^OLIAN DEPOSITS 335 



dried by the winds, and blown gradually inland, thus forming 

 Mils in some cases not less than 250 feet in height.^ In other in- 

 stances, as at Elbow Bay, on the south shore of the main island, 

 the sand, like a huge glacier, has quite filled a valley, and still 

 progressing in a mass some 25 feet in thickness, is covering 

 houses, gardens, and even woodlands, leaving, as at Lake Michi- 

 gan, only the trunks of dead trees standing partially exposed 

 in the midst of sandy plains. 



One of the most interesting and remarkable of the many 

 regions for the observation of sand dunes, lies between Bor- 

 deaux and Bayonne in Gascony, and has been admirably de- 

 scribed by Eeclus.^ The sea here throws every year upon 

 the beach along a line 100 miles in length some 5,000,000 

 cubic yards of sand. The prevailing westerly winds, contin- 

 ually picking up the surface particles from the seaward side, 

 whirl them over to the inland or leeward slope, where they 

 are again deposited, and the entire ridge by this means alone 

 moves gradually inland. In the course of years there has 

 thus been formed a complex series of dunes all approximately 

 parallel with the coast and with one another, and of all alti- 

 tudes up to 250 feet. These are still marching steadily inward, 

 though at the rate of but 3 to 6 feet annually, and whole vil- 

 lages have more than once been torn down to prevent burial, and 

 rebuilt at a distance, to be again removed within 200 years.^ 



The lithological nature of the dunes is widely variable, though 

 naturally siliceous sand is the prevailing constituent in the 

 majority of cases. J. W. Eutgers describes* the dune sands of 

 Holland as consisting principally of granules of quartz, to- 

 gether with those of garnets, augite, hornblende, tourmaline, 

 epidote, staurolite, rutile, zircon, magnetite, ilmenite, ortho- 

 clase, ealcite, and apatite; and, more rarely, microcline, cor- 

 dierite, titanite, sillimanite, olivine, kyanite, corundum, and 

 spinel. The majority of these minerals occur in the form of 

 well-rounded granules, though many of the garnets, zircons, 



* Geology of Bermuda, Bull. 25, U. S. National Museum. 

 = The Earth, Atmosphere, and Life. 



^ The ehureh of Lege, owing to the encroachment of the sand dunes, was 

 torn down in 1690, and rebuilt at a distance of 2 J miles from its first site. 

 By 1850 the dunes had traversed the intervening space, and again necessi- 

 tated its removal. 



* Neues Jahrbuch fur Mineralogie u. Geologic, etc., 1895, 1 B., 1st Heft, 



P* 22. 



