ELEVATION AND DEPRESSION OF EARTH'S CRUST. 13 7 



g,' g", each with the stumps and roots of cypress-trees as they originally 

 grew. Each one of these turf-lines is a submerged forest-ground, ex- 

 cept the uppermost, which is the present forest-ground. Precisely sim- 

 ilar phenomena have been observed in other large deltas. The deltas 

 of the Ganges and the Po have been penetrated more than 400 feet 

 without reaching bottom. In both the deposit is made up of fresh-water 

 strata alternating with dirt-beds or forest-grounds. These facts prove 

 that these great deltas have been at intervals during the whole period 

 of their formation, as they are mm, fresh-water swamps, overgrown in 

 parts with trees, etc. ; that they have steadily subsided to a depth indi- 

 cated by the thickness of the deposit containing the old forest-grounds ; 

 that the up-building by river-deposit has gone on pari passu, so as to 

 maintain nearly the same level all the time ; but that from time to 

 time the subsidence was more rapid, so that the sea gained possession 

 for a while until it was again reclaimed by river-deposit, and again 

 more slow, so that the area was again thoroughly covered with forests, 

 and so on. These facts are of great importance in geology, and will be 

 again referred to in the following pages. 



Southern Atlantic States. — Evidence of a similar kind proves that a 

 large portion of the coasts of our Southern Atlantic States is slowly 

 subsidiug at the present time, though there are also evidences, in the 

 form of raised beaches, of elevation immediately preceding the present 

 subsidence. The evidences of subsidence are most conspicuous along 

 the coast of South Carolina and Georgia. They consist of cypress- 

 stumps in situ below the present tide-level. According to Cook, late 

 Geologist of Xew Jersey, the coast from Long Island to Cape May is 

 sinking at the rate of two feet a century. 



These facts seem to point to the conclusion that subsidence is going 

 on in nearly all places where large deposits of sediment are accumulating. 



Pacific Ocean. — But by far the grandest example of subsidence 

 known is that which has been going on for thousands, probably hun- 

 dreds of thousands, of years, and is still going on in the mid-Pacific 

 Ocean. The subsiding area is situated under the equator, and is about 

 6,000 miles long by about 2,000 to 3,000 miles wide. The evidence of 

 the subsidence and its rate is entirely derived from the study of coral- 

 reefs in this region. The further discussion of the subject will be de- 

 ferred until we take up coral-reefs. 



River-Beds as Indicators of Movements. — Our examples, be it ob- 

 served, are all taken from the vicinity of coast-lines, the sea-level being 

 used as term of comparison. In the interior of continents, and in the 

 midst of the sea where there are no islands, this means of detecting 

 changes fails us, yet it is precisely there, i. e., in the middle of the rising 

 or subsiding area, that the changes are probably the greatest. In tlie 

 case of continents, however, as already explained on page 22, we have 



