Part 5

49. Overlap.--When the upper beds of a conformable group of strata spread over a wider area than the lower members of the same series, they are said to overlap. The accompanying diagram shews this appearance. An overlap proves that a gradual submergence of the land was going on at the time the strata were being accumulated. As the land disappeared below the water, the sediment gradually spread over a wider area, the more recently deposited sediment being laid down in places which existed as dry land at the time when the earliest accumulations were formed. Thus, in the accompanying illustration (fig. 16), the stratum marked 1, resting unconformably upon older strata, is overlapped by 2, as that is by 3, and so on--all the beds in succession coming to repose upon the older strata at higher and higher levels, as the old land subsided.

50. Dislocations or Faults.--When strata, once continuous, have been broken across, and displaced or shifted along the line of breakage, they are said to be faulted, the fissure along which the displacement occurs being termed a fault or dislocation. The simplest form of a fault is that shewn in the following diagram, where strata of sandstone and shale, with a coal-seam, S, have been shifted along the line f. The direction in which the fault is inclined is its hade, and the degree of vertical displacement of the beds is the amount of the dislocation. Generally, the beds seem to be pulled down in the direction of the downthrow, and drawn up on the opposite side of the fault, as shewn in the diagram. Sometimes the rocks on each side of a fault are smoothed and polished, and covered with long scratches, as if the two sides of the fissure had been rubbed together. This is the appearance called slickensides. Slickensides, however, may occur on the walls of a fissure which is not a displacement, but a mere joint or crack. A dislocation is spoken of as a downthrow or an upcast, according to the direction in which it is approached. Thus, a miner working along the coal-seam S, from a to b, would describe the fault, f, as an upcast, since he would have to mine to a higher level to catch his coal again. But, had he approached the fault from c to d, he would then have termed it a downthrow, because he would see from the hade of the fault that his coal-seam must be sought for at a lower level. Faults are of all sizes, from a foot or two up to vertical displacements of thousands of feet. Powerful dislocations can often be followed for many miles across a country, running in a more or less linear direction. Thus, one large fault has been traced across the breadth of Scotland, from near St Abb's Head, in the east, to the coast of Wigtown, in the west. Every large throw is accompanied by a number of smaller ones--some of which run parallel to the main fault, while many others seem to run out from this at various angles. Faults are of all geological ages. Some date back to a most remote antiquity, others are of quite recent origin; and no doubt faults are occurring even now. In the following diagram, the strata, a, a, have been faulted and planed away before the strata, b, were deposited. Hence, in this case, it is evident that if we know the geological age of the beds, a and b, we can have an approximation to the age of the fault. If the beds, a, be Carboniferous, and those at b Permian, then we should say the fault was post-Carboniferous or pre-Permian.

The degree of inclination is very variable. It may occur at almost any angle up to vertical. But, as a rule, the hade of the more powerful faults is steeper than that of minor displacements.

51. Metamorphic and Igneous Rocks--mode of their occurrence.--In the foregoing remarks on the structure and arrangement of rocks we have had reference chiefly to the aqueous strata--that is to say, the mechanically, chemically, and organically formed rocks. We were necessarily compelled, however, to make some reference to, and to give some description of, certain structures and arrangements which are not peculiar to aqueous strata, but characterise many metamorphic and igneous rocks as well. To avoid repetition it was also necessary, while treating of joints, &c., to give some account of certain structures which are the result of metamorphic action. But, for sake of clearness, we have reserved special account of the structure and mode of occurrence of metamorphic and igneous rocks to this place. After what has been said as to the structure and arrangement of aqueous strata, it is hardly needful to say much about the crystalline schists. These the student will understand to be merely highly altered aqueous rocks, in which the marks of their origin are still more or less distinctly traceable. As a rule, metamorphic strata are contorted, twisted, and crumpled, although here and there comparatively horizontal stretches of altered rocks may be observed. The regions in which they occur are often hilly and mountainous, but this is by no means invariably the case. The greater part of the mountainous regions of the British Islands is occupied by rocks which are more or less altered; the more crystalline rocks, such as mica-schist, gneiss, &c., being abundantly developed in the Scottish Highlands, and in the north and west of Ireland; while those which are less altered cover large areas in the south of Scotland, and in Wales and the north-west of England. Throughout these wide areas the rocks generally dip at high angles, and contortion and crumpling are of common occurrence. The finer-grained clay-rocks also exhibit fine cleavage planes, and are in some places quarried for roofing-slates--the Welsh quarries being the most famous. Here and there, bedding is entirely effaced, and the resulting rock is quite amorphous, and, becoming gradually more and more crystalline, passes at last into a rock which in many cases is true granite. The original strata have disappeared, and granite occupies their place, in such a way as to lead to the inference that the granite is merely the aqueous strata which have been fused up, as it were, in situ. At other times the granite would appear to have been erupted amongst the aqueous strata, for these are highly confused, and baked, as it were, at their junction with the granite, from which, also, long veins are seen protruding into the surrounding beds. Metamorphic granite, then, graduates, as a rule, almost imperceptibly into rocks which are clearly of aqueous origin; while on the contrary the junction-line between igneous granite and the surrounding rocks is always well marked. The origin of granite, however, is a difficult question, and one which has given rise to much discussion. Some further remarks upon the subject will be found in the sequel under the heading of Metamorphism.

Igneous rocks have also in some cases undergone considerable alteration; fine-grained tuffs, for example, occasionally assume a crystalline texture.

52. True igneous rocks occur either in beds or as irregular amorphous masses. When they occur as beds interstratified with aqueous strata, they are said to be contemporaneous, because they have evidently been erupted at the time the series of strata among which they appear was being amassed. When, on the other hand, they cut across the bedding, they are said to be subsequent or intrusive, because in this case they have been formed at a period subsequent to the strata among which they have been intruded. The bed upon which a contemporaneous igneous rock reclines, often affords marks of having been subjected to the action of heat; sandstones being hardened, and frequently much jointed and cracked, owing to the shrinking induced by the heat of the once molten rock above, and clay-rocks often assuming a baked appearance. There is generally, also, some discoloration both in the pavement of rock upon which the igneous mass lies, and in the under portions of the latter itself. The beds overlying a contemporaneous igneous rock, however, do not exhibit any marks of the action of heat; the old lava-stream having cooled before the sediment, now forming the overlying strata, was accumulated over its surface. One may often notice how the sand and mud have quietly settled down into the irregular hollows and crevices of the old lava, as in the following section, where i represents the igneous rock; a being the baked pavement of sandstone, &c.; and b the overlying sedimentary deposits. When the igneous rock itself is examined, its upper portions are often observed to be scoriaceous or cinder-like, and the under portions likewise frequently exhibit a similar appearance. It is generally most solid towards the centre of the bed. The vesicles, or pores, in the upper and lower portions are often flattened, and are frequently filled with mineral matter. Sometimes these cavities may have been filled at the time the rock was being erupted, but in most cases the mineral matter would appear to have been introduced subsequently by the action of water percolating through the rock. Occasionally we meet with igneous rocks which are more or less vesicular and amygdaloidal throughout their entire mass. Others, again, often shew no vesicular structure, but are homogeneous from top to bottom. The texture is also very variable, and this even in the same rock-mass; some portions being compact or fine-grained, and others coarsely crystalline. As a rule the rock is most crystalline towards the centre, and gets finer-grained as the top and bottom of the bed are approached. Not unfrequently, however, an igneous rock will preserve the same texture throughout. The jointing is also highly irregular as a rule. But in many cases, especially when the rock is fine-grained, the jointing is very regular. The basaltic columns of the Giants' Causeway and the Isle of Staffa are well-known examples of such regularly jointed masses. Igneous rocks frequently decompose into a loose earthy mass (wacké), and this is most markedly the case with those belonging to the basic group.

53. Contemporaneous igneous rocks are frequently associated with more or less regular beds of breccia, conglomerate, ash, tuff, &c. These are evidently the loose volcanic ejectamenta which accompanied former eruptions of lava, and have been arranged by the action of water. Beds of such materials, however, frequently occur without any accompanying lava-form rocks. Nor are they always arranged in bedded masses. They sometimes appear filling vertical pipes which seem to have been the funnels of old volcanoes. The following section exhibits the general appearance of one of these volcanic necks. They are very common in some parts of Scotland, as in Ayrshire, and are frequently ranged along the line of a fault in the strata. Fig. 21 shews such a neck of ejectamenta, made up of fragments of various kinds of rock, such as sandstone, shale, limestone, coal, &c., sometimes without any admixture of igneous rocks. The strata through which the pipe has been pierced usually dip in towards the latter, and at their junction with the coarse agglomerate often shew marks of the action of heat, coal-seams having sometimes been 'burned' useless for a number of yards away from the 'neck.'

54. Intrusive igneous rocks occur as sheets, dykes, and necks. The sheets frequently conform for long distances to the bedding of the strata among which they occur, and are thus liable to be mistaken for contemporaneous rocks. But when they are closely examined, it will be seen that they not only bake or alter the beds above and below them, but seldom keep precisely to one horizon or level--occasionally rising to a higher, or sinking to a lower position in the strata, as shewn in the following diagram-section. Dykes are wall-like masses of igneous strata which cut across the strata, generally at a high angle (see d, d, fig. 22). In the neighbourhood of a recent volcanic orifice, numerous dykes are seen ramifying in all directions. In the British Islands some dykes have been followed in a linear direction for very long distances. Sometimes these occupy the sites of large dislocations, at other times they have cut through the strata without displacing them. Occasionally they appear to have been the feeders of the great sheets of igneous rock which here and there occur in their vicinity. The phenomena presented by the necks of intrusive rock do not differ from those characteristic of agglomerate or tuff necks. The strata are bent down towards the central plug of igneous rock, and are generally more or less altered at the line of junction.

55. Intrusive rocks offer, as a rule, some contrasts in texture to contemporaneous masses. They are seldom amygdaloidal, but when they are so it is generally towards the centre of the mass. The kernels are usually minute and more or less spherical.

The diagram (fig. 23) shews the general mode of occurrence of igneous rocks on the large scale. The stratified aqueous deposits are indicated at a, a. These are overlaid by a series of alternating beds of crystalline (c) and fragmental (t) igneous rocks. An irregular intrusive sheet at i cuts across the beds a, a. At p, another intrusive mass is seen rising in a pipe, as it were, and overflowing the beds a, a, so as to form a cap. A volcanic neck filled with angular stones intersects the strata at n, and two dykes, approaching the vertical, traverse the bedded rocks at d, d. It will be noticed that the contemporaneous igneous rocks form a series of escarpments rising one above the other.

The alteration effected by igneous rocks is generally greatest in the case of intrusive masses. This is well seen in some of our coal-fields, where the coal has frequently been destroyed over large areas by the proximity of masses of what was once melted rock. It is curious to notice how the intrusive sheets in a great series of strata have forced their way along the lines of least resistance. Thus, in the Scottish coal-fields, we find again and again that intrusive sheets have been squirted along the planes occupied by coal-seams, these having been more easily attacked than beds of sandstone or shale. The coal in such cases is either entirely 'eaten up,' as it were, or converted into a black soot. At other times, however, it is changed into a kind of coke, while other seams at a greater distance from the intrusive mass have been altered into a kind of 'blind coal' or anthracite.

These remarks on the mode of occurrence of igneous rocks are meant to refer chiefly to those masses which occur in regions where volcanic action has long been extinct, as, for instance, in the British Islands. In the sequel, some account will be given of the appearances presented by modern volcanoes and volcanic rocks.

It has been usual to apply the term trap or trappean rock to all the old igneous rocks which could neither be classed with the granites and syenites, nor yet with the recent lavas, &c., which are connected with a more or less well-marked volcanic vent. The term trap (Swedish trappa, a flight of steps) was suggested by the terraced or step-like appearance presented by hills which are built up of successive beds of igneous rock. But the passage from the granitic into the so-called trap rocks, and from these into the distinctly volcanic, is so very gradual, that it is impossible to say where the one class ends and the other begins. The term trap, therefore, has no scientific precision, although it is sometimes very convenient as a kind of broad generic term to include a large number of rocks.

MINERAL VEINS.

56. The cracks and crevices and joint planes which intersect all rocks in a greater or less degree, are not unfrequently filled with subsequently introduced mineral matter, forming what are termed veins. This introduced matter may either be harder or less durable than the rock itself; in the former case, the veins will project from the surface of the stone, where that has been subjected to the weathering action of the atmosphere; in the latter case, the veins, under like circumstances, are often partially emptied of their mineral matter. Not unfrequently, however, the more or less irregularly ramifying, non-metalliferous veins appear as if they had segregated from the body of the rock in which they occur, as in the case of the quartz veins in granite. Besides these irregular veins, the rocks of certain districts are traversed in one or more determinate directions by fissures, extending from the surface down to unknown depths. These great fissures are often in like manner filled with mineral matter. The minerals are usually arranged in bands or layers which run parallel to the walls of the vein. Quartz, fluor-spar, barytes, calcite, &c. are among the commonest vein-minerals, and with these are frequently associated ores of various metals. A vein may vary in width from less than an inch up to many yards, and the arrangement of its contents is also subject to much variation. Instead of parallel layers of spars and ores, frequently a confused mass of clay and broken rocks, which are often cemented together with sparry matter, chokes up the vein. The ore in a vein may occur in one or more ribs, which often vary in thickness from a mere line up to masses several yards in width. Sometimes the rocks are dislocated along the line of fissure occupied by a great vein; at other times no dislocation can be observed. Mineral veins, however, do not necessarily occupy dislocation fissures. They often occur in cavities which have been formed by the erosive action of acidulated water, in the way described in pars. 59, 60, and 61. This is frequently the case in calcareous strata. Such veins usually coincide more or less with the bedding of the rocks, but in the case of thick limestones they not unfrequently cut across the bedding in a vertical or nearly vertical direction, forming what are termed pipe-veins.

DYNAMICAL GEOLOGY.

57. Having considered the composition, structure, and arrangement of the rock-masses which form the solid crust of our globe, we have next to inquire into the nature of those physical agencies by the action of which the rocks, as we now see them, have been produced. The work performed by the various forces employed in modifying the earth's crust is at one and the same time destructive and reconstructive. Rocks are being continually demolished, and out of their ruins new rocks are being built. In other words, matter is constantly entering into new relations--now existing as solid rock, or in solution in water, or carried as the lightest dust on the wings of the wind; now being swept down by rivers into the sea, or brought under the influence of subterranean heat--but always changing, sooner or later, slowly or rapidly, from one form to another. The great geological agents of change are these: 1. THE ATMOSPHERE; 2. WATER; 3. PLANTS AND ANIMALS; 4. SUBTERRANEAN FORCES. We shall consider these in succession.

THE ATMOSPHERE.

58. All rocks have a tendency to waste away under the influence of the atmosphere. This is termed weathering. Under the influence of the sun's heat, the external portions of a rock expand, and again contract when they cool at night. The effect of this alternate expansion and contraction is often strikingly manifest in tropical countries: some rocks being gradually disintegrated, and crumbling into grit and sand; others becoming cracked, and either exfoliating or breaking up all over their surface into small angular fragments. Again, in countries subject to alternations of extreme heat and cold, similar weathering action takes place. The chemical action of the atmosphere is most observable in the case of calcareous rocks. The carbonic acid almost invariably present acts as a solvent, so that dew and rain, which otherwise would in many cases have but feeble disintegrating power, are enabled to eat into such rocks as chalk and limestone, calcareous sandstones, &c. The oxygen of the atmosphere also unites with certain minerals, such as the proto-salts of iron, and converts them into peroxides. It is this action which produces the red and yellow ferruginous discolorations in sandstone. Chemical changes also take place in the case of many igneous rocks, the result being that a weathered 'crust' forms wherever such rocks are exposed to the action of the atmosphere. Of course, the rate at which a rock weathers depends upon its mineralogical and chemical composition. Limestones weather much more rapidly than clay-rocks; and augitic igneous rocks, as a rule, disintegrate more readily than the more highly silicated species. The weathering action of the atmosphere is also greatly aided by frost, as we shall see presently. The result of all this weathering is the formation of soil--soil being only the fine-grained débris of the weathered rocks. The angular débris found at the base of all cliffs in temperate and arctic regions, and on every hill and mountain which is subjected to alternations of extreme heat and cold, is also the effect of weathering. But these and other effects of frost will be treated of under the head of Frozen Water. The hillocks and ridges of loose sand (sand dunes) found in many places along the sea-margin, and even in the interior of some continents, as in Africa and Asia, are due to the action of the wind, which drives the loose grains before it, and piles them up. Sometimes also the wind carries in suspension the finest dust, which may be transported for vast distances before it falls to the ground. Thus, fine dust shot into the air by the volcanoes of Iceland has been blown as far as the Shetland Islands; and in tropical countries the dust of the dried-up and parched beds of lakes and rivers is often swept away during hurricanes, and carried in thick clouds for leagues. Rain falling through this dust soaks it up, and comes down highly discoloured, brown and red. This is the so-called blood-rain. Minute microscopic animal and vegetable organisms are often commingled with this dust, and falling into streams, lakes, or the sea, may thus become eventually buried in sediments very far removed from the place that gave them birth.

WATER.

59. The geological action of water in modifying the crust of the earth is twofold--namely, chemical and mechanical.

Underground Water.--All the moisture which we see falling as rain or snow does not flow immediately away by brooks and rivers to the sea. Some portion of it soaks into the ground, and finds a passage for itself by cracks and fissures in the rocks below, from which it emerges at last as springs, either at the surface of the earth, or at the bottom of the sea. Such are the more obvious courses pursued by the water--it flows off either by sub-aërial or subterranean channels. But a not inconsiderable portion soaks into the solid rocks themselves, which are all more or less porous and pervious. Water thus slowly soaking often effects very considerable chemical changes. Sometimes the binding matter which held the separate particles of the rock together is dissolved out, and the rock is thus rendered soft and crumbling; at other times, the reverse takes place, and the water deposits, in the minute interstitial pores, some binding matter by which the partially or wholly incoherent grains are agglutinated into a solid mass. Thus what were originally hard and tough rocks become disintegrated to such a degree, that they crumble to powder soon after they are exposed to the air; while some again are converted into a clay, and may be dug readily with a spade. And, on the other hand, loose sand is glued into a hard building-stone. There are many other changes effected upon rocks by water, in virtue of the chemical agents which it holds in solution. Indeed, it may be said that there are very few, if any, rocks in which the chemical action of interstitial water has not formerly been, or is not at present being, carried on. Besides that which soaks through the rocks themselves, there is always a large proportion of underground water, which, as we have said above, finds a circuitous route for itself by joints, cracks, and crevices. After coursing for, it may be, miles underground, such water eventually emerges as springs, which contain in solution the various ingredients which the water has chemically extracted from the rocks. These ingredients are then deposited in proportion as the mineral water suffers from evaporation. Water impregnated with carbonate of lime, for example, deposits that compound as soon as evaporation has carried off a certain percentage of the water itself, and the carbonic acid gas which it held. This is the origin of the mineral called travertine or calcareous tufa, which is so commonly met with on the margins of springs, rivers, and waterfalls.

60. Stalactites and stalagmites have been formed in a similar way. Water slowly oozing from the roof of a limestone cavern partially evaporates there, and a thin pellicle of carbonate of lime is formed; while that portion of the water which falls to the ground, and is there evaporated, likewise gives rise to the formation of carbonate of lime. By such constant dropping and evaporating, long tongue-and icicle-like pendants (stalactites) grow downwards from the roof; while at the same time domes and bosses (stalagmites) grow upwards from the floor, so as sometimes to meet the former and give rise to continuous pillars and columns. The great solvent power of carbonated water is shewn first by the chemical analysis of springs, and, secondly, by the great wasting effects which the long-continued action of these has brought about. Thus, it has been estimated that the fifty springs near Carlsbad, which yield eight hundred thousand cubic feet of water in twenty-four hours, contain in solution as much lime as would go to form a mass of stone weighing two hundred thousand pounds. Warm, or, as they are termed, thermal springs, frequently carry away with them, out of the bowels of the earth, vast quantities of mineral matter in solution. The waters at Bath, for instance, are estimated to bring to the surface an annual amount of various salts, the mass of which is not less than 554 cubic yards. One of the springs of Louèche, France, however, carries out with it no less than 8,822,400 pounds of gypsum annually, which is equal to about 2122 cubic yards.

61. It is easy to conceive, therefore, that in the course of ages great alterations must be caused by springs. Caves and winding galleries, and irregular channels, will be worn out of the rocks which are thus being dissolved. Especially will this be the case in countries where calcareous rocks abound. It is in such regions, accordingly, where we meet with the most striking examples of caves and underground river-channels. The largest cave at present known is the Mammoth Cave, in Kentucky. This remarkable hollow consists of numerous winding galleries and passages that cross and recross, and the united length of which is said to be 217 miles. In calcareous countries, rivers, after flowing for, it may be, miles at the surface, suddenly disappear into the ground, and flow often for long distances before they reappear in the light of day. In some regions, indeed, nearly all the drainage is subterranean. The surface of the ground, in calcareous countries, frequently shews circular depressions, caused by the falling in of the roofs of caverns. Sometimes, also, great masses of rock, often miles in extent, get loosened by the dissolving action of subterranean water, and crash downwards into the valleys. Such landslips, as they are called, are not, however, confined to calcareous regions. In 1806, a large section of the Rossberg, a mountain lying to the north of the Righi, consisting of conglomerate overlying beds of clay, rushed down into the plains of Goldau, overwhelming four villages and nearly a thousand inhabitants. The cause of this catastrophe was undoubtedly the softening into mud of the clay-beds on which the conglomerate rested, for the season which had just terminated when the slip took place had been very wet. The mass of material that slid down was estimated to contain upwards of fifty-four millions of cubic yards; it reached not less than two and a half miles in length, by some three hundred and fifty yards wide, and thirty-five yards thick.

62. Surface-water--Rain.--Having now learned something as to the modifications produced by underground water, we turn next to consider the action of surface-water, and the results arising from that action. Rain, when it falls to the ground, carries with it some carbonic acid gas which it has absorbed from the atmosphere. Armed with this solvent, it attacks certain rocks, more especially limestones and chalk, a certain proportion of which it licks up and delivers over to brooks and streams. Under its influence, also, the finer particles of the soil are ever slowly making their way from higher to lower levels. Rocks which are being gradually disintegrated by weathering have their finer grains and particles, thus loosened, carried away by rain. Nor is this rain-action so inconsiderable as might be supposed. In the gentler hollows of an undulating country, we frequently find accumulations of clay, loam, and brick-earth, which often reach many feet in thickness, and which are undoubtedly the results of rain washing down the particles of soil, &c. from the adjacent slopes.

63. River-action.--The water of streams and rivers almost invariably contains in solution one or more chemical compounds, and in this respect does not differ from the water of springs. Of course, this mineral matter is derived in considerable measure from springs, but is also no doubt to a large extent taken up by the rivers themselves, as they wash the rocks and soils on their journey to the sea. The amount of mineral matter thus transported must be something enormous, as is shewn by the chemical analyses of river-water. Bischof calculated that the Rhine carries in solution as much carbonate of lime as would suffice for the yearly formation of three hundred and thirty-two thousand millions of oyster-shells of the usual size--a quantity equal to a cube five hundred and sixty feet in the side, or a square bed a foot thick, and upwards of two miles in the side. But the mechanical erosion effected by running water is what impresses us most with the importance of rivers as geological agencies. This erosive action is due to the gravel, sand, and mud carried along by the water. These ingredients act as files in the hand of a workman, and grind, polish, and reduce the rocks against which they are borne. The beds of some streams that flow over solid rock are often pitted with circular holes, at the bottom of which one invariably finds a few rounded stones. These stones, kept in constant motion by the water, are the means by which the pot-holes, as they are called, have been excavated. When pot-holes are numerous, they often unite so as to form curious smooth-sided trenches and gullies. The same filing action goes on all over the bed of the stream wherever the solid rock is exposed. And while the latter is being gradually reduced, the stones and grit which act as the files are themselves worn and reduced; so that stones diminish in size, and grit passes into fine sand and mud, as they move from higher to lower levels. No doubt the erosive action of running water appears to have but small effect in a short time, and we are apt, therefore, to underestimate its power. But when our observations extend, we see it is quite otherwise, and that, so far from being unimportant, running water is really one of the most powerful of all the geological agencies that are employed in modifying the earth's crust. Even within a comparatively short time, it is able to effect very considerable changes. Thus, the river Simeto, in Sicily, having become dammed by a stream of lava flowing from Etna, succeeded, in two hundred and fifty years, in cutting through hard solid basalt a new channel for itself, which measured from twenty to fifty mètres in depth, and from twelve to eighteen in breadth. When, also, we remember the fact, that no river is absolutely free from mineral matter held in suspension, but that, on the contrary, all running water is more or less discoloured with sediment, which is merely the material derived from the disintegration of rocks, it will appear to us difficult to overestimate the power of watery erosion. To the mineral matter held in suspension, we have to add the coarser detritus, gravel and sand, which is being gradually pushed along the beds of rivers, and which, in the case of the Mississippi, has been estimated to equal a mass of seven hundred and fifty million cubic feet, discharged annually into the Gulf of Mexico. By careful measurements, it has also been ascertained that the same river carries down annually into the sea a weight of mud held in suspension which reaches the vast sum of 812,500,000,000 pounds. The total annual amount of mineral matter, whether held in suspension or pushed along the bottom of this great river, has been estimated to equal a mass 268 feet in height, with an area of one square mile.

64. Alluvium.--The sediment carried along and deposited by a river is called alluvium. Sometimes this alluvium covers wide areas, forming broad flats on one or both sides of a river, and in such cases it is due to the action of the floodwaters of the stream. Every time the river overflows the low grounds through which it passes, a layer of sediment is laid down, which has the effect of gradually raising the level of the alluvial tract. By and by a time comes when the river, which has all the while been slowly deepening its channel, is unable to flood the flats, and thereupon it begins to cut into these, and to form new flats at a somewhat lower level. In this way we often observe a series of alluvial terraces, consisting of gravel, sand, and silt, rising one above another along a river valley. Such are the terraces of the Thames and other rivers in England, and of the Tweed, Clyde, Tay, &c. in Scotland. The great plains through which the Rhine flows between Basel and Bingen, are also well-marked examples of alluvial accumulations. There are very few streams, indeed, which have not formed such deposits along some portion of their course.

65. When a river enters a lake, the motion of the water is of course checked, and hence the heavier detritus, such as gravel and coarse sand, moves more slowly forward, and at last comes to rest on the bed of the lake, at no great distance from the mouth of the river. Finer sand and mud are carried out for some distance further, but eventually they also cease to move, and sink to the bottom. When the lake is sufficiently large, it catches all or nearly all the matter brought down by the river, which, as it issues from the lower end of the lake, is bright and clear. A well-known example of this phenomenon is that of the Rhone, which enters the Lake of Geneva turbid and muddy, but rushes out quite clear at the lower end of the lake. Lakes, therefore, are all being slowly or more rapidly silted up, and this, of course, is most conspicuous at the points where they are entered by rivers. Thus, at the head of the Lake of Geneva, it is manifest that the wide flat through which the river flows before it pours into the lake, has been conquered by the Rhone from the latter. In the times of the Romans, the lake, as we know, extended for more than one mile and a half further up the valley.