26 PRINCIPLES OF GEOLOGY. 



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 neutra- 

 lized 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 channels to the rivers, which, changing at in- 

 tervals their mouths, raise towns into temporary distinction, and again, 

 if not prevented by art, take a fresh Course, and carry away at once their 

 harbours 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 deposits 

 of rivers and the tide, as along the great rivers of Yorkshire and Lincoln- 

 shire ; and sometimes they are covered by the shelly sediment of ancient 

 lakes. In many instances they are broken to fragments, and so irregu- 

 larly 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 condition of nature, such soils suit such 

 trees, these circumstances have led to an opinion that the trees grew 

 near the spots where they lie buried. If this be thought sufficiently 

 probable, we arrive at a startling conclusion : for as these trees are 

 often buried some yards below the usual level of the sea, and are some- 

 times, as on the shore of South Wales, covered thirty feet deep by the 

 tide, it would appear that the sea has risen so much on our coasts. If 

 the levels of Yorkshire were once covered with forests of oak, the sea 

 must have been debarred access to them, and it would seem, therefore* 

 that its general level has been since much raised ; for those trees 



