The Beauties of Nature, and the Wonders of the World We Live In Part 17

What, then, has that history been? The same valley may be of a very different character, and due to very different causes, in different parts of its course. Some valleys are due to folds (see Fig. 41) caused by subterranean changes, but by far the greater number are, in their present features, mainly the result of erosion. As soon as any tract of land rose out of the sea, the rain which fell on the surface would trickle downwards in a thousand rills, forming pools here and there (see Fig. 37), and gradually collecting into larger and larger streams.

Wherever the slope was sufficient the water would begin cutting into the soil and carrying it off to the sea. This action would be the same in any case, but, of course, would differ in rapidity according to the hardness of the ground. On the other hand, the character of the valley would depend greatly on the character of the strata, being narrow where they were hard and tough; broader, on the contrary, where they were soft, so that they crumbled readily into the stream, or where they were easily split by the weather. Gradually the stream would eat into its bed until it reached a certain slope, the steepness of which would depend on the volume of water. The erosive action would then cease, but the weathering of the sides and consequent widening would continue, and the river would wander from one part of its valley to another, spreading the materials and forming a river plain. At length, as the rapidity still further diminished, it would no longer have sufficient power even to carry off the materials brought down. It would form, therefore, a cone or delta, and instead of meandering, would tend to divide into different branches. These three stages, we may call those of--

1. Deepening and widening;

2. Widening and levelling;

3. Filling up;

and every place in the second stage has pa.s.sed through the first; every one in the third has pa.s.sed through the second.

A velocity of 6 inches per second will lift fine sand, 8 inches will move sand as coa.r.s.e as linseed, 12 inches will sweep along fine gravel, 24 inches will roll along rounded pebbles an inch diameter, and it requires 3 feet per second at the bottom to sweep along angular stones of the size of an egg.

When a river has so adjusted its slope that it neither deepens its bed in the upper portion of its course, nor deposits materials, it is said to have acquired its "regimen," and in such a case if the character of the soil remains the same, the velocity must also be uniform. The enlargement of the bed of a river is not, however, in proportion to the increase of its waters as it approaches the sea. If, therefore, the slope did not diminish, the regimen would be destroyed, and the river would again commence to eat out its bed. Hence as rivers enlarge, the slope diminishes, and consequently every river tends to a.s.sume some such "regimen" as that shown in Fig. 46.

Now, suppose that the fall of the river is again increased, either by a fresh elevation, or locally by the removal of a barrier. Then once more the river regains its energy. Again it cuts into its old bed, deepening the valley, and leaving the old plain as a terrace high above its new course. In many valleys several such terraces may be seen, one above the other. In the case of a river running in a transverse valley, that is to say of a valley lying at right angles to the "strike" or direction of the strata (such, for instance, as the Reuss), the water acts more effectively than in longitudinal valleys running along the strike. Hence the lateral valleys have been less deeply excavated than that of the Reuss itself, and the streams from them enter the main valley by rapids or cascades. Again, rivers running in transverse valleys cross rocks which in many cases differ in hardness, and of course they cut down the softer strata more rapidly than the harder ones; each ridge of harder rock will therefore form a dam and give rise to a rapid, or cataract. We often as we ascend a river, after a comparatively flat plain, find ourselves in a narrow defile, down which the water rushes in an impetuous torrent, but at the summit of which, to our surprise, we find another broad flat valley.

Another lesson which we learn from the study of river valleys, is that, just as geological structure was shown by Sir C. Lyell to be no evidence of cataclysms, but the result of slow action; so also the excavation of valleys is due mainly to the regular flow of rivers; and floods, though their effects are more sudden and striking, have had, after all, comparatively little part in the result.

The mouths of rivers fall into two princ.i.p.al cla.s.ses. If we look at any map we cannot but be struck by the fact that some rivers terminate in a delta, some in an estuary. The Thames, for instance, ends in a n.o.ble estuary, to which London owes much of its wealth and power. It is obvious that the Thames could not have excavated this estuary while the coast was at its present level. But we know that formerly the land stood higher, that the German Ocean was once dry land, and the Thames, after joining the Rhine, ran northwards, and fell eventually into the Arctic Ocean. The estuary of the Thames, then, dates back to a period when the south-east of England stood at a higher level than the present, and even now the ancient course of the river can be traced by soundings under what is now sea. The sites of present deltas, say of the Nile, were also once under water, and have been gradually reclaimed by the deposits of the river.

It would indeed be a great mistake to suppose that rivers always tend to deepen their valleys. This is only the case when the slope exceeds a certain angle. When the fall is but slight they tend on the contrary to raise their beds by depositing sand and mud brought down from higher levels. Hence in the lower part of their course many of the most celebrated rivers--the Nile, the Po, the Mississippi, the Thames, etc.--run upon embankments, partly of their own creation.

[Ill.u.s.tration: Fig. 48.--Diagrammatic section of a valley (exaggerated)

_R R_, rocky basis of valley; _A A_, sedimentary strata; _B_, ordinary level of river; _C_, flood level.]

The Reno, the most dangerous of all the Apennine rivers, is in some places as much as 30 feet above the adjoining country. Rivers under such conditions, when not interfered with by Man, sooner or later break through their banks, and leaving their former bed, take a new course along the lowest part of their valley, which again they gradually raise above the rest. Hence, unless they are kept in their own channels by human agency, such rivers are continually changing their course.

If we imagine a river running down a regularly inclined plane in a more or less straight line; any inequality or obstruction would produce an oscillation, which when once started would go on increasing until the force of gravity drawing the water in a straight line downwards equals that of the force tending to divert its course. Hence the radius of the curves will follow a regular law depending on the volume of water and the angle of inclination of the bed. If the fall is 10 feet per mile and the soil h.o.m.ogeneous, the curves would be so much extended that the course would appear almost straight. With a fall of 1 foot per mile the length of the curve is, according to Fergusson, about six times the width of the river, so that a river 1000 feet wide would oscillate once in 6000 feet. This is an important consideration, and much labour has been lost in trying to prevent rivers from following their natural law of oscillation. But rivers are very true to their own laws, and a change at any part is continued both upwards and downwards, so that a new oscillation in any place cuts its way through the whole plain of the river both above and below.

The curves of the Mississippi are, for instance, for a considerable part of its course so regular that they are said to have been used by the Indians as a measure of distance.

If the country is flat a river gradually raises the level on each side, the water which overflows during floods being r.e.t.a.r.ded by reeds, bushes, trees, and a thousand other obstacles, gradually deposits the solid matter which it contains, and thus raising the surface, becomes at length suspended, as it were, above the general level. When this elevation has reached a certain point, the river during some flood bursts its banks, and deserting its old bed takes a new course along the lowest accessible level. This then it gradually fills up, and so on; coming back from time to time if permitted, after a long cycle of years, to its first course.

In evidence of the vast quant.i.ty of sediment which rivers deposit, I may mention that the river-deposits at Calcutta are more than 400 feet in thickness.

In addition to temporary "spates," due to heavy rain, most rivers are fuller at one time of year than another, our rivers, for instance, in winter, those of Switzerland, from the melting of the snow, in summer.

The Nile commences to rise towards the beginning of July; from August to October it floods all the low lands, and early in November it sinks again. At its greatest height the volume of water sometimes reaches twenty times that when it is lowest, and yet perhaps not a drop of rain may have fallen. Though we now know that this annual variation is due to the melting of the snow and the fall of rain on the high lands of Central Africa, still when we consider that the phenomenon has been repeated annually for thousands of years it is impossible not to regard it with wonder. In fact Egypt itself may be said to be the bed of the Nile in flood time.

Some rivers, on the other hand, offer no such periodical differences.

The lower Rhone, for instance, below the junction with the Saone, is nearly equal all through the year, and yet we know that the upper portion is greatly derived from the melting of the Swiss snows. In this case, however, while the Rhone itself is on this account highest in summer and lowest in winter, the Saone, on the contrary, is swollen by the winter's rain, and falls during the fine weather of summer. Hence the two tend to counterbalance one another.

Periodical differences are of course comparatively easy to deal with. It is very different with floods due to irregular rainfall. Here also, however, the mere quant.i.ty of rain is by no means the only matter to be considered. For instance a heavy rain in the watershed of the Seine, unless very prolonged, causes less difference in the flow of the river, say at Paris, than might at first have been expected, because the height of the flood in the nearer affluents has pa.s.sed down the river before that from the more distant streams has arrived. The highest level is reached when the rain in the districts drained by the various affluents happens to be so timed that the different floods coincide in their arrival at Paris.

FOOTNOTES:

[52] Darwin's _Voyage of a Naturalist_.

[53] _Geol. Jour._, 1863.

[54] Favre, _Rech. Geol. de la Savoie._

[55] _Growth and Structure of the Alps._

CHAPTER IX

THE SEA

There is a pleasure in the pathless woods, There is a rapture on the lonely sh.o.r.e, There is society, where none intrudes, By the deep Sea, and music in its roar: I love not Man the less, but Nature more, From these our interviews, in which I steal From all I may be, or have been before, To mingle with the Universe, and feel What I can ne'er express, yet cannot all conceal.

Roll on, thou deep and dark-blue Ocean--roll!

BYRON.

[Ill.u.s.tration: THE LAND'S END. _To face page 337._]

CHAPTER IX

THE SEA

When the glorious summer weather comes, when we feel that by a year's honest work we have fairly won the prize of a good holiday, how we turn instinctively to the Sea. We pine for the delicious smell of the sea air, the murmur of the waves, the rus.h.i.+ng sound of the pebbles on the sloping sh.o.r.e, the cries of the sea-birds; and long to

Linger, where the pebble-paven sh.o.r.e, Under the quick, faint kisses of the Sea, Trembles and sparkles as with ecstasy.[56]

How beautiful the sea-coast is! At the foot of a cliff, perhaps of pure white chalk, or rich red sandstone, or stern grey granite, lies the sh.o.r.e of gravel or sand, with a few scattered plants of blue Sea Holly, or yellow-flowered Horned Poppies, Sea-kale, Sea Convolvulus, Saltwort, Artemisia, and Sea-gra.s.ses; the waves roll leisurely in one by one, and as they reach the beach, each in turn rises up in an arch of clear, cool, transparent, green water, tipped with white or faintly pinkish foam, and breaks lovingly on the sands; while beyond lies the open Sea sparkling in the suns.h.i.+ne.

... O pleasant Sea Earth hath not a plain So boundless or so beautiful as thine.[57]

The Sea is indeed at times overpoweringly beautiful. At morning and evening a sheet of living silver or gold, at mid-day deep blue; even

Too deeply blue; too beautiful; too bright; Oh, that the shadow of a cloud might rest Somewhere upon the splendour of thy breast In momentary gloom.[58]

There are few prettier sights than the beach at a seaside town on a fine summer's day; the waves sparkling in the suns.h.i.+ne, the water and sky each bluer than the other, while the sea seems as if it had nothing to do but to laugh and play with the children on the sands; the children perseveringly making castles with spades and pails, which the waves then run up to and wash away, over and over and over again, until evening comes and the children go home, when the Sea makes everything smooth and ready for the next day's play.

Many are satisfied to admire the Sea from sh.o.r.e, others more ambitious or more free prefer a cruise. They feel with Tennyson's voyager:

We left behind the painted buoy That tosses at the harbour-mouth; And madly danced our hearts with joy, As fast we fleeted to the South: How fresh was every sight and sound On open main or winding sh.o.r.e!

We knew the merry world was round, And we might sail for evermore.

Many appreciate both. The long roll of the Mediterranean on a fine day (and I suppose even more of the Atlantic, which I have never enjoyed), far from land in a good s.h.i.+p, and with kind friends, is a joy never to be forgotten.

To the Gulf Stream and the Atlantic Ocean Northern Europe owes its mild climate. The same lat.i.tudes on the other side of the Atlantic are much colder. To find the same average temperature in the United States we must go far to the south. Immediately opposite us lies Labrador, with an average temperature the same as that of Greenland; a coast almost dest.i.tute of vegetation, a country of snow and ice, whose princ.i.p.al wealth consists in its furs, and a scattered population, mainly composed of Indians and Esquimaux. But the Atlantic would not alone produce so great an effect. We owe our mild and genial climate mainly to the Gulf Stream--a river in the ocean, twenty million times as great as the Rhone--the greatest, and for us the most important, river in the world, which brings to our sh.o.r.es the suns.h.i.+ne of the West Indies.