Part 15

What we have now said on the great eminences of the earth, may also be observed on the greatest depths of the sea. The vast and highest seas are nearer the equator than the poles; and there results from this observation, that the greatest inequalities of the globe are in the southern climate. These irregularities on the surface of the earth, are the causes of an infinity of extraordinary effects: for example, between the Indus and the Ganges, there is a large peninsula, which is divided through its middle, by a chain of high mountains called the Gate, and which extends from north to south, from the extremities of Mount Caucasus to Cape Comorin; on one is the coast of Malabar, and the other Coromandel; on the side of Malabar, between this chain of mountains and the sea, the summer season lasts from September to April, during which the sky is serene and dry; on the other side the Coromandel the above period is their winter, and it rains every day plentifully and from the month of April to the month of September is their summer, whereas it is winter in Malabar; insomuch, that in many places, which are scarcely 20 miles distant, we may, by crossing the mountains, change seasons. It is said that the same thing takes place at Razalgat in Arabia, and at Jamaica, which is divided through its middle by a chain of mountains, whose direction is from east to west, and that the plantations to the south of these mountains feel the summer heat, at the time those to the north endure the rigor of winter.

Peru, which is situated under the line, and extends about a thousand leagues to the south, is divided into three long and narrow parts; these the natives call Lanos, Sierras, and Andes. The Lanos, which comprehends the plains, extends along the coast of the South Sea: the Sierras are hills with some vallies, and the Andes are the famous Cordeliers, the highest mountains that are known. The Lanos is about ten leagues in breadth; in many places the Sierras are twenty leagues broad, and the Andes in some places more and in some less. The breadth is from east to west, and the length from north to south. This part of the world is remarkable for the following particulars: first, in the Lanos the wind almost constantly blows from the south-west, which is contrary to what happens in the torrid zone: secondly, it never rains nor thunders in the Lanos, although there is plenty of dew: thirdly, it almost continually rains in the Andes: fourthly, in the Sierras, between the Lanos and the Andes, it rains from September to April.

It was for a long time supposed, that the chains of the high mountains run from west to east, till the contrary was found in America. But no person before M. Bourguet discovered the surprising regularity of the structure of those great masses: he found (after having crossed the Alps thirty times in fourteen different parts of it, twice over the Apennine mountains, and made divers tours in the environs of these mountains, and of Mount Jura) that all mountains are formed nearly after the manner of works of a fortification. When the body of the mountain runs from east to west, it forms prominences, which face the north and south; this wonderful regularity is so striking in vallies, that we seem to walk in a very regular covered way; if, for example, we travel in a valley from north to south, we perceive that the mountain on the right forms projections which front the east, and those of the mountain on the left front the west, so that the saliant angles of one side reciprocally answer the returning angles of the other, which are always alternatively opposed to them. The angles which mountains form in great vallies are less acute, because the direction is less steep, and they are farther distant from each other. In plains they are not so perceptible, except by the banks of rivers, which are generally in the middle of them, and whose natural windings answer the most advanced angles or striking projections of the mountains. It is astonishing so visible a thing was so long unobserved, for when in a valley the inclination of one of the mountains which border it is less steep than that of the other, the river takes its course much nearer the steepest mountain, and does not flow through its middle.

To these observations we may join other particular ones, which confirm them; for example, the mountains of Switzerland are much more steep, and their direction much greater on the south side than on the north, and on the west side than on the east. This may be perceived in the mountains of Gemmi, Brisa, and almost every other mountain in this country. The highest are those which separate Valesia and the Grisons from Savoy, Piedmont, and Tirol. These countries are only a continuation of these mountains, the chain of which extends to the Mediterranean, and continues even pretty far under the sea. The Pyrennees are also only a continuation of that vast mountain which begins in Upper Valesia, and whose branches extend very far to the west and south, preserving throughout the same great height; whereas on the side of the north and of the east these mountains grow lower by degrees, till they become plains; as we see by the large tract which the Rhine and Danube water before they reach their mouths, whereas the Rhone descends with rapidity towards the south into the Mediterranean. The same observation is found to hold good in the mountains of England and Norway; but the part of the world where this is most evidently seen is at Peru and Chili; the Cordeliers are cut very sharply on the western side, the length of the Pacific Ocean, whereas on the eastern side they lower by degrees into large plains, watered by the greatest rivers of the world.[279:A]

M. Bourguet, to whom we owe this great discovery of the correspondence of the angles of mountains, terms it "_The Key of the Theory of the Earth_;" nevertheless, it appears to me, that if he had conceived all the importance of it, he would more successfully have made use of it, by connecting it with suitable facts, and would have given a more probable theory of the earth; whereas in his treatise he presents only the skeleton of an hypothetical system, most of the conclusions of which are false or precarious. The theory we have given turns on four principal facts, which cannot be doubted, after the proofs have been examined on which they are founded. The first is, that the earth is every where, and to considerable depths, composed of parallel strata, and matters which have formerly been in a state of softness: the second, that the sea has for ages covered the earth which we now inhabit; the third, that the tides and other motions of the waters produce inequalities at the bottom of the sea; and the fourth, that the mountains have taken their form and the correspondent direction from the currents of the sea.

After having read the proofs which the following articles contain, it may be determined, whether I was wrong to assert, that these circumstances solidly established also ascertains the truth of the theory of the earth. What I have said on the formation of mountains has no need of a more ample explanation; but as it might be objected that I do not assign a reason for the formation of the peaks or points of mountains, no more than for some other particular circumstances, shall add the observations and reflections which I have made on this subject.

I have endeavoured to form a clear and general idea of the manner in which the different matters that compose the earth are arranged, and it appears to me they may be reduced into two general classes; the first includes all the matters we find placed in strata, or beds horizontally or regularly inclined; and the second comprehends all matters formed in masses, or in veins, either perpendicular or irregularly inclined. In the first class are included sands, clays, granite, flints, free-stone, coals, slates, marls, chalks, calcinable stones, marbles, &c. In the second I rank metals, minerals, crystals, precious stones and small flints: these two classes generally comprehend all the known materials of the earth. The first owe their origin to the sediments carried away and deposited by the sea, and should be distinguished into those which being assayed in the fire, calcine and are reduced into lime, and those which fuse and are convertible into glass. The materials of the second class are all vitrifiable excepting those which the fire entirely consumes by inflammation.

In the first class we distinguish two kinds of sands; the one, which is more abundant than any other matter of the globe, is vitrifiable, or rather is only fragments of actual glass; the other, whose quantity is much less, is calcinable, and must be looked upon as the powder of stone, and which differs only from gravel by the size of the grains. The vitrifiable sand is, in general, deposited in beds, which are often interrupted by masses of free-stone, granite, and flint; and sometimes these matters are also in banks of great extent.

By examining these vitrifiable matters, we find only a few sea shells there, and those not placed in beds, but dispersed about as if thrown there by chance. For example, I have never seen them in free-stone; that stone which is very plenty in certain places, is only composed of sandy parts, which are re-united, and are only met with in sandy soils; and the quarries of it are generally in peaked hills and in divided eminences. We may work these quarries in all directions, and if they are in large beds, they are much farther from each other than in quarries of calcinable stone or marble. Blocks of free-stone may be cut of all dimensions and in all directions, although it is difficult to work, it nevertheless has but a degree of hardness sufficient to resist powerful strokes without splitting; for friction easily reduces it into sand, excepting certain black pieces found therein, and which are so very hard, that the best files cannot touch them. Rock is vitrifiable as free-stone, and of the same nature, only it is harder and the parts more connected. This also contains many hard pieces, as may easily be remarked on the summits of high mountains, which cut and tear the shoes of travellers. This rocky stone, which is found at the top of high mountains, and which I look upon as a kind of granite, contains a great quantity of talky leaves, and is so hard as not to be worked but by an infinite deal of labour.

I have narrowly examined these sharp pieces which are found in free-stone and rock, and have discovered it to be a metallic matter, melted and calcined by a very violent fire, and which perfectly resembles certain substances thrown out by the volcanos, of which I saw a great quantity when I was in Italy, where the people called them Schiarri. They are very heavy black masses, on which neither water nor the file can make any impression, and the matter of which is different from that of the lava; for this is a kind of glass, whereas the other appears to be more metallic than vitreous. The sharp pieces in free-stone, and rock, resemble greatly the first matter, which seems still to prove that all these matters have been formerly liquified by fire.

We sometimes see on the upper parts of mountains, a prodigious quantity of blocks of this mixed rock; their position is so irregular that they appear to have been thrown there by chance, and it might be thought they had fallen from some neighbouring height, if the places where they are found were not raised above the other parts. But their vitrifiable nature, and their angular and square figures, like those of free-stone, discover them to be of one common origin. Thus in the great beds of vitrifiable sand, blocks of free-stone and rock are formed, whose figures and situations do not exactly follow the horizontal position of these strata. The rain, by degrees, carried away from the summits of the hills and mountains the sand which at first covered them, and then began to furrow and cut those hills into the spaces which are found between the nucleus in free-stone, as the hills of Fontainbleau are intersected. Each hilly point answers to a nucleus in a quarry of free-stone, and each interval has been excavated and loosened by the rain, which has caused the sand, they at first contained, to flow into the vallies; so likewise the highest mountains, whose summits are composed of rocks, and terminated by these angular blocks of granite, have formerly been covered with vitrifiable sand, and the rain having carried away the sand which covered them, they remained on the tops of the mountains in the position they were formed. These blocks generally present points; they increase in size in proportion as they descend; one block often rests upon another, the second upon a third, and so on, leaving irregular intervals between them: and as in time the rain washed away all the sand which covered these different parts on the top of the high mountains, they would remain naked, forming larger or lesser points; and this is the origin of the peaks or horns of mountains.

For supposing, as it is easy to prove by the marine productions we find there, that the chain of the Alps was formerly covered by the sea, and that above this chain there was a great thickness of vitrifiable sand, which rendered the whole mountains a flat and level country. In this depth of sand, there would necessarily be formed granite, free-stone, flint, and all matters which take their origin and figure in sand, nearly in a similar manner to that of the crystallisation of salts. These blocks once formed would support their original positions, after the rains and torrents had carried away the sand which surrounded them, and being left bare formed all those peaks or pointed eminences we see in so many places. This is also the origin of those high and detached rocks found in China and other countries, as in Ireland, where they are called the Devil's stones, and whose formation as well as that of the peaks of mountains, had hitherto appeared so difficult to explain; nevertheless the explanation which I have given is so natural, that it directly presents itself to the mind of those who examine these objects, and I must here quote what Father Tatre says, "From Yanchu-in-yen, we came to Hoytcheou, and on the road met with something particular, rocks of an extraordinary height, of the shape of a large square tower, and situate in the midst of vast plains: I cannot account for it, unless by supposing they were formerly mountains, from which the rain having washed away the earth that surrounded them, thus left the rocks entirely bare. What strengthens this conjecture is, that we saw some which, towards the base, are still covered with earth to a considerable height."

The summits of the highest mountains are composed of rocks, of granite, free-stone, and other hard and vitrifiable matters, and this often as deep as two or three hundred fathoms; below which we often meet with quarries of marble, or hard stone, filled with fossil-shells, and whose matter is calcinable; as may be remarked at Great Chartreuse, in Dauphiny, and on Mount Cenis, where the stone and marble, which contains shells, are some hundred fathoms below the summits, points and peaks of high mountains; although these stones are more than a thousand fathom above the level of the sea. Thus mountains, whereon we see points or peaks, are generally vitrifiable rock, and those whose summits are flat, mostly contain marble and hard stones filled with marine productions. It is the same with respect to hills, for those containing granite, or free-stone, are mostly intersected with points, eminences, cavities, depths, and small intermediate valleys; on the contrary, those which are composed of calcinable stone are nearly equal in height, and are only interrupted by greater and more regular vallies, whose angles are correspondent; and they are crowned with rocks whose position is regular and level.

Whatever difference may appear at first between these two species of mountains, their forms proceed from the same cause, as we have already observed; only it may be remarked, that the calcinable stones have not undergone any alteration nor change since the formation of the horizontal strata; whereas those of vitrifiable sand have been changed and interrupted by the posterior production of rocks and angular blocks formed within this sand. These two kinds of mountains have cracks which are almost always perpendicular in those of calcinable stones; but those of granite and free-stone appear to be a little more irregular in their direction. It is in these cracks metal, minerals, crystals, sulphurs, and all matters of the second class are found, and it is below these cracks that the water collects to penetrate the earth, and form those veins of water which are every where found below the surface.

FOOTNOTES:

[279:A] See Phil. Trans. Abr. Vol. VI. part ii. p. 153.

ARTICLE X.

OF RIVERS.

We have before said that, generally speaking, the greatest mountains are in islands and in the projections in the sea. That in the old continent the greatest chains of mountains are directed from west to east, and that those which incline towards the north or south are only branches of these principal chains; we shall likewise find that the greatest rivers are directed as the greatest mountains, and that there are but few which follow the course of the branches of those mountains. To be assured of this, we have only to look on a common globe, and trace the old continent from Spain to China. We shall find, by beginning at Spain, that the Vigo, Douro, Tagos, and Guadiana run from east to west, and the Ebro from west to east, and that there is not one remarkable river whose course is directed from south to north, or from north to south, although Spain is entirely surrounded by the sea on the west side, and almost so on the north. This observation on the directions of rivers in Spain not only proves that the mountains in this country are directed from west to east, but also that the southern lands, which border on the straits, are higher than the coasts of Portugal; and on the northern coast, that the mountains of Galicia, the Asturias, &c. are only a continuation of the Pyrennees, and that it is this elevation of the country, as well north as south, which does not permit the rivers to run into the sea that way.

It will also be seen, by looking on the map of France, that there is only the Rhone which runs from north to south, and nearly half its course, from the mountains to Lyons, is directed from the east towards the west; but that on the contrary all the other great rivers, as the Loir, the Charantee, the Garonne, and even the Seine, have a direction from east to west.

It will be likewise perceived, that in Germany there is only the Rhine, which like the Rhone shapes the greatest part of its course from north to south, but that the others, as the Danube, the Drave, and all the great rivers which fall into them, flow from the west to east into the Black Sea.

It will be perceived that this Black Sea, which should rather be considered as a great lake, has almost three times more extent from east to west than from north to south, and consequently its direction is similar to the rivers in general. It is the same with the Mediterranean, whose length from east to west is about six times greater than from north to south.

The Caspian Sea, according to the chart drawn by the order of Czar Peter I. has more extent from the south to the north than from east to west; whereas in the ancient charts it appears almost round, or rather more broad from east to west than from south to north; but if we consider the lake Aral as a part of the Caspian Sea, from which it is separated only by plains of sand, we shall find the length is from the western coast of the Caspian Sea as far as the greatest border of Lake Aral.

So likewise the Euphrates, the Persian gulph, and almost all the rivers in China run from west to east; all the rivers in Africa beyond Barbary flow from east to west, or from west to east, and there are only the rivers of Barbary and the Nile which flow from south to north. There are, in fact, great rivers in Asia which partly run from north to south, as the Wolga, the Don, &c. but by taking the whole length of their course, we find, that they only turn from the south to run into the Black and Caspian seas, which are only inland lakes.

It may therefore in general be said, that in Europe, Asia, and Africa, the rivers, and other mediterranean waters, extend more from east to west than from north to south, which proceeds from the chains of mountains being for the most part so directed, and that the whole continent of Europe and Asia is broader in this direction than the other; for there are two modes of considering the direction of mountains. In a long and narrow continent like South America, in which there is only one principal chain of mountains which stretches from south to north, the river not being confined by any parallel range, necessarily runs perpendicular to the course of the mountains, that is from east to west, or from west to east; in fact, it is in this direction all the rivers of America flow. In the old as well as the new continent most of the waters have their greatest extent from west to east, and most of the rivers flow in this direction; but yet this similar direction is produced by different causes; for instance, those in the old continent flow from east to west, because they are bounded by mountains whose direction is from west to east; whereas those in America preserve the same course from there being only one chain of mountains that extends from north to south.

In general, rivers run through the centre of vallies, or rather the lowest ground betwixt two opposite hills or mountains; if the two hills have nearly an equal inclination, the river will be nearly in the middle of the intermediate valley, let the valley be broad or narrow. On the contrary, if one of the hills has a more steep inclination than the other, the river will not be in the middle of the valley, but much nearer the hill whose inclination is greatest, and that too in proportion to the superiority of its declivity: in this case, the lowest ground is not in the middle of the valley, but inclines towards the highest hill, and which the river must necessarily occupy. In all places where there is any considerable difference in the height of the mountains, the rivers flow at the foot of the steepest hills, and follow them throughout all their directions, never quitting their course while they maintain the superiority of height. In the length of time, however, the steepest hills are diminished by the rain acting upon them with a greater degree of force, proportionate to their height, and consequently carry away the sand and gravel in more considerable quantities, and with greater violence; the river is then constrained to change its bed, and seek the lowest part of the valley: to this may be added, that as all rivers overflow at times, they transport and deposit mud and sand in different places, and that sands often accumulate in their own beds, and cause a swell of the water, which changes the direction of its course. It is very common to meet in vallies with a great number of old channels of the river, particularly if it is subject to frequent inundations, and carries off much sand and mud.