lspar, with subordinate quantities
of hornblende, biotite, iron oxides and apatite.

There are olivine-diabases and diabases without olivine;
quartz-diabases, analcite-diabases (or teschenites) and hornblende
diabases (or proterobases). Hypersthene (or bronzite) is
characteristic of another group. Many of them are ophitic, especially
those which contain olivine, but others are intersertal, like the
intersertal dolerites. The last include most quartz-diabases,
hypersthene-diabases and the rocks which have been described as
tholeites. Porphyritic structure appears in the diabase-porphyrites,
some of which are highly vesicular and contain remains of an abundant
fine-grained or partly glassy ground-mass (_diabas-mandelstein_,
amygdaloidal diabase). The somewhat ill-defined spilites are regarded
by many as modifications of diabase-porphyrite. In the intersertal and
porphyrite diabases, fresh or devitrified glassy base is not
infrequent. It is especially conspicuous in some tholeites
(hyalo-tholeites) and in weisselbergites. These rocks consist of
augite and plagioclase, with little or no olivine, on a brown,
vitreous, interstitial matrix. Devitrified forms of tachylyte
(sordawilite, &c.) occur at the rapidly chilled margins of dolerite
sills and dikes, and fine-grained spotted rocks with large spherulites
of grey or greenish felspar, and branching growths of brownish-green
augite (variolites).

To nearly every variety in composition and structure presented by the
diabases, a counterpart can be found among the Tertiary dolerites. In
the older rocks, however, certain minerals are more common than in the
newer. Hornblende, mostly of pale green colours and somewhat fibrous
habit, is very frequent in diabase; it is in most cases secondary
after pyroxene, and is then known as uralite; often it forms
pseudomorphs which retain the shape of the original augite. Where
diabases have been crushed or sheared, hornblende readily develops at
the expense of pyroxene, sometimes replacing it completely. In the
later stages of alteration the amphibole becomes compact and well
crystallized; the rocks consist of green hornblende and plagioclase
felspar, and are then generally known as epidiorites or amphibolites.
At the same time a schistose structure is produced. But transition
forms are very common, having more or less of the augite remaining,
surrounded by newly forme