170 DYNAMICAL GEOLOGY. 



with a considerable layer of peat, then more river-silt, 

 with an occasional layer of limestone, and so on, several 

 times repeated. Such old forest-grounds have been found 

 in the Mississippi delta fifty feet below sea-level, and in 

 the Ganges layers of peat fifty feet below sea-level, and 

 fresh-water shells and river-silt near four hundred feet. 

 In the delta of the Po, peaty layers are found four hun- 

 dred feet below sea-level (Lyell). 



Now, the only way possible to explain these facts is to 

 suppose a slow subsidence on the one hand and the up- 

 building by sedimentation on the other, but not always 

 absolutely at the same rate. When the upbuilding pre- 

 vailed, the area was reclaimed and overgrown with forest. 

 When the subsidence prevailed, the trees were submerged 

 and destroyed, rotted to stumps and buried in sediments. 

 Sometimes the subsidence was so rapid that salt-water 

 conditions prevailed and limestones were formed. Sub- 

 merged forests are found not only in deltas, but also on 

 many coast-lines, and are among the surest signs of sub- 

 mergence. 



3. Mid-Pacific Bottom. But the grandest example of 

 subsidence, still in progress, is undoubtedly that already 

 discussed under coral reefs. As already shown, we have 

 evidence that over an area of 10,000,000 square miles in 

 mid-Pacific there has been, in comparatively recent geo- 

 logical times, a subsidence of 10,000 feet, and that the 

 subsidence is still going on. Surely, in this case, we 

 have changes now in progress which are of the nature of 

 those by which continents and sea-bottoms were formed. 



4. River-beds. Our examples thus far are all from 

 the coast region. The phenomena are plainest there, 

 because we have the sea-level as a term of comparison. 

 But in the interior of continents we have river beds as 

 indicators of movement. We have seen (p. 28) that in a 

 rising country rivers cut deeper, while in a sinking 

 country they build up by deposit. 



