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POPULAR SCIENCE REVIEW. 
The inference as to the origin of depression, which can he 
drawn from delta and estuary areas, may equally be applied to 
coral-reefs and islands, and even to great accumulations of ice, 
as in Greenland ; for in almost all such situations there 
appears to he a nearly continuous downward tendency. There 
are even grounds for supposing that the depression generally ob- 
servable round sea-coasts may he due to a similar cause. The 
sediments from the wasting of the shore* are known to he 
thrown down almost wholly upon a belt thirty miles wide. 
The moving power of waves is not felt to a greater depth 
than forty feet ; and it is therefore difficult to explain, except 
upon the the theory of subsidence, why in the absence of 
currents, the sea in proximity to shore should ever he more than 
forty feet deep. All ancient lands should he surrounded by 
extensive shoals of uniform depth, for tides appear to have no 
permanent action in removing sediment, and shore-currents of 
the requisite power are local. The prevailing action, indeed, 
on our own coasts appears to he silting, if we may judge from 
the way wrecks become imbedded ; and the evidences of sub- 
sidence are innumerable. The records of submerged land 
vegetation are frequent, and though, on the other hand, there 
are in many places raised beaches, it should he remembered 
that while these are always conspicuous, depressed beaches can- 
not easily attract notice. 
If it were once conceded that sedimentation directly caused 
subsidence, we should discover a reason for the permanence of 
ocean basins, for deposition must have been unceasing since 
Palaeozoic times, and would to a large extent have filled in the 
depths of the ocean were this action not compensated by con- 
stant and gradual depression, exceeding perhaps the rate of 
sedimentation. The mean of four experiments made on the 
Challenger expedition, determined the quantity of carbonate of 
lime in the form of living organisms in the surface waters to 
be 2*545 grammes, so that if these animals were equally 
abundant in all depths down to 100 fathoms, it would give 16 
tons of carbonate of lime to each square mile of 100 fathoms 
depth, f 
The weight of sediment must exercise enormous pressure, 
tending to make the greatest depths of the sea permanent, and 
to continually elevate lines of least resistance into ridges or 
banks, leading where the state of tension is extreme, to 
sisdppi, 6.30, in which the lowest beds reached were turf and vegetable 
matter. The total thickness of many deltas, such as that of the Ganges, 
may be inferred from the depth of the sea in which they are accumulating. 
* The denudation has been estimated to equal nineteen feet in 1000 years. 
t In great depths shells are reduced to bicarbonate, and this may imply 
loss of material. The supply of lime does not seem, however, to be obtained 
to any great extent from dead organisms, but is probably kept up by rivers. 
