The singular tendency which Basalt evinces to assume a columnar figure is familiar to the commonest observer: and no constituent material of the solid globe has more frequently been an object of geological speculation in recent times; theories of equivalent ingenuity having contended as vehemently for its igneous, as its aqueous arrangement.
Although various parts of Europe and Asia are abundant with examples of Basaltic Columns, yet the island of Staffa and the Giants' Causeway appear to be the most striking and extensive in the known world. It is extraordinary that the former should scarcely have been known before the year 1772, when it was admirably described by Sir Joseph Banks, on his visit to the Scottish Hebrides. The latter had been better known by the previous description of the Bishop of Ossory.
Basaltic Hills are particularly characterized by a sweeping or undulating outline of their upper surfaces, which are usually clothed with rich verdure, while the sides are steep, naked, and in many instances, literally perpendicular. The most rude and massy rocks of this nature generally exhibit somewhat of a prismatic figure; and in numerous instances, Pillars are formed with the most geometrical exactness. These occur upon the lateral surface of hills, or flank their extremities, commonly perpendicular, sometimes obliquely, and sometimes curvilinear. Occasionally they form recesses, or, by uniformity in height, a causeway, of which the upper surface presents the structure of a Mosaic pavement. They are in some instances the partial courses of rivers, in others, the bosom of the main Ocean.
These columns, which, on the average, vary in height, from three or four to fifty and sixty feet, and in diameter from one foot to two, vary also in the
The laws of Attraction which naturally influence the geometrical figures of those mineral solids, commonly known as Crystals, appear to be exerted in a similar manner for effecting the prismatic character, for which basalt is so truly remarkable. The demonstrable formation of those bodies in the common processes of chemistry, no less than the natural appearances hereafter to be noticed, seem eminently calculated to shew that basalt owes its characteristic figure to the influence of water. This peculiar property of the mass frequently evinces itself in the process of exsiccation, or remains dormant until acted on by the air of the atmosphere. Contrary, then, to the probable operation of volcanic fires, it would appear that the power of heat, suddenly exerted, would not be likely to effect a figure so symmetrical as the more gradual influence of water; since, admitting the prismatic figure of basalt to be truly the result of crystallization, it will be remembered that crystals are more regular the more slowly they are formed.
It may be observed, that numerous substances, when submitted to heat crystallize with considerable symmetry, if that power be not suddenly withdrawn; but of such, the result is not so generally and so geometrically crystalline as of those which have been acted on by a watery menstruum.
Where basalt does not divide itself precisely after the manner of prismatic columns, or evince a tendency to that figure on a great and general scale, it
The latter circumstance, in addition to the occurrence of basaltic fragments, in which a sphere appears to be enveloped by a polyhedral figure, suggested the hint for an opinion which I have been led to adopt — that a compressible laminated sphere is the primitive figure of each prismatic articulation, composing a column of basalt: and that the lateral plane surfaces, as well as the corresponding concavity and convexity of the horizontal surfaces, the trihedral processes of the lateral edges, and the corresponding truncations of the superimposed joint, result from the assemblage of spheres, under the influence of gravitation, while the component particles are in a yielding state; their laminated structure being a mechanical accommodation to the filling of those interstitial spaces, which must result from an aggregation of spheres.
These ideas may be in some degree elucidated by the annexed Plate, in which the first figure exhibits a basaltic fragment from the vicinity of Belfast, where the sphere is developed by a partial decomposition of the prism; while the group shows the prevailing character of basaltic columns in general, two of the constituent prisms being detached, to show the alternate concavity and convexity of the horizontal surfaces, with the angular processes and truncations of their edges.
My view of this phenomenon is not widely different from that of Monsieur Allau, which having previously appeared in the Journal de Physique, was published, as a translation, in the 23d volume of Nicholson's Journal.
In concluding a paper on the contraction of artificial sandstones by fire, the natural division of basalt is alluded to, without any decided reference to the influence of fire or water; conceiving "that cracks will determine spheres of attraction, round which the particles will agglomerate; and the centres will be so much the more numerous, and the radii less as the attractive force is more considerable."
But according to the idea which has occurred to me, it is the central attraction of individual nuclei, which determines the number and figure of the cracks or fissures; or in other words, that laminated spheres of basalt chemically formed by the attractive power of its constituent particles, are mechanically altered by juxta-position under the same influence.
Mr. Gregory Watt by his excellent paper in the Philosophical Transactions appears to have adopted an opinion that a spheroidal figure is concerned in the determination of the basaltic column. But I am not aware that any theory has been suggested similar to that which is here attempted.
Where rocks of basalt, as is well instanced at Port Coane on the coast of Antrim, exhibit simply a spheroidal structure, it appears possible that the phenomenon may have depended upon an imperfect process of crystallization; an inference deducible from the uneven distribution of the constituent particles and, perhaps, also from consideration of the physical qualities of those which are predominant. For example, where alumine prevails, it does not seem impossible that this earth may influence a rapid crystallization. However, it is needless to observe that the characteristic figure of all solids is often either indeterminate, or mutilated, as the disposition of their parts to geometrical arrangement has been opposed or unfavoured by adventitious causes.
Those spheres of basalt, which constitute rock-masses, clearly exhibit numerous concentric laminae, after the manner of a coated bulb: hence, in the north of Ireland, they are vulgarly known as Onion-stones. The laminae are, generally, of a coarse grain, and the globular figure considerably mutilated. In short, it may be said to possess every variety of appearance which different degrees of mechanical compression, exerted on a sphere, can possibly effect and the remark applies equally to the spheroidal basaltes of the southern parts of Europe, and other districts where such minerals prevail.
On the side of Mount Pleaskin or Placekin, near the Giants' Causeway, there is a remarkable appearance of imperfect crystallization, between the two principal ranges of pillars, much resembling the diverging figure of radiated Zeolite. The varieties of zeolite are well known to abound in basaltic regions, particularly the most northern; and that alluded to, impresses the idea of a rapid crystalline process. Intense cold and frost most frequently produce a figure more or less resembling it.
The memorable frost in the beginning of 1814 covered the trees and shrubs of the plantations in London with a beautiful condensation, so exactly resembling tufts of radiated zeolite, that it does not seem unworthy of remark in this place, since I have never observed similar appearances in more open stations; and I am inclined to think the figure was much influenced by
On the islands of Staffa and Rathlin curvilinear prisms of basalt present themselves, which so much resemble the ribs of a ship, that every spectator at first sight expresses the same comparison. I think it possible that this appearance may have been effected by a successive arrangement of compressible laminated spheres, after the manner of a string of beads. Near the Cascade of Vestena, in the territory of Verona, where a similar appearance exists, irregular aggregated bodies (some approaching to a sphere, and some to an imperfect prism) rest within the uppermost segment of the circle. The influence of the laminated sphere seems no less applicable to the various degrees of inclination exhibited by basaltic columns in several parts of the world.
I would now take a comparative view of external resemblances between certain widely distributed masses, of which the constituent materials, differing not in chemical properties, but only in individual proportion, admit of their being termed, with propriety, Basaltic Rocks. Such are Green-stone, Grey-wacke, Porphyry-slate, and Schistus or Clay-slate; conceiving that analogies of external character are at all times worthy of consideration in common with those of chemical composition. The variety of basalt named green-stone is probably next to it the most remarkable for evincing a tendency to the columnar figure. The most familiar examples with which I am acquainted, in Great Britain, are afforded by the rock of Stirling-castle, and others near that town; and by some parts of the hill named Arthur's Seat, near Edinburgh. A small chain of rocks at Linton, in the North of Devon, which flanks the border of the Bristol Channel, commonly known as the Valley of Rocks, to my apprehension, affords the best English example of grey-wacke. These rocks evince a strong tendency to the prismatic figure, and appear in blocks upon the summits of the hills, much after the manner of decomposed granite: their edges and angles are, however, generally more acute, and their texture particularly
In the Philosophical Transactions for 1783, where Dr. Withering has given an account of the Rowley rag-stone, he seems to consider the prevailing figure rhomboidal. The rhomb is, of course, but a modification of the four-sided prism; and the rag-stone of Staffordshire has, in some instances, been seen in articulated columns of five and six sides, similar to the most genuine basalt.
I am not aware that marine exuviae, or other organized bodies, have been found or described in the grey-wacke of Linton; but, on a specimen, which I struck with difficulty from one of the most indurated blocks, there is distinctly the impression of a bivalve shell, much resembling the genus Cardium. Numerous other fractures were not successful in exposing similar appearances23.
Of the many varieties of schistus, or clay-slate, some in general appearance, as well as in chemical composition, have so close a resemblance to grey-wacke, that certain mineralogists have distinguished them by that name. The great repository of the metallic treasures of Cornwall has, in many instances, been so named. Certain hills near the Lizard-point, on proceeding from the town of Helstone, have broken and naked summits, remarkably like those of the Valley of Rocks, both in figure and composition, apparently passing into clay-slate, which, in a compact form, constitutes the Lizard-point, and joins a mass of Serpentine. The natural decomposition of certain rocks, near the well-known, slate quarries of Boscastle, are also worthy of notice, as resembling those of the Linton Valley.
There is certainly an analogy of external character, between tabular basalt and that form of clay-slate which is employed for the roofs of houses; and still more, between a form of clay-slate occurring near Taunton in Somersetshire, and that kind of basalt which forms the walls of Whin-dykes; the former
In whatever part of the world basalt or any of its varieties have been found, phenomena have also been observable which tend, by their similarity, to establish a general distinctive character. The ranges of columns are often accompanied by strata of an ochreous nature, which probably result from the oxydation of a large proportion of iron in their original mass. Some have the appearance of terra cotta, and abound with vesicated cavities; and some are no other than altered Amygdaloids, the base of which is essentially of a basaltic nature.
These appearances, which are common in Ireland, are described also in Auvergne, by Mons. Desmarest.
In the great basaltic district of the North of Ireland, wherever the columnar figure is not effected, the tufted masses which diversify the surfaces of the heights, show much of the tendency on a reduced scale; and such is equally remarkable on many schistose mountains in North Wales, particularly at Pont y Aberglasslyn, and over the whole surface of Cader Idris.
The basaltes which are abundant in Italy and the South of France exhibit various inclinations and curvatures in the prismatic form, like those of Antrim and the Isle of Staffa. Many of the groups are there termed Pavé des Géans. Like the former, also, they bear a relation to Limestone: and this, in the territory of Verona, is crowded with animal remains. They also frequently exhibit laminated spheres: and it is remarkable that Faujas de St. Fond, who adopts the volcanic theory, does not consider this variety as of igneous arrangement.
It appears, by tradition, that basalt was employed extensively by the Egyptians and other nations of antiquity for the purposes of statuary; and it is not improbable that in those days, when the recondite operations of Nature were less explored by man, superstition was so far credited as to have made the ancient naturalists nearly silent upon the subject of columnar basalt as an unaltered production of Nature. Pliny is, apparently, the first whose mention of it exists. He speaks of it as a hard stone of Ethiopia, of which Strabo had seen pillars and pieces of great dimensions.
The circumstance of the celebrated sculpture, which by command of Titus Vespasian was dedicated to Nilus in the Temple of Peace, having been hewn out of a single column of basalt, derives corroboration from the vast dimensions of those pillars which form the grand façade at Benmore, and also the immense prismatic divisions in the rock-mass on the west side of Mount Pleaskin, whence an extraordinary quadrangular pillar separated itself in the winter of 1811, falling on the shore beneath.
It has, not without reason, been supposed that the Lapis Lydius, and Lapis Metallorum, or touchstone of the ancients, was a kind of basalt; for it is sometimes rendered magnetic by the physical qualities of the iron, which is an essential ingredient, and under various degrees of chemical influence, the cause of those numerous shades of black, grey, and yellowish and reddish brown which it exhibits.
Basalt is the Basanos maximus Hibernicus of old times. The Greek básanos is as the Latin probatio; and Pliny, in the 8th chapter of his 33d book, defines básanos "lapis index, seu Lydius et Heraclius, quem coticulam vocant:" and a very common definition of the word is, "lapis quo probatur aurum."
I am, however, inclined to believe that the pillared structure of basalt must have been the object considered for the purposes of its appellation; and that the word basalt, possibly at first basilt, is a mutilated derivative of the Greek basiliké regia domus, and edificium amplum, since a royal residence, or other stately edifice, is rarely destitute of pillars: and besides this, the Latin word , scarcely altered from the Greek basiliké, has, among other definitions, that of a piazza, which in Italian seems to imply a colonnade, or at least a building to the character of which pillars are essential.
It is remarkable that a late writer should have been disposed to adduce "a reason why the basaltic pillars are not produced at this day as they were formerly" since instances are no where wanting of their development before living witnesses. Many facts are well authenticated of basalt, which on the removal of the superficial soil, or on being worked as a common stone-quarry, appeared a shapeless mass, having divided itself, on the free accession of atmospheric air, into prismatic columns, from a height of many feet, to small specimens fitted for the cabinet. The latter is well instanced by Mr. Kirwan in a fragment of that basaltic rock known in Ireland under the name of calp, or
By due observation it is sufficiently evident that the members of the mineral kingdom experience slow and unceasing revolutions, in concordance with the other admirable provisions of nature. While certain masses are decomposing and becoming fitted for the purposes of vegetation, others are arranging themselves, either by the acervation of materials unfitted for the service of organic life, or by the accession of sand and similar matters from the bosoms of the waters, thereby maintaining those diversities of surface essential to the health of animals at large, and delightful to the sense of man. Basaltic rocks of all others are productive of the richest soils; and they are no less remarkable for the purity and clearness of their waters. But it would seem as hopeless to expect that a mountain like Chimboraco should start up before our eyes, as that we may witness the formation of such piles as the Giants' Causeway, and the Cave of Fingal.