The following paper contains (in addition to other matter) the substance of two lectures delivered by the author at St. Helena on the 26th October, and the 21st December, 1868. The topographical details have been added for the benefit of those who are unacquainted with the island. The plan and sections alluded to have not yet been lithographed.
Ladder Hill, St. Helena,
THE GEOLOGY OF ST. HELENA,
CAPTAIN J. R. OLIVER, R.A.
On referring to the map of St. Helena it will be seen that the Island is of an irregular egg-shaped figure. Its greatest length, viz. in a line joining Barn Point and West Point, which line lies about E.N.E. and W.S.W., is 10˝ miles, and its greatest breadth about 6 miles. On approaching it from the North-West its general appearance is as follows:—The coast line rises abruptly from the sea in bare brown cliffs, varying from 450 feet to 2,400 feet in height, but of a general altitude of about 700. The face of this wall of rock is intersected by a number of deep and narrow gorges running at right angles to the coast line, in some places lying so close together as to leave but a narrow ridge between them, in others more than a mile apart. Form the summit of the wall of cliffs the ground recedes in a very regular slope of about 10° terminating in a ridge running along the centre of the Island. The general altitude of this ridge is 2,200 feet, but it is serrated in form and two of its peaks are as high as 2,700.
To a spectator standing on some point slightly raised above the sloping plain, such as Horse Pasture, and looking across it, the intersecting ravines are hardly visible, and
this side of the Island presents the appearance of an almost unbroken slope cut off abruptly by the sea cliffs. With the exception of High Knoll and High Hill, which are to a certain extent isolated peaks, and leaving out the parts removed by denudation, the plain, from Banks' Ridge to West Point, may be considered to slope with little interruption up to the central ridge. At both ends it is cut off abruptly—at the West in rear by the Man & Horse cliffs—at the East laterally by the line of cliffs joining Sugar Loaf and Flagstaff Hill.
A narrow ridge connects Flagstaff Hill with the Barn, an apparently level-topped mass of rock 2,200 feet high, forming the N.E. extremity of the Island. After rounding it we find the coast somewhat similar to what it is on the N.W. side. A gently sloping plain intersected by deep ravines terminates abruptly in lofty cliffs with terraces of alternate slopes of forty-five degrees inclination and vertical walls of rock. With the exception of Prosperous Bay Telegraph Hill, and Stonetop, two huge partially detached masses of rock, the coast maintains much the same character till we arrive off Sandy Bay, where an immense circular amphitheatre opens out. The back of this amphitheatre is formed by part of the central ridge and the sides by two of its spurs. The one running along the Diana's Peak range, the other separating from it behind West Lodge and terminating
in the Asses Ears and Castle Rock. Passing Speery Island, a lofty jutting rock, we again come to the Man & Horse cliffs.
Judging from the appearance of the Island one would expect to find deep water close under its cliffs, but such in reality is not the case. The water deepens very gradually to a distance of some two miles from the shore, more or less, where there seems to be a great ledge, the depth changing suddenly from 60 or 70 fathoms to no soundings at 150. The distance of this ledge from the shore varies from 1,200 yards of Horse Pasture to more than 4,000 yards off Speery, where there occurs a large shoal.
Thus much for the principal topographical features of St. Helena. Before entering upon its Geology it may be as well to state that the S.E. trade wind, blowing almost constantly across it, brings up clouds which condense in the shape of mist and rain over the higher grounds forming the centre of the Island, leaving the parts near the coast comparatively rainless. The consequence of this is that while the sea cliffs are bare and brown, and almost destitute of vegetation, the centre of the Island is for the greater part of the year clothed in the richest verdure. This gives rise to a practical difficulty in studying its Geology, for although near the coast plenty of good sections are obtainable, in the interior the rocks have been so much disintegrated by moisture and covered up by
debris that it is in places almost impossible to ascertain their real structure without deep quarrying.
The rocks composing St. Helena are entirely volcanic, and the place may be looked upon as the remains of what was once in all probability a much larger island, formed by a vast number of eruptions of ash, lava, and mud, and subsequently denuded to a very great extent—so much so in fact as to render it very difficult to unravel its geological history.
In this respect it differs from most other volcanic Islands, Teneriffe for instance, where we generally find the surface of the ground coinciding with that of the last flow of lava. It may be mentioned here that earthquakes very seldom occur at St. Helena and are then but slight. It is highly probable that the last eruption there took place, even geologically speaking, at a very remote epoch, so that that part of the earth's crust (supposing it to have become gradually consolidated by radiation of heat into space, or otherwise,) may be looked upon as having attained a state of permanent quiescence.
To an observer viewing the coast on the North-West side from the sea the cliffs appear regularly and horizontally stratified. But on entering one of the valleys;—that of Jamestown for instance,—it is at once seen that the strata dip towards the sea at an angle of about ten degrees, and lie parallel to the sur-
face of the sloping plain before mentioned. On close examination of these strata we find them to consist of layers of ash and rubble, hard splintery lava, and volcanic mud, superimposed on each other with great regularity. The mud deposits are perhaps in the proportion of about one to ten of the others, and are in every case more or less altered by the heat of the lava that has flowed over them, the upper part of each having been changed from a yellowish compact clay to a brick-looking substance of a bright red colour distinguishable at a great distance. This clay frequently contains embedded cinders and angular fragments of hard lava, and its discolouration is always in proportion to the propinquity and thickness of the flow of lava next above it. The lava itself, though brown outside, is dark greenish gray when broken, and appears to consist chiefly of augite. It is very compact, hard, and splintery, and exhibits a more or less columnar structure, the direction of the columns being at right angles to the planes of stratification. The uppermost part of each layer is always vesicular, and, owing to its toughness, forms a good building stone. The cavities are generally found to be compressed and elongated in the direction of the flow, and more or less filled by crystals of zeolitic minerals. The lower part of each lava bed is usually mixed up with rubble and cinder which it has apparently picked up and dragged along with
it. The incorporation is frequently so complete as to render it hard to distinguish one from the other. Where the lava has flowed directly over a clay bed a thin layer of cinders generally intervenes. The thickness of the beds, both of lava and rubble, varies considerably. These last are composed of cinders and fragments of hard lava mixed up with ash and dust.
The above description applies to the whole of the rocks of what may be designated for shortness sake the "Jamestown District" which extends from West Point to Sugar Loaf and Flagstaff, and from the N.W. coast to a mile or more inland. In places the layers of hard lava attain a great thickness, and occasionally we find the whole mass of rock intersected by dykes of other material; but in general the whole of this part of the Island exhibits great uniformity of geological and mineralogical character. The rocks of the S.E. coast, from near Sandy Bay to near Prosperous Bay (excluding the upper part of Stonetop), much resemble those of Jamestown and may possibly have been derived from the same sources.
The Geologist, on examining the North-West side of the Island finds two principal questions to decide. In the first place by what means the intersecting gorges have been produced, and in the second whether the beds of lava originally flowed as we now see them on a slope of about ten degrees, or
were first deposited at a lower angle and upheaved at some subsequent period. With regard to the first question persons but slightly acquainted with Geology are apt to imagine that all great chasms and ravines have necessarily been caused by some convulsion of nature upheaving and tearing asunder the earth's crust. Those who have any idea that such is the case as regards the Jamestown and other ravines are much mistaken, and the Geologist is forced to the conclusion that, wonderful as it may seem, these huge gorges have been scooped out by gradual denudation. It is possible that they may have originated in surface cracks, but they have most certainly been gradually enlarged during the lapse of ages. Had they been caused by "convulsions of nature" the strata, when viewed from the sea, would appear waved and contorted, whereas the contrary is the case, the different beds exhibiting a striking uniformity and regularity. The red clay bands are very conspicuous objects, and can be traced for miles in a continuous line, cropping out at nearly the same level. Again, we frequently find hard ledges of rock crossing the bottoms of the gorges, where they give rise to waterfalls. These ledges would of course have been broken across in the case of any great fracture, instead of being, as we find them, invariably continuous.
Another proof exists in the dykes, a few of which cross the valleys obliquely (of course
caused by real fractures, but in a different direction) and appear to have been formed prior to them, having been worn away along with the rocks they intersected, and showing no signs of disruption. It may be mentioned by the way that where the dykes have cut through clay beds the clay has been altered in their immediate neighbourhood,—also that it is possible to determine the relative age of some of these dykes, since they are often found cutting through the strata up to a certain point where they abruptly come to an end, thus marking what was the surface of the ground at the time they were formed.
The denudation which has produced these valleys must have been going on for a vast number of years; although, as it is probable that the amount of rainfall may have formerly been much greater than at present, we may suppose the scooping action to have once gone on more rapidly than now. At Teneriffe we have instances of torrents having cut deep valleys through volcanic deposits within the memory of man, and the same action has apparently been at work at St. Helena to a considerable extent.
The sides of the gorges lie at the natural slope of 45° (except at the faces of the hard lava ledges, which are vertical,) and meet at the water-courses, leaving in general but little space for cultivation. And this is the shape naturally assumed by torrent-formed ravines. The denudation is visibly going on.
Every heavy shower of rain not only carries out to the sea a quantity of mud washed down from the slopes, but also generally causes the fall of portions of the hard ledges by loosening their joints. These masses in falling frequently break up into small fragments, which in time are reduced by the action of the weather to a state of rubble and dust.
The second question is more difficult to answer. If it is true, as some Geologists assert, that lava will not form thick beds if it flows at a greater angle than 5°, then it is quite certain that the whole mass of beds forming the N.W. side of the Island must have been upheaved subsequently to their deposition, since some of them attain a thickness of more than a hundred feet. But then it must be remembered that these thick masses may have been caused by the lava meeting with obstacles in its way which have since disappeared, or by its filling basin-shaped cavities. Of this however we have not yet sufficient evidence, and the question must remain for the present an open one. The oblique dykes before mentioned may have been caused by a slight upheaval.
Another undetermined point is whether these beds were deposited in the open air or under water. The former at present appears the most probable.
There are a few more points connected with the structure of the Jamestown district that are worthy of notice. The valleys are some-
times so close together as to leave only a narrow ridge between them, as at Goat Pound Ridge, Friar's Ridge, and the ridge between Swanley Valley and Old Woman's Valley. Where these narrow ridges consist of hard columnar basaltic lava they necessarily have sharp projecting points such as the "Friar" which have been left standing while the surrounding parts have fallen away. Sugar Loaf Hill affords a good instance of denudation, the cap of hard rock, with nearly level base and vertical sides, forming its summit, having been part of a lava flow. At Egg Island again we find a thick mass of hard lava, while close to it Peaked Island is composed entirely of rough scorić cemented together.
The clay beds often attain a great thickness from the mud having met with hollows in the surface of the ground it flowed over. Very good sections illustrating this are to be seen at the bold cliff forming the end of Goat Pound Ridge and in Horse Pasture Hangings. At the former place a (now) conical mass of lava surmounts a thick sloping bed of clay, evidently the remains of a larger mass which over filled a hollow. While at the latter the section exhibits a sloping bed, of which the lower surface conforms to a series of hollows and mounds in a mass of lava, while the upper is a smooth inclined plane.
In many places the lava in its flow has assumed the form of spherical concentric masses. It is difficult to assign a reason why the beds
should have attained such an immense thickness at certain places. The most remarkable instances are High Knoll, Horse Pasture, and High Hill. Each of these forms a boss rising from the sloping plain,—abruptly on the upper side, but gradually blending with it on the lower—and each consists apparently of a solid mass of hard basaltic lava resting conformably on a sloping plane surface, and thinning out towards the North-West in proportion as the lava has flowed down the incline. The upper part of these masses is generally a spongy purplish-brown trachyte, and that at High Knoll furnishes a very tough building stone and is extensively quarried for the purpose. In spite of the abrupt S.E. terminations of these hills, which would lead us to suspect some great fracture, the beds on which they rest are found to be undisturbed.
The mass of hard basalt of which the crest of Friar's Ridge consists may be looked upon as of the same nature as the above.
In the lava beds of this district are found numerous very perfect tubular cavities about 14 inches in diameter by several feet in length. They have all the appearance of being casts of trunks of trees embedded within the liquid lava, the rock immediately enclosing them being marked in concentric rings as if it had rolled in a semi fluid condition. In some cases the interiors of these cavities are marked with a perfect network, much resembling the impressions of the bark of trees found in the coal
measures. Should the species ever be identified we should be enabled to assign the geological age of the lava streams in question. If any more of these cavities be opened up, either in quarrying or excavating, it would be very interesting to notice whether they contain any charcoal.
The coast cliffs of this side of the Island are undoubtedly due to the action of sea waves continued through a vast extent of time. The different beds of lava, scorić and mud must have once extended far out to what is now open sea, the result of the long-continued wearing action of the waves having been to cut away bodily a large portion of them.
In tracing out the Geological History of St. Helena we must take into consideration the form of the bottom of the sea surrounding it.
Suppose that the whole Island were to be raised 450 feet, or one-sixth of the height of its highest peaks—the whole of the sea bottom from the present coast line to the ledge before mentioned would then become dry land. The present sea cliffs would become an inland escarpment at an average distance of nearly two miles from the new coast, which would consist of a low sea cliff with very deep water at its base. So deep indeed that in one place we find the soundings drop suddenly from 528 feet into no bottom at 1500, within a horizontal distance of 600.
The newly exposed belt of dry land would slope very gradually to the sea, and wherever there was a sloping plain at the summit of the overhanging
escarpment it would be found that according to the greater or less inclination of one slope so would be that of the other, the lower terrace, however, being less inclined than the upper.
Referring to the Sections given it will also be noticed that the prolongation of the plane of the upper terrace about touches the crest of the lower ledge.
From these facts we have to deduce our conclusions as to the past history of this, the oldest part of the Island. It might be supposed that the terraces just described may have been caused by the successive subsidence of ring-shaped tracts of country subsequent to the formation of the Jamestown District but prior to that of the newer portions of the Island. This, however, besides being a phenomenon unknown in geological experience, is rendered most improbable by the fact that the lower terrace runs continuously round parts, like Castle Rock and its vicinity, where bold, highly-denuded cliffs, of quite a different nature, take the place of the sloping planes of the older deposits. The more probable explanation is that the terraces were produced by the action of sea waves consequent on successive subsidences of the land. A series of submarine eruptions may have originally piled up layer after layer of scorić and lava until an Island appeared above the surface of the water. A great central crater, probably occupying the position of what is now the Sandy Bay basin, continued to dis-
gorge showers of ashes and floods of molten rock, varied occasionally by an outpouring of torrents of mud. The products of these eruptions flowed down the slopes on all sides towards the sea, each one adding to the height and area of the Island until eventually it assumed something of the form that Teneriffe now possesses, viz: a lofty cone, many thousand feet in height, with sides sloping gradually to the sea and constituting an island very much larger than St. Helena now is. A subsidence of the whole then took place—whether very gradually or more quickly we are unable to say—and the waves immediately commenced eating into the sloping shores. A low vertical cliff was thus produced, the debris being removed by tides and currents. After the lapse of a number of years the sea would have worked its way to a considerable distance inland by the planing action now generally recognized by Geologists, until the lower ledge as it now exists had been formed. Another subsidence then took place and the same sequence of cause and effect resulted in the coast line as we now see it. Of course an indefinite lapse of time must be admitted in order to account for these effects. The whole hypothesis takes no account of the formation of the newer rocks to be presently noticed, nor would its probability be affected thereby. For aught we know this may have been completed before the subsidences commenced. It is noteworthy that if one of the
subsidences had been irregular—that is if the Island sank more on one side than the other—it would be a sufficient explanation of the greater inclination of the older strata on the North-West than on the South-East side. It should also be observed that the inclination of the slope formed by the debris under water ought to be less than that of the original strata, although varying according to its amount, which is exactly what we find to be the case.
If we draw a line from Manati Bay to Flagstaff it will cut off to the N.W. a tract almost coinciding with what we have called the Jamestown District, and (leaving out the valleys) the shape of this tract would be that of a broad short wedge lying on one of its faces, and with its edge cut off. Along the whole of this tract the strata dip with more or less regularity to the N.W. The long coast line constitutes the truncated edge, and the almost vertical cliffs, extending on the one hand from Manati Bay to West Point, and on the other from Flagstaff to Sugar Loaf Point, form the two ends of the wedge, each of these cliffs affording, on a large scale, a section nearly parallel to the dip of the strata.
One or two points remain to be noticed before leaving this part of the Island. On following any of the slopes a certain distance towards the central ridge the old lava streams are found to be overlain by newer deposits of a different nature. These beds are lighter
coloured and softer than those underneath, and consist in general of a species of felspathic claystone. They usually terminate about half way between the Sandy Bay Ridge and the sea, and appear to be the product of a comparatively recent series of eruptions. Their softness is no doubt partly due to the disintegration produced by the moist climate of the interior of the Island; but even without taking this into consideration the mineralogical distinction between the two sets of beds is well marked.
The Jamestown valley terminates at its upper end in a most remarkable cul de sac formed by a semicylindrical and vertical precipice some 250 feet in height. There are no dykes, no signs of fracture or displacement. The layers of lava and clay have all been cut clean through, and the chasm has exactly the appearance of having been scooped out by a huge boring machine. It is difficult to understand how such a state of things can have been brought about, but the fact remains patent. At the foot of the Waterfall is one of the well-mark clay beds before alluded to. It may be traced along the side of the valley and the faces of the sea cliffs for many miles, and serves as a conduit for the moisture that feeds the long beds of rushes above Maldivia.
The cliffs of Flagstaff and the Barn as seen from the sea exhibit a very remarkable section, the vertical height of which amounts in
one place to 2400 feet. On the right we have the whole series of strata of the Jamestown District dipping to the right, while on the left those of the Barn dip in the opposite direction and more rapidly. The faces of both cliffs are seamed by several vertical dykes, and the two are connected together by a lower neck of land, of which the stratification is not at first sight very evident.
The first impression received is that Flagstaff and the Barn were originally part of the same mass, and that a subsequent violent upheaval bulged up the strata in a dome-shaped form, the central part of this dome falling into the chasm below and the debris forming the connecting neck of land. But on closer inspection this theory would seem to be untenable. In the first place the rocks of the Barn appear to be harder and more silicious than those of Flagstaff. In the next, on examining the neck of land we find a series of hard beds lying parallel to each other and apparently underlying those of Flagstaff conformably. These beds seem to abut on to those of the Barn at right angles, much in the same manner as the two halves of a pack of cards may be made to lean against each other. A series of confused deposits resembling those of the neck, and intersected by numerous dykes, may be traced for many miles by the sections afforded by the sea cliffs between the Barn and Stonetop, and appear to underlie the whole of the nearly horizontal strata composing the
country between the Barn and the Diana's Peak range. It is by no means improbable that the Barn may be a fragment of some very old volcanic formation. The steep angle at which its beds dip to the N.E. indicate that the whole mass was probably tilted up after it had been formed, and the face of the hill seen from Longwood is apparently the fractured surface exposed where the strata have been broken off at right angles to their planes by a violent upheaval.
A line drawn form the summit of Flagstaff Hill along the crest of the ridge to Halley's Mount and thence following the Diana's Peak range and continued down to the sea in the neighbourhood of Deep Valley would enclose between it and the sea a tract of country which may be designated as the "Longwood District"—this term however being applied more in a topographical than a geological sense. The greater part of this area was no doubt once a plain, sloping very gradually towards the coast. Here as in the case of the Jamestown District, denudation has been busy at work, although in a different manner. Instead of the narrow, sharply-cut ravines of the N.W. side we find either undulating plains or wide valleys with gently sloping sides, alternating with rounded ridges. The rocks forming the surface of the country have been disintegrated by the action of the weather into a loose friable clay, and, as a consequence, the sides of all the steeper
slopes have become intersected by a multitude of miniature ravines each serving as a watercourse to drain the higher ground. The sides of these slopes exhibit a number of bright coloured bands running parallel to the stratification of the rocks underneath, and no doubt due to beds containing an unusual proportion of iron or manganese. These beds are, in general, of a bright red, blue or, violet colour, and form a remarkable feature in the District. In the immediate vicinity of the sea the valleys deepen into precipitous gorges much resembling those at the other side of the Island, except that, from the hard lava beds being much thicker, the scraps are bolder and on a larger scale.
Viewed from the neighbourhood of Halley's Mount it would appear at first sight as if this district had been formed simultaneously with, and from the same source as, that of Jamestown. But the very different mineralogical structure of the two sets of rocks points to a different conclusion. As we descend towards Prosperous Bay we find alternate beds of hard silicious lava and a soft white claystone. The outcrop of the former is vertical, and the precipices are seldom less than 50 feet high—generally much more. The alternating claystone beds are thinner, and weather at an angle of 45°. As their slopes are in most places covered with the brown debris of the harder rocks their real nature is only apparent at certain spots. These beds of claystone are
highly interesting because in them, and nowhere else, are found a vast number of veins of black oxide of manganese, varying from a quarter of an inch to nearly a foot in thickness. These veins seldom proceed far without swelling out into nodules, and it frequently happens that a number of them are interlaced in such a manner as to form considerable masses—never however quite free from the claystone, portions of which may always be noticed mixed up with the manganese and sometimes enclosed in it. At the surface the ore often stands out in a remarkable manner from the claystone having been washed away from around it. It has just the shape that we might suppose would be assumed by a quantity of melted lead forced into a mass of wet clay.
Were this manganese to be found in England it would be very valuable. At present, owing to the high price of labour and freight and the difficulties of land transport, it would not pay to work it. The claystone becomes hard at a short distance from the surface and the veins are very delusive, changing suddenly from large masses into mere streaks, so that a great deal of the rock has to be blasted away to obtain the ore in any quantity. Besides this, four-fifths of the manganese is of inferior quality and would never pay to export.
These veins may be traced at the outcrop of the claystone beds at intervals the whole way from Flagstaff to Stonetop, and the actual
quantity buried beneath the surface is probably enormous, since they doubtless underlie the whole district.
Scattered about the whole of the Longwood plain are found nodules of red oxide of iron which must have been left exposed on the surface by the wearing away of the softer material of the rock. They are of all sizes, and the larger ones have been used for building fences with.
The sections afforded by the sea cliffs and gorges shew us, as was mentioned before, that the series of nearly horizontal strata (their dip is about 2˝°) comprising this District are underlaid by a set of confused deposits, intersected by a number of dykes. These lower rocks have not as yet been properly studied, but they appear to extend to the neck of land at Flagstaff Bay, and to be the oldest in the Island. The great mass of Prosperous Bay Telegraph Hill with its sharply serrated, comb-like crest may possibly be an outlier of some similar old formation.
From what has been said it will be seen that the rocks of the Longwood District are composed of such different materials to those of Jamestown, that we cannot suppose them to have been emitted simultaneously from the same crater. The probability is that the former are of more recent origin than the latter. Whereabouts the crater or craters were situated is difficult to determine, owing to the great extent to which the face of the Island has been altered by denudation.
In the neighbourhood of Turk's Cap the greater part of the original plain has disappeared leaving however one long narrow strip with vertical sides, known as "Gregory's Rock," to mark it, as well as the conical mass of Turk's Cap itself, which, from some cause or other, has remained standing while all the surrounding strata have been washed away.
Continuing our examination of the coast Westwards from Prosperous Bay we find the cliffs have much the same character as those of the Jamestown District till we arrive at the entrance to Shark's Valley. The sides of this gorge are terraced with the usual parallel ledges of hard rock, the columnar structure of which is here very distinctly marked. On the East side of the valley, in one place, the strata have been bent up into the form of an arc of 90° the chord of which is inclined at an angle of 45°. But the most unique features of this part of the Island are the rocks known as Great and Little Stone Top. The former is a conical mass some 1800 feet high, rising almost vertically from the sea, at the West side of the entrance to the gorge. The lower half of the cliff consists of the outcrop of alternate ledges resembling those at the other side of the Island, but the upper half is an all but vertical precipice forming the face of a huge conical mass of hard, crystalline, felspathic rock of a light gray colour, which rests upon the older rocks beneath. This huge cliff show no signs of stratification and is ap-
parently a mere fragment of some great lava flow, a vertical columnar structure being distinctly traceable throughout the whole mass from base to summit. A little higher up the valley, and at the same side, we come to Little Stone Top, a conical sharp-pointed hill, the summit of which is formed by a fragmentary mass of the same hard gray rock, which here rests upon a thick deposit of a light coloured claystone, or sandstone as it might almost be termed, a great deal of which occurs in the neighbourhood. The sides of the slopes are in places covered with great angular masses of the gray rock which have fallen down from above. They are very hard and ring loudly when struck a hammer. It is evident, even to a casual observer, that the summits of the two Stonetops were once parts of the same great lava flow. The enormous denudation which must have occurred here would almost tempt us to give credence to the theory that St. Helena may be merely a remaining fragment of a great tract of country, formed like the plateaux of the Deccan and Abyssinia, by lava flows on a gigantic scale.
There now only remains to be considered the crater-like district known generally as "Sandy Bay," including the circular ridge which, commencing at the Asses Ears, terminates in Sandy Bay Barn. The crest of this ridge is, in places, only a few inches wide and its inner slope is very steep, and sends out a number of spurs which converge to-
wards Sandy Bay itself. This great amphitheatre has all the appearance of having once been a crater. Its sides exhibit in section the outcrop of thick masses of lava sloping away in different directions, and its interior is filled with all sorts of confused deposits, showing signs of great disturbance.
The spurs are composed generally of rather soft material—such as slaty friable lava, and hardened clay beds, without any very definite stratification, and their form appears to be due to water action. Several great dykes of harder material may be traced crossing the crater in a straight line from side to side and standing out boldly wherever they cut through the converging spurs. The great masses of "Lot," "Lot's Wife," the "Asses Ears" and Speery Island, are all portions of one of these dykes. Its thickness must be at least 500 feet and its junction with the softer rocks is very distinctly marked. It consists of a hard crystalline felspar rock resembling that of Stone Top and exhibiting much the same vertical columnar structure. The great peaks called the "Asses Ears" look as if they had been built up of so many distinct prisms, so well are their joints defined. "Lot's Wife" is a single vertical shaft 270 feet high, springing from the crest of a spur, and the great cone of "Lot" rises 500 feet above the crest of another ridge.
It seems probable that this amphitheatre may have been the site of most if not all the
eruptions which have at different times occurred. The newest formed rocks in the Island, viz. those which slope downwards to the East and North from the crest of Sandy Bay Ridge and Diana's Peak, covering the older Jamestown and other rocks, must undoubtedly have had their origin here. They are comparatively soft and felspathic and hence are most useful in the economy of nature, their capability of retaining moisture enabling them to accumulate and store up a sufficient supply of water to keep many of the springs on the hillsides flowing, almost without diminution, through long seasons of drought.
The mineralogical character of many of the rocks of the Sandy Bay District is very well marked. We find a great deal of hard lava, both in the shape of beds and dykes, consisting of a dark green paste with embedded crystals of black augite and light green olivine. The blocks which strew the ground at the place called the "Churchyard," and which are simply boulders that have rolled down from the crest of the hill above, are of this nature, though externally covered with a brown crust caused by weathering.
In other places, as in the neighbourhood of "Lot," we find a rock composed of a red felspathic paste with very large embedded crystals of augite. This rock decays rapidly when exposed, and the felspar gets washed away, leaving in time a thick bed of gravel of the liberated crystals.
At Lot's Wife Ponds we have the well known "Chimney" which stands out by itself, a fragment of a small dyke of hard silicious material. In the cliffs in the neighbourhood of Sandy Bay itself are numerous vertical dykes of two kinds. The one consisting of the black paste with olivine crystals, and separating in prisms running at right angles to the plane of the dykes. The other homogeneous and softer and of a greenish buff colour. The two kinds may often be noticed within a few feet of each other. Having necessarily had their origin in separate reservoirs of molten rock it may almost be taken for granted that the two sets were formed at different epochs. Indeed it is said that in places the dykes are found to cross each other. If this really is the case it puts the matter beyond a doubt, since the rocks through which these dykes cut are very much tumbled and confused, and appear more like heaps of debris than anything else. The frequent occurrence of dykes in this part of the Island shows how much more it has been disturbed by subterranean forces than other localities, and indicates its being at or near the centre of former volcanic action. Whether or not there may have been other centres—as near the Barn for instance—must be considered as yet undetermined.
In addition to the porous lava already mentioned as used as a building stone there is a quarry at Jamestown of a soft red stone which cuts easily and is employed for ornamental
purposes, but does not wear well. It occurs in a great rounded mass against which the lava streams have abutted, and appears to be the result of an accumulation in some way or other of a volcanic mud, subsequently hardened by heat and pressure.
The oxide of iron and manganese of the Longwood District have already been noticed. Only one other valuable mineral has been found at St. Helena, and that is the "lime-stone," a great deal of which has at different times been quarried and burnt. It occurs in the form of surface beds of light coloured calcareous sand, very slightly compacted together. These beds are found in different places near Sandy Bay, at Sugarloaf, and at one or two other spots. They are generally situated several hundred feet above the sea, but never very far from it. They are mere surface deposits of no great thickness and generally exhibit a bedding parallel to the slope of the ground on which they lie. It is difficult to account for their formation and it is said that under the microscope they show no traces of organism. They would appear to be accumulations of calcareous sand derived from the destruction of coral reefs, washed up on a sea beach and subsequently drifted by a strong trade wind into certain convenient hollows.
Reverting to the idea, previously broached, that St. Helena was once in all probability a much larger island than at present, consisting
of a regularly-sloping cone rising, perhaps, 10 or 15,000 feet into the air, it is worthy of note that the amount of rainfall would in this case be much more copious than now, so that the excavation of the gorges, as well as the denudation of the slopes, may have gone on with tolerable rapidity.
In drawing up this sketch the main object held in view has been rather to put forward facts as they are, than to attempt to deduce conclusions from them which further investigation might upset. Those parts of the Island which have been the most carefully examined have been the most minutely described. A great many localities have as yet been barely visited, and, until the whole Island has been thoroughly explored by a competent Geologist, it would be premature to lay down definite theories. This applies more particularly to the older rocks of the Longwood District, the coast from the Barn to Sandy Bay, and portions of the Sandy Bay deposits. It is to be hoped that the subject may be one day or other taken up by some scientific man, able and willing to investigate it thoroughly, and that the ascertained facts, put forward in this pamphlet, may serve as a basis on which to construct a more perfect and complete work.
Notes about this version of Oliver:
The text was scanned from an original copy. OCR software was used to generate a text file which was carefully proof-read against the original.
Contributed by Barry Weaver.
Details of the original:
Oliver, J. R. The Geology of St. Helena. Benjamin Grant, St. Helena, 1869.
Text: pp. 32.
Plates: No plates.
British Library shelfmark: 07109.f.37.
Library of Congress call number: Not in the catalogue.
Last updated: 19 December, 2011