258 THE PHYSICAL GEOGRAPHY OF THE SEA. 



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. 



732. The study of these "sunless treasures," recovered with 

 so much ingenuity from the rich bottom of the sea, suggests new 

 views concerning the physical economy of the ocean. 



733. In the chapter on the Salts of the Sea, p. 179, 1 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 harmo- 

 nies of nature are preserved. 



734. But the treasures of the lead and revelations of the micro- 

 scope 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 regulated and climates softened, but acting also as checks and 

 balances by which the equipoise between the solid and the fluid 

 matter of the earth is preserved. 



735. 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 

 offices which they perform, they assist to preserve its status by 

 secreting the salts which the rivers and the rains bring down to 

 the sea, and thus maintain the purity of its waters. 



736. 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 mat- 

 ter. They discharge annually into the sea an amount of this sol- 

 uble matter, which, if precipitated and collected into one solid 

 mass, would no doubt surprise and astonish the boldest specula- 

 tor with its magnitude. 



737. 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 (§ 502), 

 must continue to become more and more salt. 



738. Now the rivers convey to the sea this solid matter mixed 

 with fresh water, w^iich, being lighter than that of the ocean, re- 

 mains for a considerable time at or near the surface. Here the 



