THE ATMOSPHERE. 631 



and may in the course of time make deep beds. The dust that finds 

 its way through the windows into a neglected room indicates what 

 may be done in the progress of centuries, where circumstances are more 

 favorable. 



The moving sands of a desert or sea-coast are the more important 

 examples of this kind of action. 



On sea-shores, where there is a sea-beach, the loose sands composing 

 it are driven inland by the winds, into parallel ridges higher than the 

 beach, forming drift-sand hills. They are grouped somewhat irregu- 

 larly, owing to the course of the wind among them, and little inequali- 

 ties of compactness or protection from vegetation. They form espe- 

 cially (1) where the sand is almost purely siliceous, and therefore not 

 at all adhesive even when wet, and not good for giving root to grasses ; 

 and (2) on windward coasts. They are common on the windward 

 side, and especially the projecting points, even of a coral island, but 

 never occur on the leeward side, unless this side is the windward 

 during some portion of the year. On the north side of Oahu, they 

 are thirty feet high, and made of coral sand. Some of them, which 

 stand still higher (owing to an elevation of the island), have been 

 solidified ; and they show, where cut through, that they consist of thin 

 layers lapping over one another ; they evince also, by the abrupt 

 changes of direction in the layers (see Fig. 61 d), that the growing 

 hill was often cut partly down or through by storms, and again and 

 again completed itself after such disasters. 



This style of lamination and irregularity is characteristic of the 

 drift-sand hills of all coasts. On the southern shore of Long Island, 

 there are series of sand-hills, of the kind described, extending along 

 for a hundred miles, and five to thirty feet high. They are par- 

 tially anchored by straggling tufts of grass. The coast of New Jersey, 

 down to the Chesapeake, is similarly fronted by sand-hills. In Nor- 

 folk, England, between Hunstanton and Weybourne, the sand-hills 

 are fifty to sixty feet high. 



2. Additions to land, by means of drift-sands — The drift-sand hills 

 are a means of recovering lands from the sea. The appearance of a 

 bank at the water's edge, off an estuary at the mouth of a stream, is 

 followed by the formation of a beach, and then the raising of the hills of 

 sand by the winds, which enlarge till they sometimes close up the estu- 

 ary, exclude the tides, and thus aid in the recovery of the land by the 

 depositions of the river-detritus. Lyell observes that, at Yarmouth, 

 England, thousands of acres of cultivated land have thus been gained 

 from a former estuary. In all such results, the action of the waves 

 in first forming the beach is a very important part of the whole. 



3. Destructive effects of drift-sands. — Dunes. — Dunes are regions 



