sharp line, but disposed according to their relative weight. The mixture may be rapidly stirred for a few seconds, but still the coarse particles will sink and the fine refuse to do so. Let mud be substituted for the silt, and it will be seen how long a time the sediment is in forming, and how necessary perfect tranquillity is. Now change the scene to a river entering a sea or lake. The rushing torrent has brought down with it portions of rock, and quantities of fine and coarse silt and mud. The rocks and pebbles roll along the bottom, producing, as they grind against each other, finer particles, which are carried more rapidly onwards and prevented by the current from settling, leaving behind all the heavier stones and sand, and passing further away into the lake or ocean. Wherever the gravel settles are the materials for a future conglomerate; wherever the sand, of a sandstone; and the mud, settling down in quiet bays or deep places, becomes in time shale, slate, or limestone.

It is very important that the mechanical action of flowing water should be thoroughly understood; therefore, at the risk of repetition, more minute details may be entered into. It is immaterial whether the scale be great or small-the Mississippi or a rill-the manner in which the materials of our future stratified rock are collected, transported, and deposited is the same, and has been the same from the earliest times. On a wet day we may observe any rill running down a road over sloping ground to a lower level, and forming here and there little pools, or miniature lakes, as they actually are. Unless the current is very violent, the stones and

coarse sand brought down by the water will settle near the place where the rill enters the pool, while the mud will sink further away towards its lower end. Multiply the length, breadth, and power of this rill indefinitely, and we have a great river, whose delta of accumulated sand, gravel, or mud spreads over thousands of square miles, and in long ages forms banks extending many miles out into the sea. But let us trace a brook which enters a river, right up to its source in the hills, remembering that it also is fed by innumerable rills, all doing the same work. The deltas of the small feeders may be washed away in an hour, and their channels altered repeatedly in the same time, because the accumulations are so small. But the courses and deltas of the largest rivers must thus also change in the lapse of ages, though months or years make no perceptible difference in the larger mass; and all the destruction, redistribution, and rearrangement of the solid crust of the earth has been brought about slowly by the very same mechanical forces which affect the little pools, rills, and sand-heaps of the roadside in the short space of an hour. We may mark the course of our rill to-day, measure the depth of its pools and accumulations of mud, and to-morrow all will have been swept away; our miniature river has disappeared-on the very spot is the hard rock of the roadway. Thus the surface of the earth has been changed repeatedly; continents have melted away and been replaced by new dispositions of land; rivers and seas have been filled up and others formed elsewhere; vast accumulations of rock have

vanished before the might of flowing water, and their materials laid down anew by this unceasing process of destruction and reconstruction.

As we stand high up among the hills which supply the river with water, we see far away in the distance, winding through the valley, and opening out here and there into broad reaches or lakes, the fully grown stream whose cradle we wish to examine. Around us lie loose earth, angular stones, detached from the hill-side by water penetrating the crevices (in winter the water forms into ice, expanding with the irresistible force of a wedge, and splitting fragments from the face of the rock); and beneath lies rubbish of all kinds, evidently derived from the immediate neighbourhood. At the foot of every gully lies a heap of this débris, which will eventually be reduced to rounded stones, sand, and mud, in its passage down the river. On one hand is a greyish limestone rock, through a cleft in which flows a rivulet. This we will follow downwards to its point of entry into the river. All along its course are angular blocks of the stone, of all sizes up to many tons in weight. Some, which have remained there a long time, are polished on the sides by the attrition of passing stones, and others are fairly rounded by being rolled on during floods. The further we go the more rounded the largest blocks and smallest pebbles become, and we find besides a mud of the characteristic colour of the limestone rock. Continually grinding against each other and the bed of the stream, they scoop the channel deeper and wider year by year, and arrange themselves along its course according to

their weight. The boulders may remain for hundreds of years in one spot, but they, too, are wasting away; while the silt and sand go forward more rapidly, to form a bar or delta to the river; and the mud onwards still, to settle in the quiet depths of lake or ocean ;—every winter's frost and every shower of rain contributing a fresh supply of material at the base of the hills and gullies which flank the river's course.

A notable example of the transporting action of water is the Rhone. The colour of the water entering the Lake of Geneva is greyish on account of the fine limestone mud suspended in it, and contrasts strongly with the blue water of the lake, in which it settles long before it reaches the outlet. But on both banks at the entrance of the stream are deposits of coarse silt, which have usurped a mile and a half of the lake since the little place called Port Vallais was at the water's edge and was used as a landing-place by the Romans.

Such is the universal type of river action, modified by local circumstances, all the world over. Their channels alter rapidly or slowly, according to the nature of the soil on the great, almost level, pampas of South America they have moved many miles to the right or left of their course in no very long time; they may become a succession of lakes without outlet, or lose themselves in sandy tracts, or, entering caverns in limestone formations, flow underground for a long distance, and either totally disappear or at length reappear. All those which possess extensive deltas are undergoing a change which must in time greatly modify the physical

geography of the neighbouring country. The contraction of their mouths and the vast accumulations of detritus may convert their basins into wide reservoirs, diminishing the force of the current, allowing more of the water to escape by evaporation, and finally extinguishing the outlets to the sea altogether. Then an inland freshwater sea of considerable extent will be formed, a rampart of dense vegetation will be thrown up on the delta, and the accumulations will henceforth become a fresh-water instead of a marine deposit. Thus probably some of the stratified rocks of ancient times have originated.

Rivers having a broad and deep estuary may be taken as the type of those which contribute the most considerable marine formations, for they carry their sediments far out to sea. At 300 miles from its mouth the water of the Amazon discolours the sea! What must be the amount of material annually removed from the drainage area of this river, extending over 2,000,000 square miles? Sufficient to form a vast deposit in a very short time assuredly. With such an example of water-power before us, we should have no difficulty in realizing the deposition of a sedimentary rock hundreds of feet thick. The Amazon is thus spreading over the floor of what is now the Atlantic Ocean a bed of soil derived from the South American continent, which in some distant geological age will be a stratified rock, mechanically similar in all essential characters to those forming large portions of the existing land.

At low tide on a sandy shore we may observe a perfect system of rivers and lakes in miniature, doing