THE ATMOSPHERE AS A GEOLOGICAL AGENT. 33 



by trough-like depressions. Where dunes assume the form of hillocks 

 (Figs. 2 and 3, PL II), rather than ridges, the topography is even more 

 distinctive. In some regions, depressions (basins) are associated w^th 

 the dune hillocks. Occasionally they are hardly less notable than the 

 dunes themselves. A somewhat similar association of hillocks and 

 basins is locally developed by other means, but dunes are made up of 

 sand and usually of sand only, while the composition of similarly shaped 

 hillocks and depressions shaped by other agencies is notably different. 



In Fig. 1, Plate II (Five Mile Beach, 8 miles northeast of Cape May, X. J.), 

 the contour interval is 10 feet. There is here but one contour line (the 10-foot 

 contour), though this appears in several places. Since this line connects places 

 10 feet above sea-level, all places between it and the sea (or marsh) are less than 

 10 feet above the water, while all places within the lines have an elevation of 

 more than 10 feet. None of them reaches an elevation of 20 feet, since a 20-foot 

 contour does not appear. It will be seen that some of the elevations in Fig. 1 

 are elongate, while others have the form of mounds. 



Fig. 2 (PL II) shows dune topography along the Arkansas River in Kansas 

 (near Larned); Fig. 4, dune topography in Nebraska (Lat. 42°, Long. 103°), 

 not in immediate association with a valley or shore; and Fig. 3 shows irregular 

 ridge-like dunes at the head of Lake IMichigan. In Fig. 2 the contour interval 

 is 20 feet. All the small hillocks southeast of the river are dunes. Some of 

 them are represented by one contour and some by two. The altitude of the 

 region is considerable, the heavy contour representing an elevation of 2100 feet; 

 but the dunes themselves are rarely more than 20 feet above their surroundings. 

 In Fig. 4, where the contour interval is also 20 feet, there are, besides the numerous 

 hillocks, several depressions (basins). These are represented by hachures inside 

 the contour lines. In some cases there are intermittent lakes (blue) in the de- 

 pressions. The heavy contour at Spring Lakes in this figure is the contour of 

 4300 feet. There are two depression contours (4280 and 4260) below it. The 

 bottom of the depression is therefore lower than 4260, but not so low as 4240. 

 In Fig. 3 the contour interval is 10 feet, and the dune ridges north of Miller 

 are more than 50 feet high. The dune ridges here have helped to determine the 

 position of this branch of the Calumet River, and have blocked its former outlet. 

 The present drainage is to the westward. 



Migration of dunes. — By the continual transfer of sand from its 

 windward to its leeward side, a dime may be moved from one place to 

 another, though continuing to be made up, in large part, of the same 

 sand. In their migration dunes sometimes invade fertile lands, caus- 

 ing so great loss that means are devised for stopping them. The sim- 

 plest method (employed in France and Holland) is to help vegetation 

 to get a foothold in the sand. The effect of the vegetation is to pin the 



