The basalts are dense fine-grained volcanic rocks that are of very dark color, green or black. They are of basic composition and high specific gravity, representing the extrusive or volcanic equivalent of the plutonic gabbros. These rocks are formed under quick cooling near surface eruptive conditions. Their fabric is so fine that the constituent grains either cannot be perceived by the eye or with the lens, or, if seen, are too small to be recognized, and which are of stony but not of glassy texture. In the great majority of cases they do not show translucency on the edges of thin flakes as described under felsite. They commonly show cellular voids formed of contained gasses which were emitted during the cooling process. When not cellular, and very dense, they have a uniform dull, soft, almost velvety appearance, and do not show the horny, flinty, or greasy luster of many dense felsites.
When fresh, they are characterized by their black color, and minutely crystalline to compact texture and high density. When altered or decomposed by the action of the weather, they assume a reddish or greenish hue, and are much veined by decomposition products. A further stage of decomposition converts the basalt into a loose crumbling material known as wacke, which is but little removed from clay in composition. Microscopic examination of a fresh basalt shows it to be composed essentially of plagioclase (labradorite or anorthite) and augite. When olivine is present, the rock is termed olivine-basalt. In addition to the augite there are often present partially fused fragments of brown mica and of hornblende.
Basalt occurs in "flows" or beds of great extent and variable thickness, forming characteristic plateau and terraced hills (Snake River Plains of Oregon, the Deccan in India, the Western Isles of Scotland and the North of Ireland); also in dykes which represent, in many cases, the fissures through which the molten lava welled out. The fact that in these vast plateau there is no thickening toward particular centers from whence eruption might have been supposed to take place, and no accumulation of pyroclastic material, is the main evidence on which the "fissure eruption" theory is based. The "bedded" basalt is slaggy and amygdaloidal on the outer surface. Where basalt has been quickly cooled, by contact with cold rock, a thin layer or selvage of black opaque glass is invariably present.
Accessory minerals present in the basalts are ilmenite and magnetic iron-ore, in black opaque grains; apatite in colorless needles; and zircon, in conspicuous red grains. Isolated and much-corroded quartz grains do sometimes occur; but it is an open question whether they are indigenous to the basalt or are xenocrysts caught up during its eruption. Chlorite, calcite, and epidote, either disseminated or filling amygdaloidal cavities, occur abundantly in basalts that have undergone alteration. The chlorite is derived mainly from the decomposition of the augite; the calcite and epidote, from the feldspar; while the olivine is almost entirely replaced by serpentine, limonite, magnetite, or carbonates of lime and magnesia. Zeolites (natrolite, phillipsite, scolecite) and agates are seen frequently as the material cfilling the amygdale voids of the basalts.
While porphyritic varieties of basalt are not uncommon rocks it may be said in general, that this type of texture plays a far less important role in this group than in the felsites previously described, owing probably to the low freezing point and easy flow characteristics and crystallization of the magma. One exception to this, however, is in the presence of olivine, which is very apt to occur scattered through the basalt in transparent yellowish or bottle green porphyritic grains, averaging a few millimeters in size. The mineral is so common (indeed at one time a rock was not considered a basalt unless it contained olivine) and produces in general so little of a striking porphyritic effect, that it is rather the custom to ignore it in this respect, and term such rocks olivine-basalt rather than olivine-basalt-porphyry. The chief minerals as phenocrysts, when such occur, are feldspar and pyroxene; hornblende and mica are much less common. The feldspar is commonly labradorite; it occurs in elongate tabular forms, either singly, or in twinned groups. In a few basalts, these feldspar crystals can be quite large and be clear enough to use as gemstones. The pyroxene is the variety augite; it is black, sometimes shining, sometimes dull, and is in short thick prisms or prismoids, as illustrated under pyroxene. The hornblende is also black and has its usual shining and good cleavage. Biotite is in six-sided tablets.
It frequently happens that basalts, in addition to the ordinary crystals of olivine, contain nodules in yellowish, or green lumps, made up of grains of this mineral. Grains of other minerals, such as pyroxene, spinel, etc., may be present in them. These lumps may vary in size from a pea to masses as large as one's fist, or even larger. They are generally rounded, but often distinctly angular in shape. Their origin is somewhat problematical; some hold that they are merely agglomerations of the earlier formed crystals in the liquid magma, while others regard them as fragments of rock (dunite) torn off below and brought up in it.
Basalts sometimes contain unusual and exotic minerals, which do not appear in the ordinary rock, and whose origin in them must be ascribed to unusual conditions, or composition of the magma. The quartz basalt mentioned above is one of these. Another case is seen in the iron-bearing basalts of Greenland, which contain small to large masses of native iron, which is much like the iron found in meteorites. By the use of a solution of copper sulfate specks of native iron have been found in basalts from other places. The Greenland basalts also contain graphite. Corundum, in the form of sapphire, has also been found in basalts, and a dike at Yogo Gulch in Montana has furnished a quantity of valuable gems. In this connection also, may be mentioned the occurrence in places of native copper, especially in the Lake Superior district, where the metal occurs in dolerites and basalts and in connection with them, in quantities which have made it one of the most important sources of the world's copper ore supply.
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