_ possible ; and when these bars and other obstruc- 
tions occur opposite a delta, they prevent much 
of the moving alluvium from passing out to sea, 
and occasion it to settle between themselves and 
the delta, and greatly to accelerate the formation 
of new land. © 
But the direct action of the sea, both in disin- 
tegrating rocks and soils, and in forming new 
deposits, is of enormous power and wonderful ex- 
tent, and may be observed upon every district of 
coast in the world. Islands have been wholly 
destroyed ; large limbs of continents have been 
washed away; large islands or considerable sea- 
board districts of continents have been broken 
up into groups of islets; broad and extensive re- 
gions of continuous coast have been dissevered 
into series of sea-lakes and peninsulee ; mountain 
masses of cliff and coast-rampart have been torn 
into fragments,—part of them left as stalks, and 
arches, and caverns, and the remainder triturated 
into grit and powder; and an incalculably large 
ageregate of low coast lands have been formed 
by mingled deposits of these various disintegra- 
tions, and of the alluvial muds and sands brought 
down by rivers. The rocky coasts of Cornwall, 
and still more the rocky islands of Orkney and 
Shetland, afford sublime examples of the terrific 
disintegrating power of the billows. The whole 
coast of Yorkshire, from the Tees to the Humber, 
and particularly from Flamborough-Head and 
Spurn-Point, is undergoing regular demolition 
at the average rate of several feet in the year. 
| The coasts of Norfolk, of Suffolk, of Kent, of 
Sussex, and of Dorset, may almost be seen to 
crumble away beneath the eye of an occasional 
visitor, and afford many remarkable instances of 
| quite recent and comparatively extensive demoli- 
tion. A point in the harbour of Sherringham 
was, about sixty years ago, occupied by a cliff 
fifty feet in height and surmounted by houses, 
and is now a piece of sea sufficiently deep to float 
a frigate. All the site of the ancient little town 
of Cromer is now part of the bed of the German 
ocean. The town of Dunwich, formerly a place 
| of great trade, a seat of large population, and the 
most extensive sea-port on the coast of Suffolk, 
has been washed down piecemeal by the sea till 
only the tiniest and most miserable vestige of it 
remains. The church of Reculver in Kent stood 
nearly a mile from the sea in the reign of Henry 
VIII., but is now within about sixty yards. The 
site of the old town of Brighton, in front of the 
existing Brighton cliff, has been wholly sub- 
merged. In the course of twenty-four hours in 
the year 1792, a part of the coast of Dorsetshire, 
about a mile and a quarter in length, and one- 
| third of a mile in breadth, sunk fifty feet from 
its previous level. But—to quote stupendously 
larger, though not so certain instances—England 
is supposed to have formerly been united to 
France at the straits of Dover, Sicily is supposed 
to have been united to Italy at the straits of 
Messina, Ceylon is supposed to have been united 
_— 
ALLUVIUM. 
to the Indian continent across the series of shoals 
called Adam’s Bridge, and the islands of Cuba, 
Jamaica, St. Domingo, and the Caribbees are 
supposed to be the fragments of a horse-shoe 
continent which enclosed the Caribbean sea in 
the manner of the Mediterranean. A very large 
proportion of the detritus of coasts, and the dis- 
integrated contents of quondam districts and 
islands, must be supposed to be lodged in the 
deep parts of the ocean, there undergoing such 
processes of induration as formed our existing 
sedimentary rocks out of the debris of the primi- 
tive masses of our world; yet a considerable 
proportion obviously combines with comminuted 
shells and decayed fuci, and the mineral and 
vegetable silts brought down by rivers, to form 
the great tracts of rich low land which lie along 
many stretches of coast, and are occupying the 
places of former bays and gulfs. 
The composition of alluvium is exceedingly vari- 
ous, and depends partly on the mechanical condi- 
tions under which it was deposited, but principally 
on the character of the rocks, soils, and other sur- 
faces whence its materials were obtained. Such 
alluvium as was deposited in still water is much 
more argillaceous and far more finely pulverized 
than if it had been deposited by eddies, rapid 
currents, or turbulent waters; and such as was 
deposited under a slow, regular, and very pro- 
longed process, is incomparably more uniform in 
both composition and texture, than if it had been 
deposited by the bursting of a lake or any other 
single, extraordinary, or violent action. Alluvium 
from the debris of a limestone basin will be found 
predominantly calcareous; from the debris of a 
micaceous and softly granitic basin, predomi- 
nantly argillaceous; from the debris of a green- | 
stone or basaltic basin, predominantly loamy ; 
from the debris of a quartzose and sandstone 
basin, predominantly silicious; from the debris 
of a rocky basin, predominantly mineral; from | 
the debris of a diluvial basin, characteristically 
full of decayed vegetable fibre; and from the 
debris of coasts and islands, characteristically 
marked with calcareous sand or comminuted 
shells. Yet the great majority of alluvial lands 
are so exceedingly diversified in the mechanical 
conditions under which they were formed, and 
particularly in the sources whence their mate. 
rials were derived, and the agencies by which 
these materials were pulverized, compounded, 
and modified, that they cannot be referred to 
any one type, but may be regarded as severally 
representing nearly all the varieties of simple 
soils. Most are fertile; a large proportion are 
eminently rich; and not a few are wonderfully 
deep. No definite number of analyses, of course, 
can afford an index to the character of each par- 
ticular alluvial formation ; yet three analyses 
made by Sir Humphrey Davy may be quoted as 
illustrations, in a general manner, of the wealth 
of alluvial soils. A specimen from the banks of 
the river Parrett, in Somersetshire, yielded eighty 
ee { 
see 
i nn 
