160 



THE POPULAR EDUCATOE. 



have been due to volcanic upheaval of the horizontal strata, or 

 to the eroding action of the sea ere the continent was raised 

 out of the water. This latter action may be seen at work upon 

 any mud-flat, when the tide has receded. The surface of the 

 flat is intersected by innumerable channels which drain off the 

 water. These channels are not obliterated by the next tide, 

 but rather deepened, and in time become decided undulations. 



When the mud-flat is permanently raised beyond the reach 

 of the tidal waters, then these irregularities of the surface 

 become water-courses, which erode their respective valleys. 



In speaking of the eroding and transporting power of water, 

 we must bear in mind that all earths and stones lose one-third of 

 their weight when immersed in water. The mechanical force of 

 water depends upon its velocity, and it increases as the sixth 

 power of that velojity that is to say, if the velocity be doubled, 

 the motive power becomes sixty-four times greater ; if trebled, 

 the transporting force is increased by 729 times. This will 

 readily account for the masses of rock which are known to have 

 been transported by floods. Frequently bridges are borne 

 down by a swollen river, and the blocks of which breakwaters 

 were composed removed many yards by the force of a storm; 

 but the great work carriad on by rivers is not by the spasmodic 

 efforts of a flood, but by the continued load of mud which they 

 carry down to the sea. 



This being well nigh the most important of geological agents, 

 we shall first consider its simplest case -when a river enters a 

 lake in the basin of which it deposits the mineral matter it 

 holds in suspension. 



An admirable instance is offered to 

 us in the Lake of Geneva. At the 

 upper end of the lake the Rhone enters 

 discoloured by mud ; but, when it 

 leaves the lake, its waters are a trans- 

 parent blue the mud has been de- 

 posited in the lake. As this has been 

 going on for centuries, we may expect 

 to find some evidence of the work of 

 the river. This is given us in the 

 alluvial tract which stretches from | 

 the head of the lake for some six or I ^^L ~ 



seven miles. It is a marshy plain, I 

 higher than the level of the water, and | 



occupying what once was the bed of "~ - ^^^^^g 

 the lake. If this state of things con- Fit 



tinue, the Rhone will entirely fill up 



the lake. The rate of the advance of the delta may be gathered 

 from the fact that the Roman town, Portus Valesia, which stood 

 on the margin of the lake, is now more than a mile and a half 

 .inland, the river having added to its delta this quantity in about 

 eight centuries. By soundings it is found that the mud deposit 

 roaches some two miles from the river's mouth. 



The great difference between a lacustrine and a marine delta 

 will be found in the fossils preserved ; in the one will be the 

 remains of such animals and plants as lived in the river and 

 the country it drained ; in the other will be found a prepon- 

 derance of the fossils of marine life. 



Almost every river enters the sea through a delta ; that is, 

 its main stream divides into two or more channels, thus making 

 a triangular island, approaching to the shape of the Greek 

 capital A (delta). A glance at a map of the world will furnish 

 many instances of this. 



Egypt, from the vast antiquity of its history and monuments, 

 offers peculiar opportunities for the measurement of fluvial 

 depositions. The Nile as Herodotus observed has probably 

 made Egypt by filling up an arm of the Mediterranean. It 

 must have commenced its work at an age very remote, for the rate 

 of its deposition seems to be three and a half inches a century. 



This is gathered from the accumulation of mud around the 

 base of the obelisk at Heliopolis, and the pedestal of the 

 statue of Rameses at Memphis. This colossal image, according 

 to Lepsius, was built 1361 years B.C. Between that time to 

 the year 1850, nine feet four inches of mud has been deposited, 

 which gives a mean increase of three and a half inches a cen- 

 tury. A boring was made in the immediate neighbourhood, 

 and at a depth of thirty-two feet a piece of burnt brick was 

 found, upon which discovery it was asserted that man inhabited 

 the Nile valley 13,496 vears ago a conclusion in which no un- 

 biassed mind will acquiesce, seeing it is based upon the fact that 



the Nile, whose waters come down loaded with loam, only deposits 

 about ith of an inch of mud in a season ! We shall allude to this 

 and similar questions in a chapter on " The Antiquity of Man." 

 The deposition made by rivers has not only an effect in forming- 

 deltas, but also tends to elevate their own channels ; so that at 

 the present time the inundating waters of the Nile reach further 

 and are reclaiming more of the desert than ever they did. 



This is remarkably exhibited in the case of the Po and the 

 Adige. Slowly traversing the flat plain of Lombardy, the Po 

 deposits much of the sediment it carries in its own channel, 

 before its waters reach the sea. The effect of this is to raise 

 the water-course above the surrounding country; frequently 

 its banks have given way, causing terrible destruction, and tho 

 river has found for itself a new channel. 



To diminish the probability of these inundations, each suc- 

 ceeding spring the bed of the river is dredged, and the mud 

 added to the banks. The consequence is that the rivers run 

 like aqueducts, much above the level of the plain. At Ferrara, 

 the surface of the Po is higher than the roofs of the houses ; 

 but at the same time this has the effect of narrowing the river's 

 channel, thus enabling the waters to carry more of the sediment 

 to the sea ; so that the delta now gains seventy metres a year ; 

 whereas, 200 years ago, it seems only to have been increased 

 by an annual addition of a third of this quantity. 



The increase of the delta is rendered evident by the fact that 

 the town of Adria, which gave its name to the gulf, was a sea- 

 port in the tims of Augustus now it is twenty Italian miles 

 from the coast. Pome indication of 

 the thickness of this alluvial deposit 

 was given by boring an Artesian well 

 at Venice. At a depth of 400 feet the 

 same kind of matter was brought up as 

 that which is now in course of depo- 

 sition. 



Tho delta of the Mississippi has an 

 areaof 12,300 square miles. The river 

 brings down f^thof its weight of solid 

 matter, or more than 6,000,000,000 

 cubic feet annually ; yet such is the 

 vast size of the delta, that Sir Charles 

 Lyell computes it has been in the 

 course of formation for 33,500 years. 



'- - "- " - ~~^ The Ganges performs even a greater 



g^ work of transportation. In the four 



rainy mouths, at 500 miles from it 



mouth, it was found to bear seawards 577 cubiifeet of solid matter 

 a second! Its annual discharge has been computed to be 

 6,368,077,440 cubic feet an amount of matter equal in weight to 

 sixty Great Pyramids of Egypt, although the base of that immense 

 pile covers eleven acres, and its apex is 500 feet above the level 

 of the plain. Such a deposit, if accumulated upon Ireland, would 

 raise the surface of the whole island one foot in 144 years. 



Yet even this does not measure the deposition which is going 

 on in the upper part of the Bay of Bengal ; for the Brahma- 

 pootra, in all probability, contributes as much as the Ganges 

 to the sedimentary accumulation. The instances which we 

 have cited are not singular, for every river bears down to the 

 ocean an amount of matter in proportion to the volume of water 

 it discharges, and the nature of the country which it drains. 



The construction of a delta depends very much upon the 

 position of the river's mouth. If the point of its embouchure 

 be swept by an ocean current, the sediment has no time to 

 settle, but is carried out to sea, where it gradually subside* 

 along the path of the current, while its lighter particles tint the 

 water for thousands of miles. This is the case with the Amazon, 

 the Rio do la Plata, the rivers of Britain and Western Europe. 

 Every oceanic current has a colour peculiar to itself, imparted to 

 it by the foreign matter which it carries ; this it deposits perhaps 

 on the opposite side of the globe to that where it was eroded. 



This mighty agent is so universal in its action, so gradual, and 

 yet so continuous, that its work can only thus be seen in its re- 

 sults ; but let the observer who watches the course of the rivulet 

 gradually widen who sees the river undermine its bank, and 

 time after time the overhanging fragments carried away by the 

 j stream remember that the same never-ceasing agent is doing the 

 same work in every streamlet and every river in the world ; and 

 he will cease to wonder that it is possible that the masses of sedi- 

 mentary strata owe their existence to the power of running water. 



