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the Atlantic the salt which the surface- the Caucasian side. But the northern current brings in. But this constant in- portion is nowhere more than 50 fee terchange between the water of the Medi- deep ; and this depth is continually bes terranean and that of the outside Ocean, ing reduced by the alluvial deposit would in time most assuredly reduce the brought down by the rivers which dis density of Mediterranean water to that of charge themselves into this part of the the Atlantic, if it were not as constantly basin, notably the Volga and the Ural. maintained ; and no other cause for its These rivers run through an immense constant maintenance can be shown, than expanse of steppes, the slope of which excess of evaporation.
towards the Caspian is almost imperBut, says Professor Huxley, it would ceptible ; so that if the level of its waseem, when we consider the enormous ters were to be raised even very slightly, amount of fresh water poured into the an expanse of land at least equal to its basin of the Mediterranean by the great present area would be covered by it. rivers which discharge themselves into Now, as the present level is about eighty it, that “the sun must have enough to do feet below that of the Black Sea, whilst to keep the level on the Mediterranean ample evidence that the steppes were fordown." This part of the question has merly overflowed by salt water is afbeen more fully and carefully investi- forded by beds of marine shells, as well gated (as I shall presently show) than my as by the persistence of numerous salt friend seems to have supposed; but be- | lakes and salt marshes, there can be no fore proceeding to discuss it, I shall question that the northern basin of the bring to bear upon it the very remarkable Caspian formerly extended over the results of the inquiries made into the whole plain of the Volga below Saratov; Physical condition of the Caspian Sea, and no other cause can be assigned for by a man whom Professor Huxley and its contraction than the excess of evaporaI hold in equal respect, — the distin- tion over the return of water by rain and guished Professor Von Baer, who was rivers. sent thither some years ago by the Rus- But such a reduction in the volume of sian Government to report upon its Fish- water as must have taken place in order eries. This, the largest existing Inland to produce this lowering of level, would Sea without any outlet, is a “survival” of have shown itself, it might be supposed, that great central sea, which, at no remote in an increase of its salinity ; whereas geological period, covered a large part of the fact is the proportion of salt (which Northern Asia ; the gradual upheaval of varies in different parts of the basin, and the land having separated it from the also at different seasons) is on the averEuxine on the one side, and from the age only about one-fourth of that which Sea of Aral on the other, as well as from is found in Oceanic water, and does not the Arctic Sea with which this marine much exceed one-half of the proportion province was formerly in communication. contained in the water of the Euxine. How small an elevation has sufficed to This reduction, however, is fully excut off this communication on the north- plained by the observations of Von Baer, ern side, is shown by the fact, that the who traces it to the number of shallow connection of the Dwina with the Volga, lagoons by which the basin is surroundby a system of canals, has opened a way ed, every one of which is a sort of natufor vessels to pass between the Caspian ral “salt pan” for the evaporation of the and the White Sea. Thus remaining water and the deposit of its saline matisolated in the midst of land, the Caspian ter in the solid form. This process may has undergone a series of very remark- be well studied in the neighbourhood of able changes, which can be distinctly Novo-Petrosk on the eastern coast; traced out.
where what was formerly a bay is now In the first place it is evident, (as was divided into a large number of basins, long since pointed out by Pallas) that presenting every degree of saline conthe former extent of the Caspian was centration. One of these still occasionmuch greater than its present area. The ally receives water from the sea, and has southern portion of its basin, which lies deposited on its banks only a very thin among mountains whose escarpments ex- layer of salt. A second, likewise full of tend beneath the water, is by far the water, has its bottom hidden by a thick deepest; a large part of its bottom ly-crust of rose-coloured crystals like a ing between 2000 and 3000 feet below the pavement of marble. A third exhibits a present surface of the water. The middle compact mass of salt, in which are pools portion has also a considerable depth on of water whose surface is more than a.
yard below the level of the sea. And a | lowest estimates of the degree of saltness fourth has lost all its water by evapora- of the Caspian water, the width and depth tion; and the stratum of salt left behind of the channel, and the speed of the is now covered by sand. A similar con- current, Von Baer has shown that the centration is taking place in the arm of Karaboghaz alone daily receives from the sea termed Karasu (Black Water), the Caspian the enormous quantity of which runs southwards from the north- three hundred and fifty thousand tons of east angle of the basin; for notwith salt. If such an elevation were to take standing the proximity of the mouths of place of the surface of the bar, as should the great rivers, the proportion of salt separate the Karaboghaz from the basin there rises so greatly above that of the of the Caspian, it would quickly diminish ocean, that animal' life, elsewhere ex-in extent, its banks would be converted tremely abundant, is almost or altogether into immense fields of salt, and the sheet suppressed.
of water which might remain would be This process goes on upon the great- either converted into a shallow lake – est scale, however, in the Karaboghaz, like Lake Elton, which is 200 miles from a shallow diverticulum from the eastern the present northern border of the Caspart of the middle basin, which is prob- pian; or a salt marsh — like those which ably a “survival ” of the former commu- cover extensive tracts of the steppes ; or nication between the Caspian and the might altogether disappear by drying up, Sea of Aral. This vast gulf communi- as seems to have been the case with a cates with the sea by a narrow mouth, depressed area lying between Lake Elton which is not more than about 150 yards and the River Ural, which is 79 feet bewide, and 5 feet deep; and through this low the level of the Caspian, and about channel a current is always running in- as much more below that of the Black wards with an average speed of three Sea. It is impossible that a more " pregmiles an hour. This current is accelerat-nant instance” could be adduced, of the ed by westerly and retarded by easterly effect of evaporation alone in maintaining winds; but it never flows with less rapid- a powerful current, than is afforded by ity than a mile and a half per hour. this case of the Karaboghaz. The navigators of the Caspian, and the That when the basin of the Caspian Turkoman nomads who wander on its had been once completely isolated, the shores, struck with the constant and un- level of its water was rapidly lowered by swerving, course of this current, have evaporation, until its area was so far resupposed that its waters pass down into duced as to keep down the amount of a subterranean abyss (Karaboghaz, black evaporation to that of the return of fresh gulf), through which they reach' either water by rain and rivers, is shown by Von the Persian Gulf or the Black Sea. For Baer to be an almost inevitable inference this hypothesis, however, there is not from facts of two independent orders. the least foundation. The basin, being At the height of from 65 to 80 feet above exposed to every wind and to most in- the present level, the rocks which formed tense summer heat, is subject to the loss the original seashore of the southern of an enormous quantity of water by basin have been furrowed out into toothevaporation ; and as there is very little shaped points and needles ; lower down, direct return by streams, the deficit can on the contrary, the rocks now laid bare only be supplied by a flow from the Cas- show no trace of the erosive action of the pian. The small depth of the bar seems water ; so that its level would seem to to prevent the return of a counter-cur- have sunk too rapidly to allow the waves rent of denser water ; none such having sufficient time to attack the cliff-walls been detected, although the careful in-effectively. Again, along the shallow vestigations made by Von Baer would border of the northern basin, the shore have shown its presence if it really ex- for a space of 250 miles is gashed with isted. And thus there is a progressively thousands of narrow channels, from increasing concentration of the water twelve to thirty miles in length, separated within the basin of the Karaboghaz; so by chains of hillocks, which pass inland that seals which used to frequent it are into the level ground of the steppes. In no longer found there, and its borders the neighbourhood of the mouths of the are entirely destitute of vegetation. Volga, which brings down a greatly inLayers of salt are being deposited on creased volume of water at the time of the the mud at the bottom ; and the sound- melting of the snows, the excess flows ing-line, when scarcely out of the water, into these channels, and thus tends to is covered with saline crystals. Taking the keep them open; so that, when the inundation is over, the sea again passes up the contribution of the Danube, the them. Further to the south, on the other Dnieper, the Dniester, the Don, and hand, the channels, like the intervening other rivers that empty themselves icto hillocks, are not continuous, but form the Black Sea, towards the supply of the chains of little lakes, separated by sandy Mediterranean, is only the excess which isthmuses. Although these channels run remains after compensating for the evapnearly parallel to each other, yet they oration of the Black Sea, or (assuming have a somewhat fan-like arrangement; the equality of this with the evaporation their centre of radiation being the higher of the Caspian) the excess of the volume part of the isthmus which separates the of the Black Sea rivers over that of the slope of the Caspian from that of the N.E. Caspian rivers, which (as will presently portion of the Black Sea. It is difficult appear) must be a very insignificant conto see how these channels can have been tribution to the Mediterranean in comformed, except by the furrowing of the parison with the area of the latter. soft soil during the rapid sinking of the How small that excess really is, may level of the Caspian water; as happens be gathered from the experiments on the on the muddy banks of a reservoir, in Dardanelles and Bosphorus currents, of which the water is being rapidly lowered which the particulars have already been by the opening of a sluice-gate.
given. For not only is the outward Now, since in the area of the Caspian, surface-current extremely variable in its as at present limited, an equilibrium has rate, and liable to occasional reversal, been established between the quantity of but, when it is at its strongest, its effect water lost by evaporation, and that re- is most counteracted by the inward underturned to it by rain and rivers (for chere current. The proportional force and is no reason to believe that any continu- volume of the two currents cannot be ous change of level is now going on), we estimated from these experiments with can arrive at a better idea of what the anything like certainty ; but Captain amount of such evaporation really is, from Wharton thinks that the under-current what is needed to make it good, than we sometimes carries in as much as twehave any other means of forming. The thirds of the water that the surface-current Volga is, next to the Danube, the largest carries out. That it ordinarily returns at European river, and its drainage-area is least half, may be fairly inferred from the enormous; the Ural is a considerable constant maintenance of the average sariver, probably not bringing down much linity of the Black Sea water at about less water than the Don ; whilst the Kur half that of Mediterranean water ; since and the Araxes, which drain a large part it is obvious that this proportion could of Transcaucasia, cannot together be not be kept up, unless as much salt remuch inferior to the Dnieper : and yet enters the basin by the under-current, as the whole mass of water brought down by passes out of it by the upper. Hence, as these four rivers, serves only to keep the the salinity of the under-current is twice present level of the Caspian from being that of the upper, its volume may be further lowered by evaporation.
taken at about one-half; so that the ai
tual excess of outflow will be only about Let us now compare with the Caspian one-half of the volume of water that forms the Black Sea, with which it was formerly the surface-current. And thus the whole in continuity, and which communicates contribution of the great rivers that disindirectly with the general Oceanic sys- charge themselves into the Black Sea, tem. The area of the Black Sea (includ- to the maintenance of the level of the ing the Sea of Azov) and that of the Mediterranean, is represented by an outCaspian are nearly equal ; each being flow through the Dardanelles by no means estimated at about 180,000 square miles. exceeding the amount brought down by a They lie for the most part between the single considerable river. same Annual Isotherms of 60° and 50°, the extensions of the Caspian to the south We now turn to the Mediterranean; of the former and to the north of the and shall again use the Caspian as a latter being nearly, equal; and hence basis on which we may form some kind we may conclude that the evaporation of approximative estimate as to the profrom the two seas is nearly the same. portion between the evaporation from its Now, as the whole water of the Volga and surface and the return by river-flow. of the other rivers that empty themselves In the first place, the area of the Medinto the Caspian is only sufficient to make iterranean, including the Ægean and the up for its evaporation, it is obvious that Adriatic, is between four and five times
the present area of the Caspian ; so that, assuming that, to counteract this enortaking the evaporation over equal areas mous evaporation, the volume of riverof the two seas to be the same, the quan- water poured into the Mediterranean tity of return that would be needed to ought to be at least six times that rekeep up the level of the Mediterranean, ceived by the Caspian. But what is the would be between four and five times as actual amount of that supply? Along great as that which suffices to maintain the whole African coast, from the Strait that of the Caspian. But looking to the of Gibraltar to the Nile, there is nothing fact that the principal part of the area of that can be called a large river. Around the Mediterranean lies east and west the whole Levant there is the same debetween the parallels of 32° and 40° N. ficiency. And thus, with the exception lat., whilst that of the Caspian lies north of the Nile and of the Po — a slow-flowand south between the parallels of 36° ing river of very moderate volume, no and 46°, it seems obvious that this dif- great body of water is poured into the ference alone would cause the evapora- Eastern basin of the Mediterranean, save tion of the Mediterranean to be much the overflow of the Black Sea, which greater for equal areas than that of the comes down through the Bosphorus and Caspian. The ordinary Summer temper- Dardanelles. How small a contribution ature of a considerable part of the East is made by this overflow to the mainern basin of the Mediterranean is not tenance of the general level of the Medmuch below 80°: I have myself seen it iterranean, seems apparent from the fact ranging from 75° to 80° between Malta that the specific gravity of the water of and Alexandria, in the early part of Octo- the Ægean, with which it first mingles, is ber. And, notwithstanding the curious scarcely, if at all, lowered by the internorthern bend by which the summer mixture of the half-salt stream which Isotherm of 80° is carried through Greece discharges itself into the part of it most and Asia Minor, along the southern remote from its communication with that shore of the Black Sea, it only just larger basin. Into the Western basin of touches the southern basin of the Cas- the Mediterranean, no other considerable pian; the summer temperature of nearly rivers discharge themselves than the the whole of this sea being below that of Rhone and the Ebro. Thus the sumthe northernmost parts of the Mediter- total of the supply brought into the whole ranean. The difference is far greater, Mediterranean area by great rivers, may however, during the Winter months. be expressed by the Nile, one-half of the Taking the lowest winter temperature of Dardanelles surface-current, the Po, the the Mediterranean at Professor Huxley's Rhone, and the Ebro. And if we add to average of 48° (and I have reason to these the “ten submarine springs of believe that this is some degrees too low fresh water which are known to burst up for the Eastern basin, whilst it is not at in the Mediterranean,” it seems to me all too high for the Western), we find the perfectly clear that we cannot make that January mean of the Caspian to range total anything like six times the amount from 40° at its southern extremity, to that is brought into the Caspian by the 30° in its middle başin, while its northern Volga, the Ural, and the Transcaucasian basin is crossed by the January isotherm rivers, and which has been shown to be of 20°. Hence, as regards Temperature entirely dissipated by evaporation. – It alone, the mean annual excess is largely has been estimated by two French on the side of the Mediterranean. But officers, MM. Régy and Vigan,* who there is another element not less impor- have recently compared the probable tant, — the extreme dryness of the hot evaporation of the Mediterranean with winds which blow over the Mediterranean the rain-fall over its area, that the annual (especially its Eastern basin) from the excess of the former represents a stratum great African deserts, and which take up of 4 1-2 feet; and the largest estimate of an enormous amount of moisture in their the amount brought in by rivers cannot
Having heard much of the make up a third of this quantity:t scorching power of the Sirocco, I was
Annales des Ponts et Chaussées, 1863 and 1866. surprised, when in Malta (towards which
† Sir John Herschel, adopting somewhat different this wind blows from the south-east), to data, came to a conclusion essentially the same. find that its enervating effect was due to ing in the Black Sea as part of the Mediterranean basin, its excessive humidity, derived from the siders it as traversed medially by the Isotherm of 63" extent of sea it had traversed since leav- The excess of evaporation over rain fall, for the entire
area, he reckons at 28 inches, giving 398 cubic miles to ing the Libyan deserts.
ways. Now the Nile is estimated We should not be far wrong, then, in to deliver through the year less than 22 cubic miles ;
be supplied in other
With such an adequate vera causa as | 70° Fahr, as the uniform temperature this' enormous excess of evaporation, from the surface to the bottom at 450 there is no occasion to go in search of fathoms. This February temperature any other explanation for the Gibraltar may be taken as representing the isocheiin-current. For it is obvious that if the mal of the northern part of the Red Sea: "marine water-shed” between Capes Traf- and, until evidence to the contrary shall algar and Spartel were to be raised 1000 have been obtained, we may assume that feet, so as to cut off the Mediterranean the deep temperature of no part of the basin from the Atlantic, the excess of Red Sea falls below this, unless reduced evaporation from its surface would pro- by the inflow of cold water from the deeper duce a progressive reduction of its level stratum of the Arabian Gulf.
as has happened with the Caspian,- A very interesting question here arises, until its area came to be so far restricted as to the possible influence of this unias to limit its evaporation to the amount formly elevated Temperature in the Red returned to it by rain and rivers. But so Sea, upon the growth of the Corals which long as this communication remains open, abound in its basin and form the reefs so so long will an in-current through the dangerous to the navigator. It seems to Strait of Gibraltar maintain the present be the universal opinion of those who level and area of the Mediterranean. have most carefully studied the existing That this in-current persists through the Coral Formations in the Oceanic area, winter (which is advanced by Prof. Hux- that the reef-building types do not live ley as an objection to the received doc- and grow at a greater depth than the trine) is easily explained. The tempera- twenty fathoms first assigned as their ture of the surface, though reduced to limit by Mr. Darwin. Yet as Stony 50 degrees or thereabouts, is still suffi- Corals similar to these in every Physiociently high (especially under dry African logical character, save massiveness, have winds) to maintain a considerable amount been repeatedly brought up in the Porcuof evaporation ; and it is during the sea- pine dredgings from depths of several son of this reduced evaporation, that the hundred fathoms, there seems no à priori river-supply is least. For all the great reason for the restriction of the reefrivers which discharge themselves into builders to this limited depth; and it has the Mediterranean basin are at their low- suggested itself to me, whether the limit est during the winter months, their upper is not really one of temperature. For it sources being then frozen up; and it is is pointed out by Mr. Dana in his recent with the melting of the snows that they treatise on “Corals and Coral-Islands," become filled again.
as a deduction from the Geographical
Distribution of the reef-builders, that Although I was at first inclined to re- they cannot live in any part of the Ocean gard the uniform Temperature of the of which the temperature ever falls below great mass of Mediterranean water below 68°: so that even the Galapagos islands, the variable surface-stratum, as mainly which lie under the Equator, are outside dependent on that of the subjacent crust the boundary-line of the Coral Sea ; this of the Earth, yet my later and more ex- being carried to the north of the Equator tended inquiries have led me to believe by the cold (Humboldt's) current which that the coincidence is here accidental ; comes up along the Western Coast of and that, as in the case of other Inland South America, and which I regard as Seas, the uniform temperature is mainly the indraught of the Pacific Equatorial determined by the lowest winter tempera- current. Now all we at present know of ture of the area. For I found it to be the relation of Temperature to Depth, about two degrees higher in the Eastern would indicate that even in the Interbasin than in the Western, in accordance tropical area of the open Ocean, the temwith its lower latitude. And in the Red perature at twenty fathoms may not be Sea it seems to be very considerably much above 68°, and that in the next ten above this; the Temperature-soundings fathoms it suffers a considerable reductaken by Captain Nares in the Gulf of tion; so that the bathymetrical limit of Suez, in the month of February, giving the reef-builders may really be a thermal
one. And if the temperature of the Red so that even on the extravagant supposition that each Sea everywhere and throughout the year of the other principal rivers (the Danube, Dnieper, should prove to be above that limit
, it Don, Dniester, Po, Rhone, and Ebro,) contribute as will become a most interesting question miles of river-supply, leaving 335 to be furnished by the to determine whether the reef-building Atlantic.” (Physical Geography, p. 27.)
Corals are, or are not, to be found in that