even a prejudiced observer will readily admit that the most valuable mineral on earth is mud. diamonds and rubies are just nowhere by comparison. i don't mean weight for weight, of course—mud is 'cheap as dirt,' to buy in small quantities—but aggregate for aggregate. quite literally, and without hocus-pocus of any sort, the money valuation of the mud in the world must outnumber many thousand times the money valuation of all the other minerals put together. only we reckon it usually not by the ton, but by the acre, though the acre is worth most where the mud lies deepest. nay, more, the world's wealth is wholly based on mud. corn, not gold, is the true standard of value. without mud there would be no human life, no productions of any kind: for food stuffs of every description are raised on mud; and where no mud exists, or can be made to exist, there, we say, there is desert or sand-waste. land, without mud, has no economic value. to put it briefly, the only parts of the world that count much for human habitation are the mud deposits of the great rivers, and notably of the nile, the euphrates, the ganges, the indus, the irrawaddy, the hoang ho, the yang-tse-kiang; of the po, the rhone, the danube, the rhine, the volga, the dnieper; of the st. lawrence, the mississippi, the missouri, the orinoco, the amazons, the la plata. a corn-field is just a big mass of mud; and the deeper and purer and freer from stones or other impurities it is the better.

but england, you say, is not a great river-mud field; yet it supports the densest population in the world. true; but england is an exceptional product of modern civilization. she can't feed herself: she is fed from odessa, alexandria, bombay, new york, montreal, buenos ayres—in other words, from the mud fields of the russian, the egyptian, the indian, the american, the canadian, the argentine rivers. orontes, said juvenal, has flowed into tiber; nile, we may say nowadays, with equal truth, has flowed into thames.

there is nothing to make one realize the importance of mud, indeed, like a journey up nile when the inundation is just over. you lounge on the deck of your dahabieh, and drink in geography almost without knowing it. the voyage forms a perfect introduction to the study of mudology, and suggests to the observant mind (meaning you and me) the real nature of mud as nothing else on earth that i know of can suggest it. for in egypt you get your phenomenon isolated, as it were, from all disturbing elements. you have no rainfall to bother you, no local streams, no complex denudation: the nile does all, and the nile does everything. on either hand stretches away the bare desert, rising up in grey rocky hills. down the midst runs the one long line of alluvial soil—in other words, nile mud—which alone allows cultivation and life in that rainless district. the country bases itself absolutely on mud. the crops are raised on it; the houses and villages are built of it; the land is manured with it; the very air is full of it. the crude brick buildings that dissolve in dust are nile mud solidified; the red pottery of assiout is nile mud baked hard; the village mosques and minarets are nile mud whitewashed. i have even seen a ship's bulwarks neatly repaired with mud. it pervades the whole land, when wet, as mud undisguised; when dry, as dust-storm.

egypt, says herodotus, is a gift of the nile. a truer or more pregnant word was never spoken. of course it is just equally true, in a way, that bengal is a gift of the ganges, and that louisiana and arkansas are gifts of the mississippi; but with this difference, that in the case of the nile the dependence is far more obvious, far freer from disturbing or distracting details. for that reason, and also because the nile is so much more familiar to most english-speaking folk than the american rivers, i choose egypt first as my type of a regular mud-land. but in order to understand it fully you mustn't stop all your time in cairo and the delta; you mustn't view it only from the terrace of shepheard's hotel or the rocky platform of the great pyramid at ghizeh: you must push up country early, under mr. cook's care, to luxor and the first cataract. it is up country that egypt unrolls itself visibly before your eyes in the very process of making: it is there that the full importance of good, rich black mud first forces itself upon you by undeniable evidence.

for remember that, from a point above berber to the sea, the dwindling nile never receives a single tributary, a single drop of fresh water. for more than fifteen hundred miles the ever-lessening river rolls on between bare desert hills and spreads fertility over the deep valley in their midst—just as far as its own mud sheet can cover the barren rocky bottom, and no farther. for the most part the line of demarcation between the grey bare desert and the cultivable plain is as clear and as well-defined as the margin of sea and land: you can stand with one foot on the barren rock and one on the green soil of the tilled and irrigated mud-land. for the water rises up to a certain level, and to that level accordingly it distributes both mud and moisture: above it comes the arid rock, as destitute of life, as dead and bare and lonely as the centre of sahara. in and out, in waving line, up to the base of the hills, cultivation and greenery follow, with absolute accuracy, the line of highest flood-level; beyond it the hot rock stretches dreary and desolate. here and there islands of sandstone stand out above the green sea of doura or cotton; here and there a bay of fertility runs away up some lateral valley, following the course of the mud; but one inch above the inundation-mark vegetation and life stop short all at once with absolute abruptness. in egypt, then, more than anywhere else, one sees with one's own eyes that mud and moisture are the very conditions of mundane fertility.

beyond cairo, as one descends seaward, the mud begins to open out fan-wise and form a delta. the narrow mountain ranges no longer hem it in. it has room to expand and spread itself freely over the surrounding country, won by degrees from the mediterranean. at the mouths the mud pours out into the sea and forms fresh deposits constantly on the bottom, which are gradually silting up still newer lands to seaward. slow as is the progress of this land-forming action, there can be no doubt that the nile has the intention of filling up by degrees the whole eastern mediterranean, and that in process of time—say in no more than a few million years or so, a mere bagatelle to the geologist—with the aid of the po and some other lesser streams, it will transform the entire basin of the inland sea into a level and cultivable plain, like bengal or mesopotamia, themselves (as we shall see) the final result of just such silting action.

it is so very important, for those who wish to see things "as clear as mud," to understand this prime principle of the formation of mud-lands, that i shall make no apology for insisting on it further in some little detail; for when one comes to look the matter plainly in the face, one can see in a minute that almost all the big things in human history have been entirely dependent upon the mud of the great rivers. thebes and memphis, rameses and amenhotep, based their civilisation absolutely upon the mud of nile. the bricks of babylon were moulded of euphrates mud; the greatness of nineveh reposed on the silt of the tigris. upper india is the indus; agra and delhi are ganges and jumna mud; china is the hoang ho and the yang-tse-kiang; burmah is the paddy field of the irrawaddy delta. and so many great plains in either hemisphere consist really of nothing else but mud-banks of almost incredible extent, filling up prehistoric baltics and mediterraneans, that a glance at the probable course of future evolution in this respect may help us to understand and to realize more fully the gigantic scale of some past accumulations.

as a preliminary canter i shall trot out first the valley of the po, the existing mud flat best known by personal experience to the feet and eyes of the tweed-clad english tourist. everybody who has looked down upon the wide lombard plain from the pinnacled roof of milan cathedral, or who has passed by rail through that monotonous level of poplars and vines between verona and venice, knows well what a mud flat due to inundation and gradual silting up of a valley looks like. what i want to do now is to inquire into its origin, and to follow up in fancy the same process, still in action, till it has filled the adriatic from end to end with one great cultivable lowland.

once upon a time (i like to be at least as precise as a fairy tale in the matter of dates) there was no lombardy. and that time was not, geologically speaking, so very remote; for the whole valley of the po, from turin to the sea, consists entirely of alluvial deposits—or, in other words, of alpine mud—which has all accumulated where it now lies at a fairly recent period. we know it is recent, because no part of italy has ever been submerged since it began to gather there. to put it more definitely, the entire mass has almost certainly been laid down since the first appearance of man on our earth: the earliest human beings who reached the alps or the apennines—black savages clad in skins of extinct wild beasts—must have looked down from their slopes, with shaded eyes, not on a level plain such as we see to-day, but on a great arm of the sea which stretched like a gulf far up towards the base of the hills about turin and rivoli. of this ancient sea the adriatic forms the still unsilted portion. in other words, the great gulf which now stops short at trieste and venice once washed the foot of the alps and the apennines to the superga at turin, covering the sites of padua, ferrara, bologna, ravenna, mantua, cremona, modena, parma, piacenza, pavia, milan, and novara. the industrious reader who gets out his baedeker and looks up the shaded map of north italy which forms its frontispiece will be rewarded for his pains by a better comprehension of the district thus demarcated. the idle must be content to take my word for what follows. i pledge them my honour that i'll do my best not to deceive their trustful innocence.

it may sound at first hearing a strange thing to say so, but the whole of that vast gulf, from turin to venice, has been entirely filled up within the human period by the mud sheet brought down by mountain torrents from the alps and the apennines.

a parallel elsewhere will make this easier of belief. you have looked down, no doubt, from the garden of the hotel at glion upon the lake of geneva and the valley of the rhone about villeneuve and aigle. if so, you can understand from personal knowledge the first great stage in the mud-filling process; for you must have observed for yourself from that commanding height that the lake once extended a great deal farther up country towards bex and st. maurice than it does at present. you can still trace at once on either side the old mountainous banks, descending into the plain as abruptly and unmistakably as they still descend to the water's edge at montreux and vevey. but the silt of the rhone, brought down in great sheets of glacier mud (about which more anon) from the furca and the jungfrau and the monte rosa chain, has completely filled in the upper nine miles of the old lake basin with a level mass of fertile alluvium. there is no doubt about the fact: you can see it for yourself with half an eye from that specular mount (to give the devil his due, i quote milton's satan): the mud lies even from bank to bank, raised only a few inches above the level of the lake, and as lacustrine in effect as the veriest geologist on earth could wish it. indeed, the process of filling up still continues unabated at the present day where the mud-laden rhone enters the lake at bouveret, to leave it again, clear and blue and beautiful, under the bridge at geneva. the little delta which the river forms at its mouth shows the fresh mud in sheets gathering thick upon the bottom. every day this new mud-bank pushes out farther and farther into the water, so that in process of time the whole basin will be filled in, and a level plain, like that which now spreads from bex and aigle to villeneuve, will occupy the entire bed from montreux to geneva.

turn mentally to the upper feeders of the po itself, and you find the same causes equally in action. you have stopped at pallanza—garoni's is so comfortable. well, then, you know how every alpine stream, as it flows, full-gorged, into the italian lakes, is busily engaged in filling them up as fast as ever it can with turbid mud from the uplands. the basins of maggiore, como, lugano, and garda are by origin deep hollows scooped out long since during the great ice age by the pressure of huge glaciers that then spread far down into what is now the poplar-clad plain of lombardy. but ever since the ice cleared away, and the torrents began to rush headlong down the deep gorges of the val leventina and the val maggia, the mud has been hard at work, doing its level best to fill those great ice-worn bowls up again. near the mouth of each main stream it has already succeeded in spreading a fan-shaped delta. i will not insult you by asking you at the present time of day whether you have been over the st. gothard. in this age of trains de luxe i know to my cost everybody has been everywhere. no chance of pretending to superior knowledge about japan or honolulu; the tourist knows them. very well, then; you must remember as you go past bellinzona—revolutionary little bellinzona with its three castled crags—you look down upon a vast mud flat by the mouth of the ticino. part of this mud flat is already solid land, but part is mere marsh or shifting quicksand. that is the first stage in the abolition of the lakes: the mud is annihilating them.

maggiore, indeed, least fortunate of the three main sheets, is being attacked by the insidious foe at three points simultaneously. at the upper end, the ticino, that furious radical river, has filled in a large arm, which once spread far away up the valley towards bellinzona. a little lower down, the maggia near locarno carries in a fresh contribution of mud, which forms another fan-shaped delta, and stretches its ugly mass half across the lake, compelling the steamers to make a considerable detour eastward. this delta is rapidly extending into the open water, and will in time fill in the whole remaining space from bank to bank, cutting off the upper end of the lake about locarno from the main basin by a partition of lowland. this upper end will then form a separate minor lake, and the ticino will flow out of it across the intervening mud flat into the new and smaller maggiore of our great-great-grandchildren. if you doubt it, look what the torrent of the toce, the third assailing battalion of the persistent mud force, has already done in the neighbourhood of pallanza. it has entirely cut off the upper end of the bay, that turns westward towards the simplon, by a partition of mud; and this isolated upper bit forms now in our own day a separate lake, the lago di mergozzo, divided from the main sheet by an uninteresting mud bank. in process of time, no doubt, the whole of maggiore will be similarly filled in by the advancing mud sheet, and will become a level alluvial plain, surrounded by mountains, and greatly admired by the astute piedmontese cultivator.

what is going on in maggiore is going on equally in all the other sub-alpine lakes of the po valley. they are being gradually filled in, every one of them, by the aggressive mud sheet. the upper end of lugano, for example, has already been cut off, as the lago del piano, from the main body; and the piano itself, from which the little isolated tarn takes its name, is the alluvial mud fiat of a lateral torrent—the mud flat, in fact, which the railway from porlezza traverses for twenty minutes before it begins its steep and picturesque climb by successive zigzags over the mountains to menaggio. similarly the influx of the adda at the upper end of como has cut off the lago di mezzola from the main lake, and has formed the alluvial level that stretches so drearily all around colico. slowly the mud fiend encroaches everywhere on the lakes; and if you look for him when you go, there you can see him actually at work every spring under your very eyes, piling up fresh banks and deltas with alarming industry, and preparing (in a few hundred thousand years) to ruin the tourist trade of cadenabbia and bellagio.

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 p?an 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.

in the last resort, then, it is mainly on mud that the life of humanity in all countries bases itself. every great plain is the alluvial deposit of a great river, ultimately derived from a great mountain chain. the substance consists as a rule of the débris of torrents, which is often infertile, owing to its stoniness and its purely mineral character; but wherever it has lain long enough to be covered by earth-worms with a deep black layer of vegetable mould, there the resulting soil shows the surprising fruitfulness one gets (for example) in lombardy, where twelve crops a year are sometimes taken from the meadows. everywhere and always the amount and depth of the mud is the measure of possible fertility; and even where, as in the great american desert, want of water converts alluvial plains into arid stretches of sand-waste, the wilderness can be made to blossom like the rose in a very few years by artificial irrigation. the diversion of the arkansas river has spread plenty over a vast sage scrub; the finest crops in the world are now raised over a tract of country which was once the terror of the traveller across the wild west of america.