THE ATMOSPHERE AS A GEOLOGICAL AGENT. 



35 



the dune ridge was on one side of a church, which was then in use. 

 In 1839 the ridge had been so far shifted to the leeward as to com- 

 pletely bury the church, and in 1S69, its migration had progressed 

 so far as to again discover the building.^ 



Tilien dunes migrate into a timbered region they bury and kill 

 the trees (Fig. 17). In one instance on the coast of Prussia a tall 

 pine forest, covermg hundreds of acres, was destroyed during the brief 

 period between 1S04 and 1827.' At some points in New Jersey orchards 

 haA'e been so far buried within the lifetime of their o^\'ners that only 

 the tops of the highest trees are exposed. Trees and other objects 



Fig. 18. — A resurrected forest. The dune sand after bunnng and killing the timber 

 ha^ been shifted beyond it. Dune Park, Ind. (Meyers.) 



once buried may be again discovered by farther migration of the sand 

 (Figs. IS and 19). ^ 



Eolian sand, not aggregated into distinct dunes, is often destructive. 

 Even valleys and cities are sometimes buried by it. Drifting sands 

 had so completely buried Nineveh two centuries after its destruction 

 that its site was imknoT^Ti. 



Distribution of dunes. — Dunes are likely to be developed wherever 

 dry sand is exposed to the wind. Their favorite situations are the 



* Credner. Elemente der Geologie, 6th ed., p. 271. 



' Merrill. Rocks, Rock Weathering, and Soils, p. 295. 



• Copies. The Ecological Relations of the Vegetation of the Sand Dunes of Lake 

 Michigan. Botanical Gazette, Vol. XX\TI, 1899. An excellent study of the relations 

 of sand dunes and vegetation. 



