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DOLERITE (from Gr. SoXepos, deceptive)

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Originally appearing in Volume V08, Page 387 of the 1911 Encyclopedia Britannica.
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DOLERITE (from Gr. SoXepos, deceptive) , in See also:petrology., the name given by See also:Hauy to those basaltic rocks which are comparatively coarse grained and nearly, if not. quite, holocrystalline. As may be inferred from their highly crystalline See also:state they are very often intrusive, and occur as dikes and sills, but many of them See also:form See also:lava flows. Their essential minerals are those of See also:basalt, viz. See also:olivine, See also:augite and See also:plagioclase See also:felspar, while See also:hornblende, See also:ilmenite, See also:apatite and hiotite are their commonest See also: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 See also:majority, are non-porphyritic,and these are generally coarser grained than the ground-See also:mass of the former See also:group; though See also:DOLET 3$7 Iacking their large conspicuous phenocrysts. The commonest type of structure in dolerite is the ophitic, which results from the felspar of the See also:rock having crystallized before the augite; the latter See also:mineral forms shapeless masses in which the idiomorphic felspars See also: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 See also:light, the former often behaves as a single individual over a considerable See also:area, while the latter mineral consists of See also:independent crystals. This structure may be so coarse as to be easily detected by the unaided See also:eye, or so See also: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. Ophiti'c structure is commonest in olivine-dolerites, though the olivine takes no See also:part in it.

The See also:

quartz-dolerites are an important group, hardly less See also:common than the olivine-dolerites. They contain a small amount of quartz, and often micropegmatite, as the last See also:element to consolidate, filling up little angular interspaces' between the felspars and pyroxenes, which had previously crystallized. They rarely contain olivine, but pleochroic See also:hypersthene is by no means rare in them (hypersthene-dolerites). Some contain larger individuals of See also:pale See also:green, rather pleochroic augite (the so-called sahlite), and a little See also:brown See also:mica, and brownish-green hornblende may also be See also:present. Allied to these are olivine-See also:free dolerites with more or less of interstitial glassy See also: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 See also:glass, and' pass into See also:tachylytes, in which this material preponderates. Another interesting group of doleritic rocks contains See also:analcite. They may be ophitic, though often they are not, and they usually contain olivine, while their augite has distinctly See also: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 See also:nepheline; others regard it as a See also:primary mineral. Usually it can be clearly shown to be secondary to some extent, but there is See also:reason to suppose that it is really a pneumatolytic See also:deposit.

These rocks are known as teschenites, and have a wide See also:

distribution in See also:England, See also:Scotland, on the See also:continent and in See also:America. Often they are comparatively See also:rich in brown hornblende. This last-named mineral is not usually abundant in "dolerites, but in a See also: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 See also:Tertiary volcanic districts of the See also: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 kerb-stones, though their dark See also:colour and the tendency they have to See also:weather with a dingy brown crust make them unsuitable for the better classes of architectural See also:work. (J. S.

End of Article: DOLERITE (from Gr. SoXepos, deceptive)

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DOLET, ETIENNE (1509-1546)

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