Shale

“ Ordinary clastic rocks, such as shale, sandstone, and conglomerate—altered or unaltered, as the case may be—form by far the largest proportion of our aqueous strata, and speak to us only of shallow waters. ”

“ All the phenomena, organic and inorganic, imply conditions nowhere to be met with except in the deltas of large rivers. We have to account for an abundant supply of fluviatile sediment, carried for ages towards one and the same region, and capable of forming strata of mud and sand thousands of feet, or even fathoms, in thickness, many of them consisting of laminated shale, inclosing the leaves of ferns and other terrestrial plants. ”

“ Following up through the layer of shale, we note that the ordinary bottom life gradually reappears, and shortly becomes so plentiful that the deposit resumes the character which it had before the interruption began. ”

“ The flocculating action of the salts dissolved in sea-water greatly assists the precipitation of clay before it has reached some two hundred miles from land. However, just as sandstone begets sandstone, clays or shales exposed upon a coast produce new clays close to shore. ”

“ The formation of shales will now demand our attention. When a river is carrying down with it a quantity of mud or clay, it is transported as a fine, dusty silt, and when present in quantities, gives the muddy tint to the water which is so noticeable. We can very well see how that silt will be carried down in greater quantities than sand, since nearly all rivers in some part of their course will travel through a clayey district, and finely-divided clay, being of a very light nature, will be carried forward whenever a river passes over such a district. ”

“ Such rocks are called strata. They may be of mechanical origin such as clay or mud which hardens to shale; sand, which consolidates into sandstone; and gravel, which when cemented becomes conglomerate. They may be of organic origin such as limestone, most of which is formed by the accumulation of calcareous shells; flint and chert, which are accumulations of siliceous shells; or coal, which is formed by the accumulation of partly decayed organic matter. Or, finally, they may be formed by chemical precipitation, as beds of salt, gypsum, bog iron ore, etc. ”

“ These shales have scarcely tenacity enough to hold their place in steep descents, still less, when they have been removed from their original beds, can they be made to cohere, and they form treacherous foundations for buildings erected, or for roadways constructed upon them. ”

“ In the north the hills are built of coarse sandstones, hard grey limestones, and fine black shales which hold much water, so that there are many swampy places and innumerable streams and little waterfalls. ”

“ Running water is a busy miller. It grinds earth to mud, and rocks to sand. Then, when it gets down to low ground, where it flows more slowly, it drops all this heavy matter along the river banks, in the river bed, and far out into the ocean. The gravel drops first, then the sand, then the mud. If you dig a well in the valley you will find a layer of loam on top, then sand, then gravel, on a bed of stiff clay or rock. If you scoop out a river bed you find things in the same order. In the ocean you find the gravel near the shore. It hurts your bare feet when you go in bathing. ”

“ Estuary and Shore Deposits.
Rivers which discharge themselves into the ocean, where tides and currents break with a certain regularity the quiet of its waters, exhibit always at their mouths, and often along the lower part of their channels, another set of phenomena.
Where the tide enters a river's mouth, and periodically combats the freshes, these are "backed" to certain distances, their motion is nearly destroyed for a time, and the sediment, which was only suspended by the agitation of the water, is dropped in the interval of quiescence. ”

“ Now, the power of this moving water is a mighty wide-spread agent of change in the physical condition of the globe. For wherever water is in motion over the surface of the land, whether it be a rippling stream, or a mountain torrent, or a majestic river, it is surely wearing away the channel through which it flows, and carrying along in its course particles of clay, or sand, or gravel. ”

“ As the height of the confined waters increases, the range of their journey over the bottom as the wave comes and goes rapidly increases. When they have an elevation of ten feet they can probably stir the finer mud on the ocean floor, and in shallow water move yet heavier particles. In the embayments of the land, where a great body of water journeys like an alternating river into extensive basins, the tidal action becomes intense; the current may be able to sweep along large stones quite as effectively as a mountain torrent. ”

“ The greater part of the water which enters the soil does not penetrate to any great depth in the underlying rocks, but finds its way to the surface after no long journey in the form of small springs. Generally those superficial springs do not emerge through distinct channels, but move, though slowly, in a massive way down the slopes until they enter a water course. Along the banks of any river, however small, or along the shores of the sea, a pit a few inches deep just above the level of the water will be quickly filled by a flow from this sheet which underlies the earth. ”

“ When we come to examine more closely the work which rivers do in removing mineral substances from the land by washing particles of them from the surface, we find that the records they leave in geological history must be plainly marked. Every stream, large or small, is always busy carrying mud, sand, or gravel. ”

“ There is no miraculous springing up of water out of the ground at our feet; but every fountain and well is supplied from reservoirs among the hills, so placed as to involve some slight fall or pressure enough to secure the constant flowing of the stream; and the incalculable blessing of the power given to us, in most valleys, of reaching by excavation some point whence the water will rise to the surface of the ground in perennial flow, is entirely owing to the concave dispositions of the beds of clay or rock raised from beneath the bosom of the valley into ranks of enclosing hills. ”

“ Compared with the former, these recent formations are of very limited extent; they consist of the sand and stones that are accumulated on the sea coast by tides and currents; of the land washed away from one bank of a river, and thrown up on the opposite bank by the winding stream; of the earth and gravel, and fragments of rock, carried down by all rivers, and forming deposits at their mouths; and of the constant increase of marsh land, in consequence of the growth of aquatic plants. ”

“ What we may term the littoral or shore zone of the sea occupies a belt of prevailingly shallow water, varying in width from a few score to a few hundred miles. Where the bottom descends steeply from the coast, where there are no strong off-shore setting currents, and where the region is not near the mouth of a large river which bears a great tide of sediment to the sea, the land waste may not affect the bottom for more than a mile or two from the shore. ”

“ A normal river—in fact, all the greater streams of the earth—originates in high country, generally in a region of mountains. Here, because of the elevation of the region, the streams have cut deep gorges or extensive valleys, all of which have slopes leading steeply downward to torrent beds. Down these inclined surfaces the particles worn off from the hard rock by frost and by chemical decay gradually work their way until they attain the bed of the stream. The agents which assist gravitation in bearing this detritus downward are many, but they all work together for the same end. ”

“ A river flowing to the sea or emptying itself into an inland lake, carries its burden of gravel, sand, and mud, and sooner or later, as the rate of flow slackens, it deposits the materials in the river-bed or on the floor of the sea or lake. ”

“ The transmission of water through a porous medium being so rapid, we may easily understand why springs are thrown out on the side of a hill, where the upper set of strata consist of chalk, sand, and other permeable substances, whilst those lying beneath are composed of clay or other retentive soils. ”

“ When this mechanical power of running water is considered, we are prepared for the transportation before alluded to of large quantities of gravel, sand, and mud, by torrents which descend from mountainous regions. But a question naturally arises, How the more tranquil rivers of the valleys and plains, flowing on comparatively level ground, can remove the prodigious burden which is discharged into them by their numerous tributaries, and by what means they are enabled to convey the whole mass to the sea? ”

“ The rainfall may be divided into three parts: the first DRIES UP, being discharged into the air by evaporation either directly from the soil or through vegetation; the second RUNS OFF over the surface to flood the streams; the third SOAKS IN the ground and is henceforth known as GROUND or UNDERGROUND WATER.
THE DESCENT OF GROUND WATER. Seeping through the mantle of waste, ground water soaks into the pores and crevices of the underlying rock. All rocks of the upper crust of the earth are more or less porous, and all drink in water. ”

“ The study of any considerable river basin enables us to trace the more important steps which lead to the destructure and renovation of the earth's detrital coating. In such an interpretation we note that everywhere the rocks which were built on the sea bottom, and more or less made over in the great laboratory of the earth's interior, are at the surface, when exposed to the conditions of the atmosphere, in process of being taken to pieces and returned to the sea. ”

“ First, we have to note that in the valley or region drained by a river there is another degrading or down-wearing action than that which is accomplished by the direct work of the visible stream. All over such a valley the underground waters, soaking through the soil and penetrating through the underlying rock, are constantly removing a portion of the mineral matter which they take into solution and bear away to the sea. ”

“ The sea has washed the land into them. But more, the sea-currents do not allow the sands of the æstuary to escape freely out to sea. They pile it up in shifting sand-banks about the mouth of the æstuary. The prevailing sea-winds, from whatever quarter, catch up the sand, and roll it up into sand-hills. Those sand-hills are again eaten down by the sea, and mixed with the mud of the tide-flats, and so is formed a mingled soil, partly of clayey mud, partly of sand; such a soil as stretches over the greater part of all our lowlands. ”

“ The water in passing through the sand leaves behind upon the sand grains the extremely small particles which were too fine to settle out in the settling-basin, and is quite clear as it goes from the gravel to the drains and the pumps, which forward it to the reservoir or city. ”

“ Back of the granite barriers the river spread right and left, and then went out to sea in a deep and narrow stream, curiously free from ice. Indeed, there was but little ice in the main basin, and a kind of steam hung over it so that the Poor Boy was compelled and delighted to conclude (with the aid of his companions) that the river toward its mouth must be swollen by warm springs. ”

“ To explain them, we must suppose that the ground went on sinking till the sea covered the whole place where once the swamp had been, and then sea-sand would be thrown down over the clay and gradually pressed down by the weight of new sand above, till it formed solid sandstone and our coal-bed became buried deeper and deeper in the earth. ”

“ PART I EXTERNAL GEOLOGICAL AGENCIES CHAPTER I THE WORK OF THE WEATHER In our excursion to the valley with sandstone ledges we witnessed a process which is going forward in all lands. Everywhere the rocks are crumbling away; their fragments are creeping down hillsides to the stream ways and are carried by the streams to the sea, where they are rebuilt into rocky layers. When again the rocks are lifted to form land the process will begin anew; again they will crumble and creep down slopes and be washed by streams to the sea. ”

“ The vast volume of pent-in water meeting all manner of submerged obstacles dashes in zigzag from shore to shore, cannoning off walls of rock until the whole gorge becomes one rushing, gyrating mass of angry water. ”

“ All shores are swept also by currents produced by winds and tides. These are usually far too gentle to transport of themselves the coarse materials of which beaches are made. But while the wave stirs the grains of sand and gravel, and for a moment lifts them from the bottom, the current carries them a step forward on their way. ”

“ We must suppose that at the bottom of the sea, as well as in the beds of rivers, the motions of waves, currents, and eddies often cause mud, sand, and gravel to be thrown down in heaps on particular spots, instead of being spread out uniformly over a wide area. ”

“ The pot-holes of which people talk are deep, rounded cavities, ground out of the rocky stream-bed by the scouring of sand and loose stones driven by whirling eddies in shallow basins. Every year deepens each pot-hole until some change in the stream-bed shifts the eddy to another place. ”

“ The mechanical wear of streams is performed by the aid of hard mineral fragments which are carried along by the current. The effective force is that of the current; the tools are mud, sand, and bowlders. The most important of them is sand; it is chiefly by the impact and friction of grains of sand that the rocky beds of streams are disintegrated. ”

“ There are lots of wells, in which the water is very hard, and people say it is sea-water filtered through the soil; and it must be so, for at high tide the wells are at their fullest, and quite brackish. ”

“ The shifting action of tidal currents succeeds in giving rise to very irregular deposits in shallow seas. The soundings often shew sudden changes from gravel to sand and mud, nor can there be any doubt that, could we lay bare the sea-bottom, we should often observe gravel shading off into sand, and sand into mud, and vice versâ. ”