CHAPTER y. 



THE SAiroS. 



Origin of Sand — Sand now Carried to the Sea — Beach-Sands of Northern 

 Africa— Sands of Egypt— Sand-Dunes and Sand-Plains— Coast-Dunes— 

 Sand-Banks— Character of Dune Sand— Interior Structure of Dunes- 

 Geological Importance of Dunes — Dunes on American Coasts — Dunes 

 of Western Europe — Age, Character and Permanence of Dunes — Dunes 

 as a Barrier against the Sea — Encroachments of the Sea — Liimf jord — 

 Coasts of Schleswig-Holstein, Netherlands and France — Movement of 

 Dimes — Control of Dunes by Man — Inland Dunes — Inland Sand-Plains. 



Origvn of Sa/nd. 



Sand, wkicli is found in beds or strata at the bottom of the 

 sea or in the channels of rivers, as well as in extensive deposits 

 upon or beneath the surface of the dry land, appears to consist 

 essentially of the detritus of rocks. It is not always by any 

 means clear through what agency the solid rock has been re- 

 duced to a granular condition ; for there are beds of quartzose 

 sand, where the sharp, angular shape of the particles renders it 

 highly improbable that they have been formed by gradual abra- 

 sion and attrition, and where the supposition of a crushing 

 mechanical force seems equally inadmissible. In common sand, 

 the quartz grains are the most numerous ; but this is not a proof 

 that the rocks from which these particles were derived were 

 wholly, or even chiefly, quartzose in character; for, in many 

 composite rocks, as for example in the granitic group, the mica, 

 felspar and hornblende are more easily decomposed by chemical 

 action, or disintegrated, comminuted and reduced to an impal- 

 pable state by mechanical force, than the quartz. In the destruc- 

 tion of such rocks, therefore, the quartz would survive the other 

 ingredients, and remain unmixed, when they had been decom- 

 posed and recomposed into new mineralogical or chemical com- 



(535) 



