Dolerite

Dolerite (from Gr. δολερός, deceptive), in petrology, the name given by Haüy to those basaltic rocks which are comparatively coarse grained and nearly, if not quite, holocrystalline. As may be inferred from their highly crystalline state they are very often intrusive, and occur as dikes and sills, but many of them form lava flows. Their essential minerals are those of basalt, viz. olivine, augite and plagioclase felspar, while hornblende, ilmenite, apatite and biotite are their commonest accessory ingredients. The chemical and microscopic features of these minerals agree generally with those presented in the basalts, and only their exceptional peculiarities need be mentioned here. Many dolerites are porphyritic and carry phenocrysts of olivine, augite and plagioclase felspar (or of one or more of these). Others, probably the majority, are non-porphyritic, and these are generally coarser grained than the ground-mass of the former group, though lacking their large conspicuous phenocrysts. The commonest type of structure in dolerite is the ophitic, which results from the felspar of the rock having crystallized before the augite; the latter mineral forms shapeless masses in which the idiomorphic felspars lie. The augite enclosing the felspars is well crystallized, though its continuity is interrupted more or less completely by the numerous crystals of felspar which it envelops, and in polarized light the former often behaves as a single individual over a considerable area, while the latter mineral consists of independent crystals. This structure may be so coarse as to be easily detected by the unaided eye, or so fine that it cannot be seen except in microscopic sections. Some of the porphyritic dolerites have ophitic ground-masses; in others this structure is imperfect (subophitic); while in many the augite, like the felspar, occurs as small and distinct individuals, which react differently on polarized light, and have the outlines of more or less perfectly shaped crystals. Ophitic structure is commonest in olivine-dolerites, though the olivine takes no part in it.

The quartz-dolerites are an important group, hardly less common than the olivine-dolerites. They contain a small amount of quartz, and often micropegmatite, as 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 are much employed for road-mending and for kerbstones, though their dark colour and the tendency they have to weather with a dingy brown crust make them unsuitable for the better classes of architectural work.

(J. S. F.)

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