s the last element to consolidate, filling up little
angular interspaces between the felspars and pyroxenes, which had
previously crystallized. They rarely contain olivine, but pleochroic
hypersthene is by no means rare in them (hypersthene-dolerites). Some
contain larger individuals of pale green, rather pleochroic augite (the
so-called sahlite), and a little brown mica, and brownish-green
hornblende may also be present.

Allied to these are olivine-free dolerites with more or less of
interstitial glassy base (tholeites, &c.). In the rocks of this group
ophitic structure is typically absent, and the presence of an
interstitial finely crystalline or amorphous material gives rise to the
structure which is known as "intersertal." Transitions to the
porphyritic dolerites and basalts arise by increase in the proportion of
this ground-mass. The edges of dolerite sills and dikes often contain
much dark brown glass, and pass into tachylytes, in which this material
preponderates.

Another interesting group of doleritic rocks contains analcite. They may
be ophitic, though often they are not, and they usually contain olivine,
while their augite has distinctly purple shades, and a feeble dichroism.

Their characteristic feature is the presence of a small amount of
analcite, which never shows crystalline outlines but fills up the
interspaces between the other minerals. Some writers held that this
mineral has resulted from the decomposition of nepheline; others regard
it as a primary mineral. Usually it can be clearly shown to be secondary
to some extent, but there is reason to suppose that it is really a
pneumatolytic deposit. These rocks are known as teschenites, and have a
wide distribution in England, Scotland, on the continent and in America.
Often they are comparatively rich in brown hornblende. This last-named
mineral is not usually abundant in dolerites, but in a special group,
the proterobases, it to a large extent replaces the customary augite. A
few dolerites contain much brown mica (mica-dolerites). Nepheline may
appear in these rocks, as in the basalts. Typical nepheline-dolerites
are scarce, and consist of idiomorphic augite, surrounded by nepheline.
Examples are known from the Tertiary volcanic districts of the Rhine.

Dolerites have a very wide distribution, as they are found wherever
basalts occur in any number. It is superfluous to cite localities for
them as they are among the commonest of igneous rocks. They a