ed 
we oF 
Principles of Geology. 23 
latter part of this process has proceeded so far that the marshes are 
dry at intervals, man exerts his enterprising industry, and defends the 
new land by a bank. If this be made too abrupt, the ocean indig- 
nantly washes it away, and reclaims his ancient domain ; but a long 
gradual slope of pebbles and sand averts the fury of the sea, and 
protects, though with a moving barrier, the lands within, above which, 
in storms, the waves hang suspended and threatening destruction, but 
dash their spray and fling their foam in vain. 
Action or RIvERS, &c.—Imperceptible as is the reduction of 
mountains and hills by rains and rivulets, yet the matter thus collect- 
ed, by constant attrition, assumes an important character, when con- 
centrated along the margins of rivers, and changes the appearance 
of the vallies. In proportion to the magnitude of the stream, the 
altitude of its sources, and the nature of the country through which 
it flows, the effects are more or less considerable. But the every 
where tend to the same result; the raising of the level of the valley 
by horizontal layers of sediment. This accumulation is most rapid 
where rivers approach the sea, because there the current is languid, 
and often Weakened or neutralized by the opposition of the tide. 
tom the point where the tide ceases, to the sea, the natural tendency 
of every land flood, and every muddy tide, is to heighten and extend 
the low alluvial lands, whilst, by the same process, the bed of the 
ver 1s raised, and its mouth carried further into the sea. _ The “1 Ww 
nd” thus produced, being but feebly consolidated, opens new ch 
nels to the 
towns into 
take a fresh Course, and carry away at once their harbors and their 
opulence, eis 
SUBTERRANEAN ForEsts, &c.—Under the alluvial deposits of silt 
mo tis common to find, at various depths, great quantities of 
oo kinds, in different states of preservation. They are 
._.) &cCompanied by peat: sometimes they lie under the de- 
Pos of tivers and the tide, as along the great rivers of Yorkshire 
. ncolnshire ; and sometimes they are covered by the shelly 
ie ts ancient lakes. In many instances they are broken to 
hen poy and so irregularly disposed, as to make it probable they 
ss “eR regnas by violent land-floods ; but in other cases they 
Yaty in th x . regularly prostrated in a particular direction, and to 
on which re inds according to the nature of the subterranean soil 
on clay. <4, %¢ Placed. It is reported that oaks are found lying 
¥, and firs, alder, and birch upon sand ; and, as in the present 
