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 j 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 of 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, M^hen 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 they 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. 

 From 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 

 river is raised, and its mouth carried further into the sea. The "new 

 land" thus produced, being but feebly consolidated, opens new chan- 

 nels to the rivers, which, changing at intervals their mouths, raise 

 towns into temporary distinction, and again, if not prevented by art,, 

 take a fresh course, and carry away at once their harbors and their 

 opulence. 



Subterranean forests, &c. — Under the alluvial deposits of silt 

 or clay, it is common to find, at various depths, great quantities of 

 trees of several kinds, in different states of preservation. They are 

 frequently accompanied by peat : sometimes they lie under the de- 

 posits of rivers and the tide, as along the great rivers of Yorkshire 

 and Lincolnshire ; and sometimes they are covered by the shelly 

 sediment of ancient lakes. In many instances they are broken to 

 fragments, and so irregularly disposed, as to make it probable they 

 were swept together by violent land-floods ; but in other cases they 

 are stated to be regularly prostrated in a particular direction, and to 

 vary in their kinds according to the nature of the subterranean soil 

 on which they are placed. It is reported that oaks are found lying 

 on clay, and firs, alder, and birch upon sand ; and, as in the present 



