628 DYNAMICAL GEOLOGY. 



clayey layers. They arise from the solidification commencing first 

 around the circumference of the concretions, and then the circle 

 thus begun acting as a nucleus about which the concreting is con- 

 tinued. 



III. THE ATMOSPHERE. 



The following are some of the mechanical effects connected with 

 the movements of the atmosphere. 



1. Destructive effects from the transportation of sand, dust, etc. — The 

 streets of most cities, as well as the roads of the country, in a dry 

 summer day, afford examples of the drift of dust by the winds. 

 The dust is borne most abundantly in the direction of the preva- 

 lent winds, 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 cir- 

 cumstances 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 com- 

 posing it are driven inland by the winds into parallel ridges higher 

 than the beach, forming drift-sand hills. They are grouped some- 

 what irregularly, owing to the course of the wind among them, and 

 little inequalities of compactness or protection from vegetation. 

 They form especially (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 com- 

 mon 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 ; and they 

 evince also, by the abrupt changes of direction in the layers (see 

 fig. 61/), 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 one 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- 



