214 THE PHYSICAL GEOGRAPHY OF THE SEA. 
paring the ingredients for the fruitful soil of a land that some 
earthquake or upheaval, in ages far away in the future, may be 
sent to cast up from the bottom of the sea for man's use. 
The study of these "sunless treasures," recovered v^ith so much 
ingenuity from the rich bottom of the sea, suggests new views 
concerning the physical economy of the ocean. 
457. In the chapter on the Salts of the Sea, p. 150, I endeav- 
ored to show how sea-shells and marine insects may, by reason 
of the offices which they perform, be regarded as compensations 
in that exquisite system of physical machinery by which the har- 
monies of nature are preserved. 
But the treasures of the lead and revelations of the microscope 
present the insects of the sea in a new and still more striking light. 
We behold them now serving not only as compensations by which 
the motions of the water in its channels of circulation are regu- 
lated and climates softened, but acting also as checks and bal- 
ances by which the equipoise between the solid and the fluid 
matter of the earth is preserved. 
Should it be established that these microscopic creatures live 
at the surface, and are only buried at the bottom of the sea, we 
may then view them as conservators of the ocean ; for, in the of- 
fices which they perform, they assist to preserve its status by 
maintaining the purity of its waters. 
It is admitted 343) that the salts of the sea come from the 
land, and that they consist of the soluble matter which the rains 
wash out from the fields, and which the rivers bring down to the 
sea. 
The waters of the Mississippi and the Amazon, together with 
all the streams and rivers of the world, both great and small, hold 
in solution large quantities of lime, soda, iron, and other matter. 
They discharge annually into the sea an amount of this soluble 
matter which, if precipitated and collected into one solid mass, 
would no doubt surprise and astonish the boldest speculator with 
its magnitude. 
458. This soluble matter can not be evaporated. Once in the 
ocean, there it must remain ; and as the rivers are continually 
pouring in fresh supplies of it, the sea, it has been argued, must 
continue to become more and more salt. 
Now the rivers convey to the sea this solid matter mixed with 
