eh eee — —_- 
130 
tropical regions of Africa; and, when the season 
of the tropical rains arrives, the stream increases 
in both volume and silt, and continues day by 
day to rise in its channel and become turbid in 
its waters, till it spreads like a sea over all the 
cultivated lands of Lower Egypt, refreshing the 
parched ground with moisture, and renewing the 
fertility of the exhausted soil by a new deposit 
of rich alluvial loam. Even the Ganges, though 
more remarkable for its delta and its submarine 
deposits than for its fertilizing inundations, an- 
nually overflows a vast tract of low country on 
both sides of its banks, renovating the land with 
a thick and fertilizing top-dressing of fresh allu- 
vium, stimulating all the far-spread expanse into 
the utmost luxuriance of vegetation, and ena- 
bling it to produce valuable and varied crops of 
rice, wheat, barley, indigo, tobacco, cotton, and 
other cultivated plants. 
Rivers which slowly descend to the sea across 
a low, broad, slowly-shelving seaboard, and which 
have little or no conflict with tidal currents, but 
enjoy permission to make their final deposits of 
sand and silt with deliberation and tranquillity,— 
such rivers have, in many instances, made large 
encroachments on the sea, and divided them- 
selves into a divergent series of terminating 
streams, and formed, all round their streams and 
athwart their mouths, great expanses of low, 
level; alluvial land. In some instances, the two 
outer streams of the divided river constantly 
diverge from one another till they fall into the 
sea, and give to the district which they enclose 
the triangular outline of the Greek letter delta, A ; 
and hence first the enclosed districts, and next the 
entire expanses of lowalluvial land at the mouths of 
rivers, are technically designated deltas; yet some 
of these expanses have a totally different outline 
from the triangular, and others are traversed and 
cut into a kind of stupendous network by in- 
numerable subdivisions and cross-streams of the 
original river. The delta of the Ganges measures 
two hundred miles along its base, and the same 
distance from its apex to the sea; and, through- 
out its lower region, called the Sunderbunds, it 
is a wilderness of the rankest vegetation, in- 
fested by alligators and tigers. The delta of 
the Rhine is identical with a very large pro- 
portion of Holland; the delta of the Nile is 
identical with the main part of Lower Egypt; 
and the deltas of the Rhone, the Danube, the 
Po, the Indus, the Wolga, the Orinoco, and sev- 
eral other great rivers, are identical with some 
of the richest and most extensive regions of the 
inhabited and cultivated seaboards of the coasts 
to which they respectively belong. All the deltas 
are encroachments on the sea, and all the lacus- 
trine alluvial lands are encroachments upon 
lakes ; and several of both classes of formations 
have very greatly extended themselves within 
the period of European record. The city of Ra- 
venna, for example, was formerly a sea-port of 
the Adriatic, but now stands four miles inland; 
ALLUVIUM. 
and the town of Port-Vallais stood, about eight 
centuries ago, on the margin of the lake of Geneva, 
but is now about a mile and a half distant from 
the nearest part of the lake. 
The amount of alluvial deposits carried by 
rivers to the ocean is evidently enormous, and 
far exceeding what any superficial thinker would 
suspect; but, in the present blank condition of 
the statistics of physical geography, it cannot be 
proximately estimated. The Ganges colours the 
sea with mud to the distance of sixty miles from 
the coast, and is supposed to deposit daily be- | 
neath the tide a quantity of matter of equal bulk | 
to the great pyramid of Egypt. The river Ama- 
zon, which is said to be forty miles wide at its 
mouth, and is computed to drain an extent of 
territory equal to five-sixths of Europe, colours | 
the ocean with the mud of its waters to the dis- 
tance of about three hundred miles from the | 
coast. The river Orinoco discolours the ocean | 
to probably two-thirds the same extent as the 
Ganges. Many broad bands of existing low 
country upon the coasts of continents may easily | 
be seen, by their position, their elevation, their 
mineral constitution, and their quite recent 
marine remains, to have been formed by the 
fluviatile deposition of alluvium beneath the sea, 
and to have been, only one or two thousand 
years ago, or even but a few centuries ago, the 
beds of bays, estuaries, and far-extending inlets 
of the sea. Various bands, or long and sometimes 
curved belts of low land also, may easily be proved 
to have, at a comparatively recent period, been | 
the bottom of straits and sounds, and to have 
been gradually filled up by fluviatile deposits 
till they united islands to a neighbouring conti- | 
nent. Existing bays, estuaries, marine inlets, 
and even seas are known to actual observation 
to be so rapidly and regularly filling up, that | 
proximate calculations can be made of the future 
century or decade when all or parts of some will 
become impracticable to navigation, and when 
all or parts of others will be easily convertible 
into dry land by embankment, or will, without 
artificial aid, rise permanently above the tide. 
Several of the largest marine inlets of Great Bri- | 
tain, as the estuaries of the Forth, the Tay, and 
the Humber, are thus known to be rapidly fill- | 
ing up; and even the bed of the German ocean 
is ascertained to be steadily increasing in eleva- 
tion. The alluvial silts held in mechanical mix- 
ture by tidal currents, and giving a constant tint 
of earthy redness to the waters of such estuaries 
as the Humber and the Solway, are often artifi- 
cially made to overflow level coast-lands lying at | 
an elevation below high-water mark, and .are 
found to deposit, in a brief period, such a thick 
bed of fine alluvium as constitutes an entirely 
new and very fertile soil. See Warpine. Bars, 
shoals, and sand-banks are known by every 
mariner to be formed athwart the embouchure 
= 
if 
of every sea-entering river,—often with the effect 
of rendering navigable entrance 
difficult or im- | 
