DUNES ON SHORES OF TIDELESS WATERS. 543 



also extensive ranges of dunes upon the eastern shores of the 



Caspian, and at the southern, or rather southeastern, extremity 



of Lake Michio^an.^ There is no doubt that this latter lake for- 

 es 



merly extended much farther in that direction, but its southern 

 portion has gradually shoaled, and at last been converted into solid 

 land, in consequence of the prevalence of the northwest winds. 

 These blow over the lake a large part of the year, and create a 

 southwardly set of the currents which wash up sand from the 

 bed of the lake and tkrow it on shore. Sand is taken up from 

 the beach at Michigan city by every wind from that quarter, and, 

 after a heavy blow of some hours' duration, sand ridges may be 

 observed on the north side of the fences, like the snow wreaths 

 deposited by a drifting wind in winter. Some of the particles 

 axe carried back by contrary winds, but most of them lodge on 

 or behind the dunes, or in the moist soil near the lake, or are en- 

 tangled by vegetables, and tend permanently to elevate the level. 

 Like effects are produced by constant sea-winds, and dunes will 

 generally be formed on all low coasts where such prevail, whether 

 in tideless or in tidal waters. 



Jobard thus describes the modus opercmdi, under ordinary cir- 

 cumstahces, at the mouths of the Nile, where a tide can scarcely 

 be detected : " When a wave breaks, it deposits an almost imper- 

 ceptible line of fine sand. The next wave brings also its contri- 

 bution, and shoves the preceding line a little higher. As soon as 

 the particles are fairly out of the reach of the water they are 

 dried by the heat of the burning sun, and immediately seized by 

 the wind and rolled or borne farther inland. The gravel is not 

 thrown out by the waves, but rolls backwards and forwards until 

 it is worn down to the state of fine sand, when it, in its turn, is 

 cast upon the land and taken up by the wind." f This descrip- 

 tion applies only to the common every-day action of wind and 

 water ; but just in proportion to the increasing force of the wind 

 and the waves, there is an increase in the quantity of sand, and 



* The careful observations of Colonel J. D. Graham, of the United States 

 Army, show a tide of about three inches in Lake Michigan. See " A Lunai 

 Tidal Wave in the North American Lakes," demonstrated by Lieut. -Colonel 

 J. D. Graham, in the fourteenth volume of the Proceedings of the American 

 Association for the Advancement of Science. 

 f Staking, De Bodem van Nederland, i., p. 327, note. 



