ACTION OF THE WAVES AND OF THE TIDES. 135 



brought down by rivers. In this way banks and beaches, of greater 

 or less extent, are formed on coasts, the finer parts of which, car- 

 ried inland by the wind, form dunes (see page 125). There are 

 many places where accumulations of this kind are daily formed, 

 and many points of coast have been invaded by deposits from the 

 sea from remotest times: sometimes, by a single irruption, en'.ire 

 kingdoms have been covered by sand, and fertile countries changed 

 to arid plains, either in extraordinary tides, or in tempests, or by 

 the sudden displacement of waters consequent on earthquakes 

 Low countries, exposed to these alluvions^ daily grow at the ex- 

 pense of the waters, and, at certain points, this growth has been 

 estimated at several yards a year. Bays and ports have been filled 

 up in this way ; buildings and towns, formerly situated on the sea 

 shore, are now far from it ; Jakes have been transformed into 

 marshes, marshes into solid land, and islands joined to the main by 

 sands deposited around them. The sea, in some instances, contri- 

 butes to the growth of deltas. 



28. Torrents and rivers transport not only mineral debris, but 

 also organic remains, immense masses of plants, detached from 

 ravines, or by falls. Here and there great masses of materials are 

 formed, especially in rivers which are bordered by immense forests. 

 Great deposits of debris of this kind are formed in the Mississippi 

 and its tributaries ; they there form immense rafts of trunks of trees, 

 interlaced, which are stopped here and there by the sands, and 

 finally are buried under the enormous alluvions daily deposited. 

 The mass of plants that the river carries is so considerable, that it 

 has been estimated at several thousands of cubic yards per hour. 



29. Currents of the sea also often transport immense masses of 

 various vegetables, marine plants, and organic debris of every kind, 

 and from all climates, which are here and there deposited in the 

 bays these currents meet in their course. This is especially the 

 case as regards the great Atlantic current, the Gulf Stream, the 

 strongest and most considerable of all, which extends along the 

 coast of North America to the icy regions, where the polar currents 

 accumulate these debris with those of other parts of the world. 



We cannot doubt, on reflecting on the quantity of debris borne by the 

 waters, that lakes which receive rivers are filled up, little by little, by tho 

 matters daily brought into them ; this is evident, in some places, where 

 marshes and considerable alluvions are thus formed. The same must be truo 

 of the bottom of the sea, where all waters finallv come; it is easv to con- 

 ceive there must be daily formed considerable deposits of all the substances 

 which are carried there, as well as of those washed away by the waves t and 

 of all the remains of animals which perish in this vast abyss. 



30. Deposits of substances held in solution. Waters degrade 



28. Are all the materials, transported by waters, of a mineral origin ? How 

 are the rails in the Mississippi formed ? ^^*MMMBIM^^^ 



29. What effects are due to currents of the sea ? A *^^V 



Of THE * X 



VNIYERSiTYJ 

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