Science in Arcady

by Grant Allen

If we turn from the lakes themselves to the Lombard plain at large, which is an immensely older and larger basin, we see traces of the same action on a vastly greater scale. A glance at the map will show the intelligent and ever courteous reader that the 'wandering Po'—I drop into poetry after Goldsmith—flows much nearer the foot of the Apennines than of the Alps in the course of its divagations, and seems purposely to bend away from the greater range of mountains. Why is this, since everything in nature must needs have a reason? Well, it is because, when the mud first began to accumulate in the old Lombard bay of the Adriatic, there was no Po at all, whether wandering or otherwise: the big river has slowly grown up in time by the union of the lateral torrents that pour down from either side, as the growth of the mud flat brought them gradually together. Careful study of a good map will show how this has happened, especially if it has the plains and mountains distinctively tinted after the excellent German fashion. The Ticino, the Adda, the Mincio, if you look at them close, reveal themselves as tributaries of the Po, which once flowed separately into the Lombard bay; the Adige, the Piave, the Tagliamento farther along the coast, reveal themselves equally as tributaries of the future Po, when once the great river shall have filled up with its mud the space between Trieste and Venice, though for the moment they empty themselves and their store of detritus into the open Adriatic.

Fix your eyes for a moment on Venetia proper, and you will see how this has all happened and is still happening. Each mountain torrent that leaps from the Tyrolese Alps bring down in its lap a rich mass of mud, which has gradually spread over a strip of sea some forty or fifty miles wide, from the base of the mountains to the modern coast-line of the province. Near the sea—or, in other words, at the temporary outlet—it forms banks and lagoons, of which those about Venice are the best known to tourists, though the least characteristic. For miles and miles between Venice and Trieste the shifting north shore of the Adriatic consists of nothing but such accumulating mud banks. Year after year they push farther seaward, and year after year fresh islets and shoals grow out into the waves beyond the temporary deltas. In time, therefore, the gathering mud banks of these Alpine torrents must join the greater mud bank that runs rapidly seaward at the delta of the Po. As soon as they do so the rivers must rush together, and what was once an independent stream, emptying itself into the Adriatic, must become a tributary of the Po, helping to swell the waters of that great united river. The Adige has now just reached this state: its delta is continuous with the delta of the Po, and their branches interosculate. The Mincio and the Adda reached it ages since: the Piave and the Livenia will not reach it for ages. In Roman days Hatria was still on the sea: it is now some fifteen miles inland.

From all this you can gather why the existing Po flows far from the Alps and nearer the base of the Apennines. The Alpine streams in far distant days brought down relatively large floods of glacial mud; formed relatively large deltas in the old Lombard bay; filled up with relative rapidity their larger half of the basin. The Apennines, less lofty, and free from glaciers, sent down shorter and smaller torrents, laden with far less mud, and capable therefore of doing but little alluvial work for the filling in of the future Lombardy. So the river was pushed southward by the Alpine deposits of the northern streams, leaving the great plains of Cisalpine Gaul spread away to the north of it.

And this land-making action is ceaseless and continuous. About Venice, Chioggia, Maestra, Comacchio, the delta of the Po is still spreading seaward. In the course of ages—if nothing unforeseen occurs meanwhile to prevent it—the Alpine mud will have filled in the entire Adriatic; and the Ionian Isles will spring like isolated mountain ridges from the Adriatic plain, as the Euganean hills—those 'mountains Euganean' where Shelley 'stood listening to the pæan with which the legioned rocks did hail the sun's uprise majestical'—spring in our own time from the dead level of Lombardy. Once they in turn were the Euganean islands, and even now to the trained eye of the historical observer they stand up island-like from the vast green plain that spreads flat around them.

Perhaps it seems to you a rather large order to be asked to believe that Lombardy and Venetia are nothing more than an outspread sheet of deep Alpine mud. Well, there is nothing so good for incredulity, don't you know, as capping the climax. If a man will not swallow an inch of fact, the best remedy is to make him gulp down an ell of it. And, indeed, the Lombard plain is but an insignificant mud flat compared with the vast alluvial plains of Asiatic and American rivers. The alluvium of the Euphrates, of the Mississippi, of the Hoang Ho, of the Amazons would take in many Lombardies and half-a-dozen Venetias without noticing the addition. But I will insist upon only one example—the rivers of India, which have formed the gigantic deep mud flat of the Ganges and the Jumna, one of the very biggest on earth, and that because the Himalayas are the highest and newest mountain chain exposed to denudation. For, as we saw foreshadowed in the case of the Alps and Apennines, the bigger the mountains on which we can draw the greater the resulting mass of alluvium. The Rocky Mountains give rise to the Missouri (which is the real Mississippi); the Andes give rise to Amazons and the La Plata; the Himalayas give rise to the Ganges and the Indus. Great mountain, great river, great resulting mud sheet.

At a very remote period, so long ago that we cannot reduce it to any common measure with our modern chronology, the southern table-land of India—the Deccan, as we call it—formed a great island like Australia, separated from the continent of Asia by a broad arm of the sea which occupied what is now the great plain of Bengal, the North-West, and the Punjaub. This ancient sea washed the foot of the Himalayas, and spread south thence for 600 miles to the base of the Vindhyas. But the Himalayas are high and clad with gigantic glaciers. Much ice grinds much mud on those snow-capped summits. The rivers that flowed from the Roof of the World carried down vast sheets of alluvium, which formed fans at their mouths, like the cones still deposited on a far smaller scale in the Lake of Geneva by little lateral torrents. Gradually the silt thus brought down accumulated on either side, till the rivers ran together into two great systems—one westward—the Indus, with its four great tributaries, Jhelum, Chenab, Ravee, Sutlej; one eastward, the Ganges, reinforced lower down by the sister streams of the Jumna and the Brahmapootra. The colossal accumulation of silt thus produced filled up at last all the great arm of the sea between the two mountain chains, and joined the Deccan by slow degrees to the continent of Asia. It is still engaged in filling up the Bay of Bengal on one side by the detritus of the Ganges, and the Arabian Sea on the other by the sand-banks of the Indus.

In the same way, no doubt, the silt of the Thames, the Humber, the Rhine, and the Meuse tend slowly (bar accidents) to fill up the North Sea, and anticipate Sir Edward Watkin by throwing a land bridge across the English Channel. If ever that should happen, then history will have repeated itself, for it is just so that the Deccan was joined to the mainland of Asia.

One question more. Whence comes the mud? The answer is, Mainly from the detritus of the mountains. There it has two origins. Part of it is glacial, part of it is leaf-mould. In order to feel we have really got to the very bottom of the mud problem—and we are nothing if not thorough—we must examine in brief these two separate origins.

The glacier mud is of a very simple nature. It is disintegrated rock, worn small by the enormous millstone of ice that rolls slowly over the bed, and deposited in part as 'terminal moraine' near the summer melting-point. It is the quantity of mud thus produced, and borne down by mountain torrents, that makes the alluvial plains collect so quickly at their base. The mud flats of the world are in large part the wear and tear of the eternal hills under the planing action of the eternal glaciers.

But let us be just to our friends. A large part is also due to the industrious earth-worm, whose place in nature Darwin first taught us to estimate at its proper worth. For there is much detritus and much first-rate soil even on hills not covered by glaciers. Some of this takes its origin, it is true, from disintegration by wind or rain, but much more is caused by the earth-worm in person. That friend of humanity, so little recognized in his true light, has a habit of drawing down leaves into his subterranean nest, and there eating them up, so as to convert their remains into vegetable mould in the form of worm-casts. This mould, the most precious of soils, gets dissolved again by the rain, and carried off in solution by the streams to the sea or the lowlands, where it helps to form the future cultivable area. At the same time the earthworms secrete an acid, which acts upon the bare surface of rock beneath, and helps to disintegrate it in preparation for plant life in unfavourable places. It is probable that we owe almost more on the whole to these unknown but conscientious and industrious annelids than even to those 'mills of God' the glaciers, of which the American poet justly observes that though they grind slowly, yet they grind exceedingly small.