334 THE EEGOLITH 



SO loose and fine as to rise in clouds at the merest puff of 

 wind, and gives the tables on the preceding page to show their 

 chemical and physical natures. These he regards as fairly typ- 

 ical for soils of the arid regions of the United States.^ 



Sand Dunes. — The influence of the wind in the formation 

 of sand hills or dunes, as they are commonly called, has received 

 attention on p. 163. A few words more regarding their physical 

 qualities and lithological nature are here essential. 



The effect of the single whirlwind or it may be that of the 

 more constant air current for days, weeks, or even months, 

 may be from a geological standpoint comparatively insignifi- 

 cant; but they are, nevertheless, interesting, and at times 

 important. In certain regions of the West, and notably in 

 parts of the Colorado desert, as described by W. P. Blake, in 

 1853, all the fine loose sand on the surface of the ground is 

 blown away, leaving every pebble and boulder standing out in 

 strong relief from the hard sun-baked soil;, or ledge of bed-rock. 



Under favorable conditions the material thus blown along 

 may gather in the form of dunes, which themselves travel 

 slowly across the country, ever changi!ag their outlines like 

 drifts of snow. A few miles north of Winnemucca Lake, in 

 western Nevada, is a belt of these dunes described by geologist 

 EusselP as fully 75 feet in thickness and about 40 miles in 

 length by 8 miles in breadth. These, under the restless 

 goading of the winds, are constantly varying in shape, and 

 though moving in ma^s probably but a few feet a year have 

 already, in more than one instance, made necessary the splicing 

 of telegraph poles to prevent the burial of the wires. Another 

 range of sand dunes, at least 20 miles in length, and forming 

 hills 200 to 300 feet high, occurs on the eastern end of Alkali 

 Lake in the same state. On the eastern shore of Lake Michi- 

 gan are also dunes of sand sometimes 200 feet in height, and 

 which at Grand Haven and Sleeping Bear have drifted over 

 the adjacent woodlands, leaving only the dead tops of trees 

 exposed. Similar dunes occur frequently on the Atlantic 

 coast, as at Hatteras, Long Island, and Cape Cod. The island 

 of Bermuda is made up almost altogether of coral and shell 

 fragments. These are washed by the waves upon the beaches, 



iBull. No. 3, Weather Bureau, U. S. JDept. of Agriculture, 1892. 

 « Geological History of Lake Laliontan, Monograph XI, U. S. Geol. Sur- 

 vey, 1885. 



