(FIGURE 6. OPAQUE BROWN SPHAERULITES, drawn on an enlarged scale. The upper ones are externally marked with parallel ridges. The internal radiating structure of the lower ones, is much too plainly represented.
FIGURE 7. A LAYER FORMED BY THE UNION OF MINUTE BROWN SPHAERULITES, INTERSECTING TWO OTHER SIMILAR LAYERS: the whole represented of nearly the natural size.)
One of the commonest transitional rocks deserves in several respects a further description. It is of a very complicated nature, and consists of numerous thin, slightly tortuous layers of a pale-coloured feldspathic stone, often pa.s.sing into an imperfect pitchstone, alternating with layers formed of numberless little globules of two varieties of obsidian, and of two kinds of sphaerulites, embedded in a soft or in a hard pearly base. The sphaerulites are either white and translucent, or dark brown and opaque; the former are quite spherical, of small size, and distinctly radiated from their centre. The dark brown sphaerulites are less perfectly round, and vary in diameter from the twentieth to the thirtieth of an inch; when broken they exhibit towards their centres, which are whitish, an obscure radiating structure; two of them when united sometimes have only one central point of radiation; there is occasionally a trace of or a hollow crevice in their centres. They stand either separately, or are united two or three or many together into irregular groups, or more commonly into layers, parallel to the stratification of the ma.s.s. This union in many cases is so perfect, that the two sides of the layer thus formed, are quite even; and these layers, as they become less brown and opaque, cannot be distinguished from the alternating layers of the pale-coloured feldspathic stone. The sphaerulites, when not united, are generally compressed in the plane of the lamination of the ma.s.s; and in this same plane, they are often marked internally, by zones of different shades of colour, and externally by small ridges and furrows. In the upper part of Figure 6, the sphaerulites with the parallel ridges and furrows are represented on an enlarged scale, but they are not well executed; and in the lower part, their usual manner of grouping is shown. In another specimen, a thin layer formed of the brown sphaerulites closely united together, intersects, as represented in Figure 7, a layer of similar composition; and after running for a short s.p.a.ce in a slightly curved line, again intersects it, and likewise a second layer lying a little way beneath that first intersected.
The small nodules also of obsidian are sometimes externally marked with ridges and furrows, parallel to the lamination of the ma.s.s, but always less plainly than the sphaerulites. These obsidian nodules are generally angular, with their edges blunted: they are often impressed with the form of the adjoining sphaerulites, than which they are always larger; the separate nodules seldom appear to have drawn each other out by exerting a mutually attractive force. Had I not found in some cases, a distinct centre of attraction in these nodules of obsidian, I should have been led to have considered them as residuary matter, left during the formation of the pearlstone, in which they are embedded, and of the sphaerulitic globules.
The sphaerulites and the little nodules of obsidian in these rocks so closely resemble, in general form and structure, concretions in sedimentary deposits, that one is at once tempted to attribute to them an a.n.a.logous origin. They resemble ordinary concretions in the following respects: in their external form,--in the union of two or three, or of several, into an irregular ma.s.s, or into an even-sided layer,--in the occasional intersection of one such layer by another, as in the case of chalk-flints,- -in the presence of two or three kinds of nodules, often close together, in the same basis,--in their fibrous, radiating structure, with occasional hollows in their centres,--in the co-existence of a laminary, concretionary, and radiating structure, as is so well developed in the concretions of magnesian limestone, described by Professor Sedgwick.
("Geological Transactions" volume 3 part 1 page 37.) Concretions in sedimentary deposits, it is known, are due to the separation from the surrounding ma.s.s of the whole or part of some mineral substance, and its aggregation round certain points of attraction. Guided by this fact, I have endeavoured to discover whether obsidian and the sphaerulites (to which may be added marekanite and pearlstone, both of them occurring in nodular concretions in the trachytic series) differ in their const.i.tuent parts, from the minerals generally composing trachytic rocks. It appears from three a.n.a.lyses, that obsidian contains on an average 76 per cent of silica; from one a.n.a.lysis, that sphaerulites contain 79.12; from two, that marekanite contains 79.25; and from two other a.n.a.lyses, that pearlstone contains 75.62 of silica. (The foregoing a.n.a.lyses are taken from Beudant "Traite de Mineralogie" tome 2 page 113; and one a.n.a.lysis of obsidian from Phillips "Mineralogy.") Now, the const.i.tuent parts of trachyte, as far as they can be distinguished consist of feldspar, containing 65.21 of silica; or of albite, containing 69.09; of hornblende, containing 55.27 (These a.n.a.lyses are taken from Von Kobell "Grundzuge der Mineralogie" 1838.), and of oxide of iron: so that the foregoing gla.s.sy concretionary substances all contain a larger proportion of silica than that occurring in ordinary feldspathic or trachytic rocks. D"Aubuisson ("Traite de Geogn." tome 2 page 535.), also, has remarked on the large proportion of silica compared with alumina, in six a.n.a.lyses of obsidian and pearlstone given in Brongniart"s "Mineralogy." Hence I conclude, that the foregoing concretions have been formed by a process of aggregation, strictly a.n.a.logous to that which takes place in aqueous deposits, acting chiefly on the silica, but likewise on some of the other elements of the surrounding ma.s.s, and thus producing the different concretionary varieties. From the well-known effects of rapid cooling (This is seen in the manufacture of common gla.s.s, and in Gregory Watts"s experiments on molten trap; also on the natural surfaces of lava- streams, and on the side-walls of dikes.) in giving gla.s.siness of texture, it is probably necessary that the entire ma.s.s, in cases like that of Ascension, should have cooled at a certain rate; but considering the repeated and complicated alterations of nodules and thin layers of a gla.s.sy texture with other layers quite stony or crystalline, all within the s.p.a.ce of a few feet or even inches, it is hardly possible that they could have cooled at different rates, and thus have acquired their different textures.
The natural sphaerulites in these rocks very closely resemble those produced in gla.s.s, when slowly cooled. (I do not know whether it is generally known, that bodies having exactly the same appearance as sphaerulites, sometimes occur in agates. Mr. Robert Brown showed me in an agate, formed within a cavity in a piece of silicified wood, some little specks, which were only just visible to the naked eye: these specks, when placed by him under a lens of high power, presented a beautiful appearance: they were perfectly circular, and consisted of the finest fibres of a brown colour, radiating with great exactness from a common centre. These little radiating stars are occasionally intersected, and portions are quite cut off by the fine, ribbon-like zones of colour in the agate. In the obsidian of Ascension, the halves of a sphaerulite often lie in different zones of colour, but they are not cut off by them, as in the agate.) In some fine specimens of partially devitrified gla.s.s, in the possession of Mr. Stokes, the sphaerulites are united into straight layers with even sides, parallel to each other, and to one of the outer surfaces, exactly as in the obsidian. These layers sometimes interbranch and form loops; but I did not see any case of actual intersection. They form the pa.s.sage from the perfectly gla.s.sy portions, to those nearly h.o.m.ogeneous and stony, with only an obscure concretionary structure. In the same specimen, also, sphaerulites differing slightly in colour and in structure, occur embedded close together. Considering these facts, it is some confirmation of the view above given of the concretionary origin of the obsidian and natural sphaerulites, to find that M. Dartigues ("Journal de Physique" tome 59 1804 pages 10, 12.), in his curious paper on this subject, attributes the production of sphaerulites in gla.s.s, to the different ingredients obeying their own laws of attraction and becoming aggregated. He is led to believe that this takes place, from the difficulty in remelting sphaerulitic gla.s.s, without the whole be first thoroughly pounded and mixed together; and likewise from the fact, that the change takes place most readily in gla.s.s composed of many ingredients. In confirmation of M. Dartigues" view, I may remark, that M. Fleuriau de Bellevue (Idem tome 60 1805 page 418.) found that the sphaerulitic portions of devitrified gla.s.s were acted on both by nitric acid and under the blowpipe, in a different manner from the compact paste in which they were embedded.
COMPARISON OF THE OBSIDIAN BEDS AND ALTERNATING STRATA OF ASCENSION, WITH THOSE OF OTHER COUNTRIES.
I have been struck with much surprise, how closely the excellent description of the obsidian rocks of Hungary, given by Beudant ("Voyage en Hongrie" tome 1 page 330; tome 2 pages 221 and 315; tome 3 pages 369, 371, 377, 381.), and that by Humboldt, of the same formation in Mexico and Peru ("Essai Geognostique" pages 176, 326, 328.), and likewise the descriptions given by several authors (P. Scrope "Geological Transactions" volume 2 second series page 195. Consult also Dolomieu "Voyage aux Isles Lipari" and D"Aubuisson "Traite de Geogn." tome 2 page 534.) of the trachytic regions in the Italian islands, agree with my observations at Ascension. Many pa.s.sages might have been transferred without alteration from the works of the above authors, and would have been applicable to this island. They all agree in the laminated and stratified character of the whole series; and Humboldt speaks of some of the beds of obsidian being ribboned like jasper.
(In Mr. Stokes" fine collection of obsidians from Mexico, I observe that the sphaerulites are generally much larger than those of Ascension; they are generally white, opaque, and are united into distinct layers: there are many singular varieties, different from any at Ascension. The obsidians are finely zoned, in quite straight or curved lines, with exceedingly slight differences of tint, of cellularity, and of more or less perfect degrees of gla.s.siness. Tracing some of the less perfectly gla.s.sy zones, they are seen to become studded with minute white sphaerulites, which become more and more numerous, until at last they unite and form a distinct layer: on the other hand, at Ascension, only the brown sphaerulites unite and form layers; the white ones always being irregularly disseminated. Some specimens at the Geological Society, said to belong to an obsidian formation from Mexico, have an earthy fracture, and are divided in the finest parallel laminae, by specks of a black mineral, like the augitic or hornblendic specks in the rocks at Ascension.) They all agree in the nodular or concretionary character of the obsidian, and of the pa.s.sage of these nodules into layers. They all refer to the repeated alterations, often in undulatory planes, of gla.s.sy, pearly, stony, and crystalline layers: the crystalline layers, however, seem to be much more perfectly developed at Ascension, than in the above-named countries. Humboldt compares some of the stony beds, when viewed from a distance, to strata of a schistose sandstone. Sphaerulites are described as occurring abundantly in all cases; and they everywhere seem to mark the pa.s.sage, from the perfectly gla.s.sy to the stony and crystalline beds. Beudant"s account (Beudant "Voyage" tome 3 page 373.) of his "perlite lithoide globulaire" in every, even the most trifling particular, might have been written for the little brown sphaerulitic globules of the rocks of Ascension.
From the close similarity in so many respects, between the obsidian formations of Hungary, Mexico, Peru, and of some of the Italian islands, with that of Ascension, I can hardly doubt that in all these cases, the obsidian and the sphaerulites owe their origin to a concretionary aggregation of the silica, and of some of the other const.i.tuent elements, taking place whilst the liquified ma.s.s cooled at a certain required rate.
It is, however, well-known, that in several places, obsidian has flowed in streams like lava; for instance, at Teneriffe, at the Lipari Islands, and at Iceland. (For Teneriffe see von Buch "Descript. des Isles Canaries"
pages 184 and 190; for the Lipari Islands see Dolomieu "Voyage" page 34; for Iceland see Mackenzie "Travels" page 369.) In these cases, the superficial parts are the most perfectly gla.s.sy, the obsidian pa.s.sing at the depth of a few feet into an opaque stone. In an a.n.a.lysis by Vauquelin of a specimen of obsidian from Hecla, which probably flowed as lava, the proportion of silica is nearly the same as in the nodular or concretionary obsidian from Mexico. It would be interesting to ascertain, whether the opaque interior portions and the superficial gla.s.sy coating contained the same proportional const.i.tuent parts: we know from M. Dufrenoy ("Memoires pour servir a une Descript. Geolog. de la France" tome 4 page 371.) that the exterior and interior parts of the same stream of lava sometimes differ considerably in their composition. Even should the whole body of the stream of obsidian turn out to be similarly composed with nodular obsidian, it would only be necessary, in accordance with the foregoing facts, to suppose that lava in these instances had been erupted with its ingredients mixed in the same proportion, as in the concretionary obsidian.
LAMINATION OF VOLCANIC ROCKS OF THE TRACHYTIC SERIES.
We have seen that, in several and widely distant countries, the strata alternating with beds of obsidian, are highly laminated. The nodules, also, both large and small, of the obsidian, are zoned with different shades of colour; and I have seen a specimen from Mexico in Mr. Stokes" collection, with its external surface weathered (MacCulloch states "Cla.s.sification of Rocks" page 531 that the exposed surfaces of the pitchstone dikes in Arran are furrowed "with undulating lines, resembling certain varieties of marbled paper, and which evidently result from some corresponding difference of laminar structure.") into ridges and furrows, corresponding with the zones of different degrees of gla.s.siness: Humboldt ("Personal Narrative" volume 1 page 222.), moreover, found on the Peak of Teneriffe, a stream of obsidian divided by very thin, alternating, layers of pumice.
Many other lavas of the feldspathic series are laminated; thus, ma.s.ses of common trachyte at Ascension are divided by fine earthy lines, along which the rock splits, separating thin layers of slightly different shades of colour; the greater number, also, of the embedded crystals of gla.s.sy feldspar are placed lengthways in the same direction. Mr. P. Scrope ("Geological Transactions" volume 2 second series page 195.) has described a remarkable columnar trachyte in the Panza Islands, which seems to have been injected into an overlying ma.s.s of trachytic conglomerate: it is striped with zones, often of extreme tenuity, of different textures and colours; the harder and darker zones appearing to contain a larger proportion of silica. In another part of the island, there are layers of pearlstone and pitchstone, which in many respects resemble those of Ascension. The zones in the columnar trachyte are generally contorted; they extend uninterruptedly for a great length in a vertical direction, and apparently parallel to the walls of the dike-like ma.s.s. Von Buch ("Description des Iles Canaries" page 184.) has described at Teneriffe, a stream of lava containing innumerable thin, plate-like crystals of feldspar, which are arranged like white threads, one behind the other, and which mostly follow the same direction. Dolomieu ("Voyage aux Isles de Lipari" pages 35 and 85.) also states, that the grey lavas of the modern cone of Vulcano, which have a vitreous texture, are streaked with parallel white lines: he further describes a solid pumice-stone which possesses a fissile structure, like that of certain micaceous schists. Phonolite, which I may observe is often, if not always, an injected rock, also, often has a fissile structure; this is generally due to the parallel position of the embedded crystals of feldspar, but sometimes, as at Fernando Noronha, seems to be nearly independent of their presence. (In this case, and in that of the fissile pumice-stone, the structure is very different from that in the foregoing cases, where the laminae consist of alternate layers of different composition or texture. In some sedimentary formations, however, which apparently are h.o.m.ogeneous and fissile, as in glossy clay-slate, there is reason to believe, according to D"Aubuisson, that the laminae are really due to excessively thin, alternating, layers of mica.) From these facts we see, that various rocks of the feldspathic series have either a laminated or fissile structure, and that it occurs both in ma.s.ses which have injected into overlying strata, and in others which have flowed as streams of lava.
The laminae of the beds, alternating with the obsidian at Ascension, dip at a high angle under the mountain, at the base of which they are situated; and they do not appear as if they had been inclined by violence. A high inclination is common to these beds in Mexico, Peru, and in some of the Italian islands (See Phillips "Mineralogy" for the Italian Islands page 136. For Mexico and Peru see Humboldt "Essai Geognostique." Mr. Edwards also describes the high inclination of the obsidian rocks of the Cerro del Navaja in Mexico in the "Proc. of the Geolog. Soc." June 1838.): on the other hand, in Hungary, the layers are horizontal; the laminae, also, of some of the lava-streams above referred to, as far as I can understand the descriptions given of them, appear to be highly inclined or vertical. I doubt whether in any of these cases, the laminae have been tilted into their present position; and in some instances, as in that of the trachyte described by Mr. Scrope, it is almost certain that they have been originally formed with a high inclination. In many of these cases, there is evidence that the ma.s.s of liquified rock has moved in the direction of the laminae. At Ascension, many of the air-cells have a drawn out appearance, and are crossed by coa.r.s.e semi-gla.s.sy fibres, in the direction of the laminae; and some of the layers, separating the sphaerulitic globules, have a scored appearance, as if produced by the grating of the globules. I have seen a specimen of zoned obsidian from Mexico, in Mr. Stokes" collection, with the surfaces of the best-defined layers streaked or furrowed with parallel lines; and these lines or streaks precisely resembled those, produced on the surface of a ma.s.s of artificial gla.s.s by its having been poured out of a vessel. Humboldt, also, has described little cavities, which he compares to the tails of comets, behind sphaerulites in laminated obsidian rocks from Mexico, and Mr. Scrope has described other cavities behind fragments embedded in his laminated trachyte, and which he supposes to have been produced during the movement of the ma.s.s. ("Geological Transactions" volume 2 second series page 200 etc. These embedded fragments, in some instances, consist of the laminated trachyte broken off and "enveloped in those parts, which still remained liquid." Beudant, also, frequently refers in his great work on "Hungary" tome 3 page 386, to trachytic rocks, irregularly spotted with fragments of the same varieties, which in other parts form the parallel ribbons. In these cases, we must suppose, that after part of the molten ma.s.s had a.s.sumed a laminated structure, a fresh irruption of lava broke up the ma.s.s, and involved fragments, and that subsequently the whole became relaminated.) From such facts, most authors have attributed the lamination of these volcanic rocks to their movement whilst liquified. Although it is easy to perceive, why each separate air-cell, or each fibre in pumice-stone (Dolomieu "Voyage"
page 64.), should be drawn out in the direction of the moving ma.s.s; it is by no means at first obvious why such air-cells and fibres should be arranged by the movement, in the same planes, in laminae absolutely straight and parallel to each other, and often of extreme tenuity; and still less obvious is it, why such layers should come to be of slightly different composition and of different textures.
In endeavouring to make out the cause of the lamination of these igneous feldspathic rocks, let us return to the facts so minutely described at Ascension. We there see, that some of the thinnest layers are chiefly formed by numerous, exceedingly minute, though perfect, crystals of different minerals; that other layers are formed by the union of different kinds of concretionary globules, and that the layers thus formed, often cannot be distinguished from the ordinary feldspathic and pitchstone layers, composing a large portion of the entire ma.s.s. The fibrous radiating structure of the sphaerulites seems, judging from many a.n.a.logous cases, to connect the concretionary and crystalline forces: the separate crystals, also, of feldspar all lie in the same parallel planes. (The formation, indeed, of a large crystal of any mineral in a rock of mixed composition implies an aggregation of the requisite atoms, allied to concretionary action. The cause of the crystals of feldspar in these rocks of Ascension, being all placed lengthways, is probably the same with that which elongates and flattens all the brown sphaerulitic globules (which behave like feldspar under the blowpipe) in this same direction.) These allied forces, therefore, have played an important part in the lamination of the ma.s.s, but they cannot be considered the primary force; for the several kinds of nodules, both the smallest and largest, are internally zoned with excessively fine shades of colour, parallel to the lamination of the whole; and many of them are, also, externally marked in the same direction with parallel ridges and furrows, which have not been produced by weathering.
Some of the finest streaks of colour in the stony layers, alternating with the obsidian, can be distinctly seen to be due to an incipient crystallisation of the const.i.tuent minerals. The extent to which the minerals have crystallised can, also, be distinctly seen to be connected with the greater or less size, and with the number, of the minute, flattened, crenulated air-cavities or fissures. Numerous facts, as in the case of geodes, and of cavities in silicified wood, in primary rocks, and in veins, show that crystallisation is much favoured by s.p.a.ce. Hence, I conclude, that, if in a ma.s.s of cooling volcanic rock, any cause produced in parallel planes a number of minute fissures or zones of less tension (which from the pent-up vapours would often be expanded into crenulated air-cavities), the crystallisation of the const.i.tuent parts, and probably the formation of concretions, would be superinduced or much favoured in such planes; and thus, a laminated structure of the kind we are here considering would be generated.
That some cause does produce parallel zones of less tension in volcanic rocks, during their consolidation, we must admit in the case of the thin alternate layers of obsidian and pumice described by Humboldt, and of the small, flattened, crenulated air-cells in the laminated rocks of Ascension; for on no other principle can we conceive why the confined vapours should through their expansion form air-cells or fibres in separate, parallel planes, instead of irregularly throughout the ma.s.s. In Mr. Stokes"
collection, I have seen a beautiful example of this structure, in a specimen of obsidian from Mexico, which is shaded and zoned, like the finest agate, with numerous, straight, parallel layers, more or less opaque and white, or almost perfectly gla.s.sy; the degree of opacity and gla.s.siness depending on the number of microscopically minute, flattened air-cells; in this case, it is scarcely possible to doubt but that the ma.s.s, to which the fragment belonged, must have been subjected to some, probably prolonged, action, causing the tension slightly to vary in the successive planes.
Several causes appear capable of producing zones of different tension, in ma.s.ses semi-liquified by heat. In a fragment of devitrified gla.s.s, I have observed layers of sphaerulites which appeared, from the manner in which they were abruptly bent, to have been produced by the simple contraction of the ma.s.s in the vessel, in which it cooled. In certain dikes on Mount Etna, described by M. Elie de Beaumont ("Mem. pour servir" etc. tome 4 page 131.), as bordered by alternating bands of scoriaceous and compact rock, one is led to suppose that the stretching movement of the surrounding strata, which originally produced the fissures, continued whilst the injected rock remained fluid. Guided, however, by Professor Forbes"
("Edinburgh New Phil. Journal" 1842 page 350.) clear description of the zoned structure of glacier-ice, far the most probable explanation of the laminated structure of these feldspathic rocks appears to be, that they have been stretched whilst slowly flowing onwards in a pasty condition (I presume that this is nearly the same explanation which Mr. Scrope had in his mind, when he speaks ("Geolog. Transact." volume 2 second series page 228) of the ribboned structure of his trachytic rocks, having arisen, from "a linear extension of the ma.s.s, while in a state of imperfect liquidity, coupled with a concretionary process."), in precisely the same manner as Professor Forbes believes, that the ice of moving glaciers is stretched and fissured. In both cases, the zones may be compared to those in the finest agates; in both, they extend in the direction in which the ma.s.s has flowed, and those exposed on the surface are generally vertical: in the ice, the porous laminae are rendered distinct by the subsequent congelation of infiltrated water, in the stony feldspathic lavas, by subsequent crystalline and concretionary action. The fragment of gla.s.sy obsidian in Mr. Stokes" collection, which is zoned with minute air-cells must strikingly resemble, judging from Professor Forbes" descriptions, a fragment of the zoned ice; and if the rate of cooling and nature of the ma.s.s had been favourable to its crystallisation or to concretionary action, we should here have had the finest parallel zones of different composition and texture. In glaciers, the lines of porous ice and of minute crevices seem to be due to an incipient stretching, caused by the central parts of the frozen stream moving faster than the sides and bottom, which are r.e.t.a.r.ded by friction: hence in glaciers of certain forms and towards the lower end of most glaciers, the zones become horizontal. May we venture to suppose that in the feldspathic lavas with horizontal laminae, we see an a.n.a.logous case? All geologists, who have examined trachytic regions, have come to the conclusion, that the lavas of this series have possessed an exceedingly imperfect fluidity; and as it is evident that only matter thus characterised would be subject to become fissured and to be formed into zones of different tensions, in the manner here supposed, we probably see the reason why augitic lavas, which appear generally to have possessed a high degree of fluidity, are not, like the feldspathic lavas, divided into laminae of different composition and texture. (Basaltic lavas, and many other rocks, are not unfrequently divided into thick laminae or plates, of the same composition, which are either straight or curved; these being crossed by vertical lines of fissure, sometimes become united into columns.
This structure seems related, in its origin, to that by which many rocks, both igneous and sedimentary, become traversed by parallel systems of fissures.) Moreover, in the augitic series, there never appears to be any tendency to concretionary action, which we have seen plays an important part in the lamination of rocks, of the trachytic series, or at least in rendering that structure apparent.
Whatever may be thought of the explanation here advanced of the laminated structure of the rocks of the trachytic series, I venture to call the attention of geologists to the simple fact, that in a body of rock at Ascension, undoubtedly of volcanic origin, layers often of extreme tenuity, quite straight, and parallel to each other, have been produced;--some composed of distinct crystals of quartz and diopside, mingled with amorphous augitic specks and granular feldspar,--others entirely composed of these black augitic specks, with granules of oxide of iron,--and lastly, others formed of crystalline feldspar, in a more or less perfect state of purity, together with numerous crystals of feldspar, placed lengthways. At this island, there is reason to believe, and in some a.n.a.logous cases, it is certainly known, that the laminae have originally been formed with their present high inclination. Facts of this nature are manifestly of importance, with relation to the structural origin of that grand series of plutonic rocks, which like the volcanic have undergone the action of heat, and which consist of alternate layers of quartz, feldspar, mica and other minerals.
CHAPTER IV.--ST. HELENA.
Lavas of the feldspathic, basaltic, and submarine series.
Section of Flagstaff Hill and of the Barn.
Dikes.
Turk"s Cap and Prosperous Bays.
Basaltic ring.
Central crateriform ridge, with an internal ledge and a parapet.
Cones of phonolite.
Superficial beds of calcareous sandstone.
Extinct land-sh.e.l.ls.
Beds of detritus.
Elevation of the land.
Denudation.
Craters of elevation.
The whole island is of volcanic origin; its circ.u.mference, according to Beatson, is about twenty-eight miles. (Governor Beatson "Account of St.
Helena.") The central and largest part consists of rocks of a feldspathic nature, generally decomposed to an extraordinary degree; and when in this state, presenting a singular a.s.semblage of alternating, red, purple, brown, yellow, and white, soft, argillaceous beds. From the shortness of our visit, I did not examine these beds with care; some of them, especially those of the white, yellow, and brown shades, originally existed as streams of lava, but the greater number were probably ejected in the form of scoriae and ashes: other beds of a purple tint, porphyritic with crystal- shaped patches of a white, soft substance, which are now unctuous, and yield, like wax, a polished streak to the nail, seem once to have existed as solid claystone-porphyries: the red argillaceous beds generally have a brecciated structure, and no doubt have been formed by the decomposition of scoriae. Several extensive streams, however, belonging to this series, retain their stony character; these are either of a blackish-green colour, with minute acicular crystals of feldspar, or of a very pale tint, and almost composed of minute, often scaly, crystals of feldspar, abounding with microscopical black specks; they are generally compact and laminated; others, however, of similar composition, are cellular and somewhat decomposed. None of these rocks contain large crystals of feldspar, or have the harsh fracture peculiar to trachyte. These feldspathic lavas and tuffs are the uppermost or those last erupted; innumerable dikes, however, and great ma.s.ses of molten rock, have subsequently been injected into them.
They converge, as they rise, towards the central curved ridge, of which one point attains the elevation of 2,700 feet. This ridge is the highest land in the island; and it once formed the northern rim of a great crater, whence the lavas of this series flowed: from its ruined condition, from the southern half having been removed, and from the violent dislocation which the whole island has undergone, its structure is rendered very obscure.
BASALTIC SERIES.
The margin of the island is formed by a rude circle of great, black, stratified, ramparts of basalt, dipping seaward, and worn into cliffs, which are often nearly perpendicular, and vary in height from a few hundred feet to two thousand. This circle, or rather horse-shoe shaped ring, is open to the south, and is breached by several other wide s.p.a.ces. Its rim or summit generally projects little above the level of the adjoining inland country; and the more recent feldspathic lavas, sloping down from the central heights, generally abut against and overlap its inner margin; on the north-western side of the island, however, they appear (judging from a distance) to have flowed over and concealed portions of it. In some parts, where the basaltic ring has been breached, and the black ramparts stand detached, the feldspathic lavas have pa.s.sed between them, and now overhang the sea-coast in lofty cliffs. The basaltic rocks are of a black colour and thinly stratified; they are generally highly vesicular, but occasionally compact; some of them contain numerous crystals of gla.s.sy feldspar and octahedrons of t.i.taniferous iron; others abound with crystals of augite and grains of olivine. The vesicles are frequently lined with minute crystals (of chabasie?) and even become amygdaloidal with them. The streams are separated from each other by cindery matter, or by a bright red, friable, saliferous tuff, which is marked by successive lines like those of aqueous deposition; and sometimes it has an obscure, concretionary structure. The rocks of this basaltic series occur nowhere except near the coast. In most volcanic districts the trachytic lavas are of anterior origin to the basaltic; but here we see, that a great pile of rock, closely related in composition to the trachytic family, has been erupted subsequently to the basaltic strata: the number, however, of dikes, abounding with large crystals of augite, with which the feldspathic lavas have been injected, shows perhaps some tendency to a return to the more usual order of superposition.
BASAL SUBMARINE LAVAS.
The lavas of this basal series lie immediately beneath both the basaltic and feldspathic rocks. According to Mr. Seale, they may be seen at intervals on the sea-beach round the entire island. ("Geognosy of the Island of St. Helena." Mr. Seale has constructed a gigantic model of St.
Helena, well worth visiting, which is now deposited at Addis...o...b.. College, in Surrey.) In the sections which I examined, their nature varied much; some of the strata abound with crystals of augite; others are of a brown colour, either laminated or in a rubbly condition; and many parts are highly amygdaloidal with calcareous matter. The successive sheets are either closely united together, or are separated from each other by beds of scoriaceous rock and of laminated tuff, frequently containing well-rounded fragments. The interstices of these beds are filled with gypsum and salt; the gypsum also sometimes occurring in thin layers. From the large quant.i.ty of these two substances, from the presence of rounded pebbles in the tuffs, and from the abundant amygdaloids, I cannot doubt that these basal volcanic strata flowed beneath the sea. This remark ought perhaps to be extended to a part of the superinc.u.mbent basaltic rocks; but on this point, I was not able to obtain clear evidence. The strata of the basal series, whenever I examined them, were intersected by an extraordinary number of dikes.
FLAGSTAFF HILL AND THE BARN.
(FIGURE 8. FLAGSTAFF HILL AND THE BARN. (Section West (left) to East (right)) Flagstaff Hill, 2,272 feet high to The Barn, 2,015 feet high.
The double lines represent the basaltic strata; the single, the basal submarine strata; the dotted, the upper feldspathic strata; the dikes are shaded transversely.)
I will now describe some of the more remarkable sections, and will commence with these two hills, which form the princ.i.p.al external feature on the north-eastern side of the island. The square, angular outline, and black colour of the Barn, at once show that it belongs to the basaltic series; whilst the smooth, conical figure, and the varied bright tints of Flagstaff Hill, render it equally clear, that it is composed of the softened, feldspathic rocks. These two lofty hills are connected (as is shown in Figure 8) by a sharp ridge, which is composed of the rubbly lavas of the basal series. The strata of this ridge dip westward, the inclination becoming less and less towards the Flagstaff; and the upper feldspathic strata of this hill can be seen, though with some difficulty, to dip conformably to the W.S.W. Close to the Barn, the strata of the ridge are nearly vertical, but are much obscured by innumerable dikes; under this hill, they probably change from being vertical into being inclined into an opposite direction; for the upper or basaltic strata, which are about eight hundred or one thousand feet in thickness, are inclined north-eastward, at an angle between thirty and forty degrees.
This ridge, and likewise the Barn and Flagstaff Hills, are interlaced by dikes, many of which preserve a remarkable parallelism in a N.N.W. and S.S.E. direction. The dikes chiefly consist of a rock, porphyritic with large crystals of augite; others are formed of a fine-grained and brown- coloured trap. Most of these dikes are coated by a glossy layer, from one to two-tenths of an inch in thickness, which, unlike true pitchstone, fuses into a black enamel; this layer is evidently a.n.a.logous to the glossy superficial coating of many lava streams. (This circ.u.mstance has been observed (Lyell "Principles of Geology" volume 4 chapter 10 page 9) in the dikes of the Atrio del Cavallo, but apparently it is not of very common occurrence. Sir G. Mackenzie, however, states (page 372 "Travels in Iceland") that all the veins in Iceland have a "black vitreous coating on their sides." Captain Carmichael, speaking of the dikes in Tristan d"Acunha, a volcanic island in the Southern Atlantic, says ("Linnaean Transactions" volume 12 page 485) that their sides, "where they come in contact with the rocks, are invariably in a semi-vitrified state.") The dikes can often be followed for great lengths both horizontally and vertically, and they seem to preserve a nearly uniform thickness ("Geognosy of the Island of St. Helena" plate 5.): Mr. Seale states, that one near the Barn, in a height of 1,260 feet, decreases in width only four inches,--from nine feet at the bottom, to eight feet and eight inches at the top. On the ridge, the dikes appear to have been guided in their course, to a considerable degree, by the alternating soft and hard strata: they are often firmly united to the harder strata, and they preserve their parallelism for such great lengths, that in very many instances it was impossible to conjecture, which of the beds were dikes, and which streams of lava. The dikes, though so numerous on this ridge, are even more numerous in the valleys a little south of it, and to a degree I never saw equalled anywhere else: in these valleys they extend in less regular lines, covering the ground with a network, like a spider"s web, and with some parts of the surface even appearing to consist wholly of dikes, interlaced by other dikes.
From the complexity produced by the dikes, from the high inclination and anticlinal dip of the strata of the basal series, which are overlaid, at the opposite ends of the short ridge, by two great ma.s.ses of different ages and of different composition, I am not surprised that this singular section has been misunderstood. It has even been supposed to form part of a crater; but so far is this from having been the case, that the summit of Flagstaff Hill once formed the lower extremity of a sheet of lava and ashes, which were erupted from the central, crateriform ridge. Judging from the slope of the contemporaneous streams in an adjoining and undisturbed part of the island, the strata of the Flagstaff Hill must have been upturned at least twelve hundred feet, and probably much more, for the great truncated dikes on its summit show that it has been largely denuded. The summit of this hill now nearly equals in height the crateriform ridge; and before having been denuded, it was probably higher than this ridge, from which it is separated by a broad and much lower tract of country; we here, therefore, see that the lower extremities of a set of lava-streams have been tilted up to as great a height as, or perhaps greater height than, the crater, down the flanks of which they originally flowed. I believe that dislocations on so grand a scale are extremely rare in volcanic districts. (M. Constant Prevost "Mem. de la Soc. Geolog." tome 2 observes that "les produits volcaniques n"ont que localement et rarement meme derange le sol, a travers lequel ils se sont fait jour.") The formation of such numbers of dikes in this part of the island shows that the surface must here have been stretched to a quite extraordinary degree: this stretching, on the ridge between Flagstaff and Barn Hills, probably took place subsequently (though perhaps immediately so) to the strata being tilted; for had the strata at that time extended horizontally, they would in all probability have been fissured and injected transversely, instead of in the planes of their stratification. Although the s.p.a.ce between the Barn and Flagstaff Hill presents a distinct anticlinal line extending north and south, and though most of the dikes range with much regularity in the same line, nevertheless, at only a mile due south of the ridge the strata lie undisturbed. Hence the disturbing force seems to have acted under a point, rather than along a line. The manner in which it has acted, is probably explained by the structure of Little Stony-top, a mountain 2,000 feet high, situated a few miles southward of the Barn; we there see, even from a distance, a dark-coloured, sharp, wedge of compact columnar rock, with the bright-coloured feldspathic strata, sloping away on each side from its uncovered apex. This wedge, from which it derives its name of Stony-top, consists of a body of rock, which has been injected whilst liquified into the overlying strata; and if we may suppose that a similar body of rock lies injected, beneath the ridge connecting the Barn and Flagstaff, the structure there exhibited would be explained.
TURK"S CAP AND PROSPEROUS BAYS.
(FIGURE 9. PROSPEROUS HILL AND THE BARN. (Section S.S.E. (left) to N.N.W.
(right) Prosperous Hill through Hold-fast-Tom and Flagstaff Hill to The Barn.
The double lines represent the basaltic strata; the single, the basal submarine strata; the dotted, the upper feldspathic strata.)
Prosperous Hill is a great, black, precipitous mountain, situated two miles and a half south of the Barn, and composed, like it, of basaltic strata.
These rest, in one part, on the brown-coloured, porphyritic beds of the basal series, and in another part, on a fissured ma.s.s of highly scoriaceous and amygdaloidal rock, which seems to have formed a small point of eruption beneath the sea, contemporaneously with the basal series. Prosperous Hill, like the Barn, is traversed by many dikes, of which the greater number range north and south, and its strata dip, at an angle of about 20 degrees, rather obliquely from the island towards the sea. The s.p.a.ce between Prosperous Hill and the Barn, as represented in Figure 9, consists of lofty cliffs, composed of the lavas of the upper or feldspathic series, which rest, though unconformably, on the basal submarine strata, as we have seen that they do at Flagstaff Hill. Differently, however, from in that hill, these upper strata are nearly horizontal, gently rising towards the interior of the island; and they are composed of greenish-black, or more commonly, pale brown, compact lavas, instead of softened and highly coloured matter. These brown-coloured, compact lavas, consist almost entirely of small glimmering scales, or of minute acicular crystals, of feldspar, placed close by the side of each other, and abounding with minute black specks, apparently of hornblende. The basaltic strata of Prosperous Hill project only a little above the level of the gently-sloping, feldspathic streams, which wind round and abut against their upturned edges. The inclination of the basaltic strata seems to be too great to have been caused by their having flowed down a slope, and they must have been tilted into their present position before the eruption of the feldspathic streams.
BASALTIC RING.
Proceeding round the Island, the lavas of the upper series, southward of Prosperous Hill, overhang the sea in lofty precipices. Further on, the headland, called Great Stony-top, is composed, as I believe, of basalt; as is Long Range Point, on the inland side of which the coloured beds abut. On the southern side of the island, we see the basaltic strata of the South Barn, dipping obliquely seaward at a considerable angle; this headland, also, stands a little above the level of the more modern, feldspathic lavas. Further on, a large s.p.a.ce of coast, on each side of Sandy Bay, has been much denuded, and there seems to be left only the basal wreck of the great, central crater. The basaltic strata reappear, with their seaward dip, at the foot of the hill, called Man-and-Horse; and thence they are continued along the whole north-western coast to Sugar-Loaf Hill, situated near to the Flagstaff; and they everywhere have the same seaward inclination, and rest, in some parts at least, on the lavas of the basal series. We thus see that the circ.u.mference of the island is formed by a much-broken ring, or rather, a horse-shoe, of basalt, open to the south, and interrupted on the eastern side by many wide breaches. The breadth of this marginal fringe on the north-western side, where alone it is at all perfect, appears to vary from a mile to a mile and a half. The basaltic strata, as well as those of the subjacent basal series, dip, with a moderate inclination, where they have not been subsequently disturbed, towards the sea. The more broken state of the basaltic ring round the eastern half, compared with the western half of the island, is evidently due to the much greater denuding power of the waves on the eastern or windward side, as is shown by the greater height of the cliffs on that side, than to leeward. Whether the margin of basalt was breached, before or after the eruption of the lavas of the upper series, is doubtful; but as separate portions of the basaltic ring appear to have been tilted before that event, and from other reasons, it is more probable, that some at least of the breaches were first formed. Reconstructing in imagination, as far as is possible, the ring of basalt, the internal s.p.a.ce or hollow, which has since been filled up with the matter erupted from the great central crater, appears to have been of an oval figure, eight or nine miles in length by about four miles in breadth, and with its axis directed in a N.E. and S W.
line, coincident with the present longest axis of the island.
THE CENTRAL CURVED RIDGE.