Basalt

“ All formations are, therefore, common to every quarter of the globe and assume the like forms. Everywhere basalt rises in twin mountains and truncated cones; everywhere trap-porphyry presents itself to the eye under the form of grotesquely-shaped masses of rock, while granite terminates in gently rounded summits. ”

“ Basalt is a heavy igneous rock that is black, dark gray, or dark brown. This rock is made up chiefly of a feldspar mineral, such as labradorite, and a pyroxene mineral, such as augite. Other minerals may be present.
The mineral grains of some basalts are so small that you cannot distinguish them even with a magnifying glass. Other basalts, however, are porphyritic, which means that they contain larger, easily seen crystals and grains of feldspar and pyroxene scattered either through a mass of the small mineral grains or through glassy material. ”

“ Among the numerous species of the trap family here existing, basalt is the most conspicuous, and it occurs almost every where, alternating in an irregular manner with all the other species or varieties. It is most frequently amorphous, displaying at the same time so great a variety both in its natural mode of breaking, in its external appearance on weathering, and in its texture and colour, as to form a great number of subordinate varieties much more remarkable in their natural situations than when broken into hand specimens. ”

“ Many of the boulders have been rounded by weathering, and their shape, together with their dark color, has stimulated the local name “niggerhead” for them.
Basalt in hand specimen.
Basalt is a strong, tough, well-knit rock that will withstand heavy blows from a sledge hammer, which usually rebounds upon striking. Except for use as rubble stones, basalt has no commercial value. It weathers characteristically to a yellowish, brownish, or reddish surface coating of iron oxide and clay. ”

“ A conspicuous band of coarse basalt-agglomerate, containing blocks of compact and amygdaloidal basalt a yard or more in diameter, shows by the excessive induration of its dull-green matrix the general alteration which the rocks of the basalt-plateau have here undergone. An almost incredible number of veins of fine basalt, porphyry and felsite has been injected into these rocks—a structure which is precisely a counterpart of what occurs under the main body of gabbro in Skye, Ardnamurchan and Rum. ”

“ Early work in France involved substituting melted basalt for glass and was not directed toward improving.the product over the raw material. German researchers advanced another step by evolving a technology for recrystallizing the melt and casting it into simple shapes. The Soviet Union began experimentation with basalt in the 1930s at the Moscow Rock Foundry Works. Processed basalt currently is being manufactured either on a pilot or factory scale in Czechoslovakia, Poland, Sweden, Italy, and the United States. ”

“ On the lower slopes of the mountain as high as 1,100 or 1,200 feet is exposed a porphyritic doleritic basalt showing semi-ophitic augite and abundant interstitial glass. Its specific gravity is 2·8, but there is no olivine. It belongs to a type of basalt described under genus 9, sub-genus A, of the augite-andesites. The upper double-peaked portion rises precipitously, displaying bare rocky cliff-faces with a drop of 100 or 150 feet, and formed in mass of a grey andesitic rock with a specific gravity of 2·71 and showing abundant small porphyritic crystals of hornblende and rhombic pyroxene. ”

“ Basalt is the prevailing rock, assuming the prismatic form, and passing into greenstone, and the various modifications of amygdaloid and porphyry. The general direction of the mountain ranges inclines to the S.W. and N.E., varying in height generally from 500 to 2500 feet. Many of the hills are intersected by trap dykes; these dykes are of very frequent occurrence, and are usually of basalt. ”

“ Veins of perfectly characterized basalt also occur in some places, and these, or fragments of them may in particular be observed in great quantity strewing the top of the first summit, (that one which is marked by two cairns,) and laying fair claim to an equal antiquity with the veins of porphyry. It is easy to procure detached specimens of the junction between the granite and basalt in great variety and abundance. The line of junction is in all cases clear and well defined, but does not admit of ready separation even after long exposure to weather. ”

“ Both these rocks are composed of triclinic feldspar and augite with more or less olivine, magnetic or titaniferous oxide of iron, and usually a little nepheline, leucite, and apatite; basalt usually contains considerably more olivine than melaphyre, but chemically they are closely allied, although the melaphyres usually contain more silica and alumina, with less oxides of iron, lime, and magnesia, than the basalts. ”

“ The major advantages of basalt are its easy availability, its tolerance of machining, good compressive strength, and high density in some uses. Anticipated applications include machine support bases, furnace support walls, robot manipulator tools (to avoid vacuum welding) , and other special parts where weight is not a problem. Because plaster fuses at 1720 K - very near the melting point of basalt - and loses its water of crystallization around 475 K, it cannot be used to make basalt castings. ”

“ The basaltic hills, in those parts which I have visited, are covered, as in the neighbourhood of Hobarton, with a good-bodied clay, chiefly of a red tint, encumbered with masses of basalt, in a more or less forward state of degeneration. The chief ranges are of this description at the summit. In the lower levels sandstone often crops out, having the basalt overlying and subjacent. Indeed, basalt may be said to be the prevalent rock. The peninsula of Point Puer is formed of an indurated clay, containing fossil remains, consisting chiefly of marine shells, gorgoniæ, corals, &c. ”

“ The basalt, which is one of the igneous rocks, has been erupted into the coal-seam in a state of fusion, and the heat contained in it has been sufficient to cause the disentanglement of the gases, the extraction of which from the coal brings about the condition of anthracite and graphite. ”

“ It may be an interesting matter of speculation to inquire by what power the vicinity of trap operates in influencing the change from shale to Lydian stone, as well as in producing the much better known changes which occur in sandstone bordering on trap. If basalts have been in any case produced by the fusion of beds of slate, the necessary analogy between the Lydian stone and basalt will appear conspicuous, and we have only to consider it as a shale brought into the state of basalt by fusion, without such further disturbance as to destroy its original stratification. ”

“ I have here, as on other occasions, applied to these veins the general term trap, for the same reasons which I assigned in speaking of the rocks of this class, namely, that they vary in composition, although basalt is certainly the prevalent substance in them here and every where else. The order which I have allotted for them in this description is also the order which they hold in nature, since they traverse every rock that lies in their way from the most ancient to the most recent, seldom suffering any change either of direction or composition in this varying course. ”

“ The principal varieties of trappean rock are basalt, green stone, and trachyte. In basalt, augite, or, in some cases, hornblende, is the predominant mineral. It is a heavy, close-grained rock, of a black or dark brown color. Greenstone differs from basalt in containing a much larger proportion of felspar. Its structure is more granular, and frequently it assumes so much of the crystalline form as to pass insensibly into syenite or granite. ”

“ Intrusive basalt sheets have been intercalated between some of the Carboniferous strata, and the superior resisting power of this rock has been the cause of the existence of West Lomond, Benarty, Cleish Hills and Bishop Hill, which are formed of soft marls and sandstones capped by basalt. ”

“ Near the base of this hill, I observed beds of white tuff, intersected by numerous dikes, some of amygdaloidal basalt and others of trachyte; and beds of slaty phonolite with the planes of cleavage directed N.W. and S.E. Parts of this rock, where the crystals were scanty, closely resembled common clay-slate, altered by the contact of a trap-dike. The lamination of rocks, which undoubtedly have once been fluid, appears to me a subject well deserving attention. On the beach there were numerous fragments of compact basalt, of which rock a distant façade of columns seemed to be formed. ”

“ Its lower part is composed of stratified Palagonite clay, which higher up becomes a friable conglomerate, embedding compact and cellular basalt, mostly in small fragments. The heaps at the base are simply slippings, disposed at the natural angle, and they are garnished with many blocks the size of an Iceland room. Above them rise the organs, buttresses, and flying buttresses, resembling pillars of mud, several exceeding 300 feet; the material assumes the most fantastic shapes: in one place I found a perfect natural arch resting upon heat-altered basalt. ”

“ It has been observed that the inhabitants of basaltic localities are more generally natural clairvoyants than others. Basalt is an igneous rock composed largely of augite and felspar, which are silicate crystals of calcium, potassium, alumina, etc., of which the Moonstone is a variety. ”

“ They are usually dull granular quartz-porphyries, or granophyres, often only two or three feet in thickness, and interposed between the beds of basalt that form the mass of the hills. Along the crest of the ridge that stretches through Beinn Chreagach Mhor to Mainnir nam Fiadh they take a prominent place among the ledges of basalt, basalt-conglomerate and dolerite. The largest sheet in Mull is probably that which has thrust itself between the base of the basalts and the underlying Jurassic strata and crystalline-schists on the shore of the Sound of Mull at Craignure. ”

“ The compressional strength and elastic modulus are quite high; that is, basalt as a construction material is far more rigid than other substances listed, a quality of some importance in large space structures. One drawback is tensile strength, roughly an order Of magnitude lower for basalt than other typical construction materials. This problem can be overcome either by designing structures such that basalt components are not exposed to high tensile or extensional stress states or by producing a compound basalt reinforced with fibers. ”

“ In this last direction, all the strata attain their greatest height, the chalk standing in Slieve Gallion at 1460 feet and upwards, and the basalt in Cragnashoack at 1864 feet. It is rather remarkable that the cap of basalt grows gradually thinner in proceeding towards the same point. On the north-east edge of this basin, the inferior limit of the basalt, or its junction with the chalk, is 500 feet lower, but the thickness of the cap of trap, increasing in the inverse ratio of this diminution of level, raises the summit of Knocklead till it nearly rivals Cragnashoack. ”

“ When leucite or nepheline replaces plagioclase, the rock becomes a leucite-basalt, [1] or nepheline-basalt. Some basalts have a glass paste, or "ground-mass," in which the minerals are enclosed.
The lava of Vesuvius may be regarded as a variety of basalt in which leucite replaces plagioclase, although this latter mineral [Pg 261] is also present. Zirkel calls it "Sanidin-leucitgestein," as both the macroscopic and microscopic structure reveal the presence of leucite, sanidine, plagioclase, nephiline, augite, mica, olivine, apatite, and magnetite. ”

“ The palagonitic breccias, which stretch in an irregular belt across the island, are younger than the basalt. In the north-west, north and east the coasts are formed of basalt, and rise in steep, gloomy walls of rock to altitudes of 3000 ft. and more above sea-level. Deposits of clay, with remains of plants of the Tertiary period, lignite and tree-trunks pressed flat, which the Icelanders call surtarbrandur, occur in places in the heart of the basalt formation. These fossiliferous strata are developed in greatest thickness in the north-west peninsula. ”

“ Some assert that the basalt and the nickel-iron have been expelled together from great depths below the earth's surface, while others consider that the nickel-iron is due to the reduction of the iron-compounds in the basalt by the passage of the lava through the beds of lignite and other vegetable matter found in the vicinity. ”

“ I do not possess artificial basalt resembling the natural columnar kind; yet the slags obtained at the furnaces of Sahla resemble basalt so perfectly, as to deceive the most experienced eye, especially as their cavities contain crystals of augite. But I have found at Fahlun a bisilicate of protoxide of iron, which has in consequence a composition analogous to that of basalt, and which has distinct joints. In this slag we perceive that the joints, which are parallel to the axis of the prism and to the lateral planes of the crystals, are always perpendicular to the plane of cooling. ”

“ Coxe's Rough, Basalt Quarry.
If these beds be connected with the basalt, which seems almost certain, it shows that the basalt was an eruptive [4] rock, poured out at the surface in the form of a sheet of lava, whether subaqueous or subaerial, immediately after the formation of the part of the Coal-measures which lie below it. These trappean breccias or conglomerates belong to and pass into the Coal-measures, and therefore the basalt of the Rowley Hills also belongs to and forms part and parcel of the Coal-measure series. ”