous
rocks, so called because they contain a large proportion of the
heavier bases, such as _magnesia_, _lime_, oxides of iron and
manganese, &c. Igneous rocks vary in texture from homogeneous,
compact, and finely crystalline masses up to coarsely crystalline
aggregates, in which the crystals may be more than an inch in
diameter. Sometimes they are dull and earthy in texture, at other
times vesicular. When the vesicles are filled up with some mineral,
the rock is said to be _amygdaloidal_, from the almond shape assumed
by the kernels filling the cavities. When single crystals of any
mineral are scattered through a rock, so as to be readily
distinguished from the compact or crystalline base, the rock becomes
_porphyritic_.