GIBRALTAR

GIBRALTAR is a fortress of immense strength, of which a very full account has been given in the Encyclopaedia. cyclopaedia. Nothing, however, is in that article said of the natural history of the mountain on which the fortress is built, though, to men of science, that subject must be as interesting as a detail of sieges. This defect we are enabled to supply by means of Major Imrie's mineralogical description of Gibraltar, which is published in the fourth volume of the Transactions of the Royal Society of Edinburgh; and, we are persuaded, the following abstract of that elegant memoir will afford rational entertainment to many of our readers. "The form of this mountain is oblong; its summit a sharp craggy ridge; its direction is nearly from north to south; and its greatest length, in that direction, falls very little short of three miles. Its breadth varies with the indentations of the shore, but it nowhere exceeds three quarters of a mile. The line of its ridge is undulating, and the two extremes are somewhat higher than its centre. "The summit of the Sugar Loaf, which is the point of its greatest elevation towards the south, is 1439 feet; the Rock Mortar, which is the highest point to the north, is 1350; and the Signal House, which is nearly the central point between these two, is 1276 feet above the level of the sea. The western side of the mountain is a series of rugged slopes, interspersed with abrupt precipices. Its northern extremity is perfectly perpendicular, except towards the north-west, where what are called the Lines intervene, and a narrow passage of flat ground that leads to the isthmus, and is entirely covered with fortification. The eastern side of the mountain mostly consists of a range of precipices; but a bank of sand, rising from the Mediterranean in a rapid acclivity, covers a third of its perpendicular height. Its southern extremity falls, in a rapid slope from the summit of the Sugar Loaf, into a rocky flat of considerable extent, called Windmill Hill. "The principal mass of the mountain rock consists of a grey, dense (what is generally called primary) marble; the different beds of which are to be examined in a face of 1350 feet of perpendicular height, which it presents to Spain in a conical form. These beds, or strata, are of various thickness, from 20 to upwards of 40 feet, dipping in a direction from east to west, nearly at an angle of 35 degrees. In some parts of the solid mass of this rock are found testaceous bodies entirely transmuted into the constituent matter of the rock, and their interior hollows filled up with calcareous spar; but these do not occur often in its composition, and its beds are not separated by any intermediate strata. "The caves of Gibraltar are many, and some of them of great extent. That which most deserves attention and examination is called St. Michael's Cave, which is situated upon the southern part of the mountain, almost equally distant from the Signal Tower and the Sugar Loaf. Its entrance is 1000 feet above the level of the sea: This entrance is formed by a rapid slope of earth, which has fallen into it at various periods, and which leads to a spacious hall, mercurated with spar, and apparently supported in the centre by a large massy stalactitical pillar. To this succeeds a long series of caves of difficult access. In these cavernous recesses, the formation and process of stalactites is to be traced, from the slimy quilt-like cone, suspended from the roof, to the robust trunk of a pillar, three feet in diameter, which rises from the floor, and seems intended by Nature to support the roof from which it originated. Gibraltar. "The only inhabitants of these caves are bats, some of which are of a large size. The soil, in general, upon the mountain of Gibraltar is but thinly sown; and in many parts that thin covering has been washed off by the heavy autumnal rains, which have left the superfluous of the rock, for a considerable extent, bare and open to inspection. In those situations, an observing eye may trace the effects of the flow, but constant, decomposition of the rock, caused by its exposure to the air, and the corrosion of sea-salts, which, in the heavy gales of easterly winds, are deposited with the spray on every part of the mountain. Those uncovered parts of the mountain rock also expose to the eye a phenomenon worthy of some attention, as it tends clearly to demonstrate, that, however high the surface of this rock may now be elevated above the level of the sea, it has once been the bed of agitated waters. This phenomenon is to be observed in many parts of the rock, and is constantly found in the beds of torrents. It consists of pot-like holes, of various sizes, hollowed out of the solid rock, and formed apparently by the attrition of gravel or pebbles, set in motion by the rapidity of rivers or currents in the sea. "Upon the west side of the mountain, towards its base, some strata occur, which are heterogenous to the mountain rock: the first, or highest, forms the segment of a circle; its convex side is towards the mountain, and it slopes also in that direction. This stratum consists of a number of thin beds; the outward one, being the thinnest, is in a state of decomposition, and is mouldering down into a blackish brown or ferruginous coloured earth. The beds, inferior to this, progressively increase in breadth to 17 inches, where the stratification rests upon a rock of an argillaceous nature. "This last bed, which is 17 inches thick, consists of quartz of a blackish blue colour, in the septa or cracks of which are found fine quartz crystals, colourless, and perfectly transparent. These crystals are composed of 18 planes, disposed in hexangular columns, terminated at both extremities by hexangular pyramids. The largest of those that Major Imrie saw did not exceed one-fourth of an inch in length: They, in general, adhere to the rock by the sides of the column, but are detached without difficulty. Their great degree of transparency has obtained them the name of Gibraltar diamonds." Much has been said of the fossil bones found in the rock of Gibraltar; and the general idea which exists concerning them is, that they are found in a petrified state, and inclosed in the solid calcareous rock; but this, says Major Imrie, is a mistake, which could arise only from inaccurate observation and false description. "In the perpendicular fissures of the rock, and in some of the caverns of the mountain (all of which afford evident proofs of their former communication with the surface), a calcareous concretions is found, of a reddish brown ferruginous colour, with an earthy fracture, and considerable induration, inclosing the bones of various animals, some of which have the appearance of being human. These bones are of various sizes, and lie in all directions, intermixed with shells of snails, fragments of the calcareous rock, and particles of spar; all of which materials are still to be seen in their natural uncombined state, Gibraltar. flates, partially scattered over the surface of the mountain. These having been swept, by heavy rains at different periods, from the surface into the situations above described, and having remained for a long series of years in those places of rest, exposed to the permeating action of water, have become enveloped in, and cemented by, the calcareous matter which it deposits. "The bones, in this composition, have not the smallest appearance of being petrified; and if they have undergone any change, it is more like that of calcination than that of petrification, as the most solid parts of them generally admit of being cut and scraped down with the same ease as chalk. "Bones combined in such concretions are not peculiar to Gibraltar: they are found in such large quantities in the country of Dalmatia, and upon its coasts in the islands of Cherso and Ofero, that some naturalists have been induced to go so far as to assert, that there has been a regular stratum of such matter in that country, and that its present broken and interrupted appearance has been caused by earthquakes, or other convulsions, experienced in that part of the globe. But, of late years, a traveller (Abbé Alberto Fortis) has given a minute description of the concretion in which the bones are found in that country: And by his account it appears, that with regard to situation, composition, and colour, it is perfectly similar to that found at Gibraltar. By his description, it also appears that the two mountain rocks of Gibraltar and Dalmatia consist of the same species of calcareous stone; from which it is to be presumed, that the concretions in both have been formed in the same manner and about the same periods. "Perhaps if the fissures and caves of the rock of Dalmatia were still more minutely examined, their former communications with the surface might yet be traced, as in those described above; and, in that case, there would be at least a strong probability, that the materials of the concretions of that country have been brought together by the same accidental cause which has probably collected those found in the caverns of Gibraltar. Major Imrie traced, in Gibraltar, this concretion, from the lowest part of a deep perpendicular fissure, up to the surface of the mountain. As it approached to the surface, the concretion became less firmly combined, and, when it had no covering of the calcareous rock, a small degree of adhesion only remained, which was evidently produced by the argillaceous earth, in its composition, having been moistened by rain and baked by the sun. "The depth at which these materials had been penetrated by that proportion of silicate matter, capable of giving to the concretion its greatest adhesion and solidity, he found to vary according to its situation, and to the quantity of matter to be combined. In fissures, narrow and contracted, he found the concretion possessing a great degree of hardness at six feet from the surface; but in other situations more extended, and where a larger quantity of the materials had been accumulated, he found it had not gained its greatest degree of adhesion at double that depth. In one of the caves, where the mass of concretion is of considerable size, he perceived it to be divided into different beds, each bed being covered with a crust of the silicate spars, from one inch to an inch and a half in thickness, which seems to indicate, that the materials have been carried in at various periods, and that those periods have been very remote from each other. "At Rosia Bay, upon the west side of Gibraltar, this concretion is found in what has evidently been a cavern, originally formed by huge unshapely masses of the rock which have tumbled in together. The fissure, or cavern, formed by the disruption and subsidence of those masses, has been entirely filled up with the concretion, and is now exposed to full view by the outward mass having dropped down in consequence of the encroachments of the sea. It is to this spot that strangers are generally led to examine the phenomenon; and the composition, having here attained to its greatest degree of hardness and solidity, the hasty observer seeing the bones inclosed in what has so little the appearance of having been a vacancy, examines no further, but immediately adopts the idea of their being incased in the solid rock. The communication from this former chasm, to the surface from which it has received the materials of the concretion, is still to be traced in the face of the rock, but its opening is at present covered by the base of the line wall of the garrison. Here bones are found that are apparently human; and those of them that appear to be of the legs, arms, and vertebrae of the back, are scattered among others of various kinds and sizes, even down to the smallest bones of small birds. Major Imrie found here the complete jaw-bone of a sheep; it contained its full complement of teeth, the enamel of which was perfect, and its whiteness and lustre in no degree impaired. In the hollow parts of some of the large bones was contained a minute crystallization of pure and colourless calcareous spar; but, in most, the interior part consisted of a sparry crust of a reddish colour, scarcely in any degree transparent. "At the northern extremity of the mountain, the concretion is generally found in perpendicular fissures. The miners there, employed upon the fortifications in excavating one of those fissures, found, at a great depth from the surface, two skulls, which were supposed to be human; but, to the Major, one of them, if not both, appeared to be too small for the human species. The bone of each was perfectly firm and solid; from which it is to be presumed, that they were in a state of maturity before they were inclosed in the concretion. Had they appertained to very young children, perhaps the bone would have been more porous, and of a less firm texture. The probability is, that they belonged to a species of monkey, which still continues to inhabit, in considerable numbers, those parts of the rock which are to us inaccessible. "This concretion varies, in its composition, according to the situation in which it is found. At the extremity of Princes Lines, high in the rock which looks towards Spain, it is found to consist only of a reddish calcareous earth, and the bones of small birds cemented thereby. The rock around this spot is inhabited by a number of hawks, that, in the breeding season, nestle here and rear their young; the bones in this concretion are probably the remains of the food of those birds. At the base of the rock, below King's Lines, the concretion consists of pebbles of the prevailing calcareous rock. In this concretion, at a very considerable depth under the surface, was found the under parts of a glass bottle, uncommonly shaped, and of great thickness; the colour of the glass was of a dark green." Major Imrie makes an apology for giving so minute a description of these fossil bones; but, in our opinion, the public is indebted to him for bestowing so much attention on a subject which all must admit to be curious, and which, from the strange inferences drawn from similar phenomena by modern philosophers, has become important as well as curious. We cannot dismiss this article without noticing the subterraneous galleries constructed in the rock not only for the protection of the men during a siege, but also for placing cannon, to annoy the enemy, in situations inaccessible but by such means. The idea of forming these galleries was conceived by the late Lord Heathfield when governor, and by him, in some measure, carried into execution; though the plan was not completed till lately by General O'Hara. Of these galleries we have in the Monthly Magazine for April 1798 an animated account, which we shall insert in the writer's own words. "The subterraneous galleries are very extensive, pierce the rock in several places and in various directions, and at various degrees of elevation; all of them have a communication with each other, either by flights of steps cut in the rock, or by wooden stairs where the passages are required to be very perpendicular. "The sentinels may now be relieved during a siege from one post to another in perfect safety; whereas, previously to the constructing of these galleries, a vast number of men were killed by the Spaniards while marching to their several stations. The width of these galleries is about twelve feet, their height about fourteen. The rock is broken through in various places, both for the purpose of giving light and for placing the guns to bear on the enemy. In different parts there are spacious recesses, capable of accommodating a considerable number of men. To these recesses they give names, such as St Patrick's Chamber, St George's Hall, &c. The whole of these singular structures have been formed out of the solid rock by blasting with gunpowder. Through the politeness of an officer on duty, a place called Smart's Reservoir was opened for our inspection, which is a great curiosity, and not generally permitted to be shewn. It is a spring at a considerable depth in the body of the rock, and is above 700 feet above the level of the sea; we descended into the cavern that contains it by a rope ladder, and with the aid of lighted candles proceeded through a narrow passage over crystallized protuberances of the rock till we came to a hollow, which appears to have been opened by some convulsion of Nature. Here, from a bed of gems, arises the salutary fount, clear as the brilliant of the east, and cold as the icicle. We hailed the nymph of the grot, and, prostrating ourselves, quaffed hygean nectar from her sparry urn. When restored to the light of day, we obtained, through the medium of the same gentleman, the key of St George's Hall, at which we arrived by a very intricate and gloomy path to the spacious excavation, which is upwards of an hundred feet in length, its height nearly the same. It is formed in a semicircular part of the rock; spacious apertures are broken through, where cannons of a very large calibre command the isthmus, the Spanish lines, and a great part of the bay. The top of the rock is pierced through, so as to introduce sufficient light to enable you to view every part of it. It appears almost incredible that so large an excavation could be formed by gunpowder, without blowing up the whole of that part of the rock, and still more so, that they should be able to direct the operations of such an instrument, so as to render it subservient to the purpose of elegance. We found in the hall a table, placed, I suppose, for the convenience of those who are traversing the rock. The cloth was spread, the wine went round, and we made the vaulted roof resound with the accents of mirth and the songs of conviviality." These excavations are indeed very extraordinary works; but as the whole rock abounds with caverns, we wish that our author had inquired more particularly than he seems to have done, whether St George's Hall be wholly the work of art. From one of the passages which we have extracted from Major Imrie's memoir, we are led to think that it is not, or, at least, that the concretion removed had not acquired the consistence of the more solid parts of the rock. If this was the case, much of the wonder will vanish, since the pick-axe and chisel were probably employed to give elegance to the vault, and even, in some degree, to direct the operation of the gunpowder.