DUNES ON SHORES OF LAKE MICHIGAN. 
467 
quency, duration, or strength than the sea winds, the sands 
left by the retreating wave will be constantly blown back into 
the water ; but if the prevailing air currents are in the oppo¬ 
site direction, the sands will soon be carried out of the reach 
of the highest waves, and transported continually farther and 
farther into the interior of the land, unless obstructed by high 
grounds, vegetation, or other obstacles. 
The tide, though a usual, is by no means a necessary con¬ 
dition for the accumulations of sand out of which dunes are 
formed. The Baltic and the Mediterranean are almost tideless 
seas, but there are dunes on the Russian and Prussian coasts 
of the Baltic, and at the mouths of the Rile and many other 
points on the shores of the Mediterranean. The vast shoals in 
the latter sea, known to the ancients as the Greater and Lesser 
Syrtis, are of marine origin. They are still filling up with 
sand, washed up from greater depths, or sometimes drifted 
from the coast in small quantities, and will probably be con¬ 
verted, at some future period, into dry land covered with sand 
hills. There are also extensive ranges of dunes upon the east¬ 
ern shores of the Caspian, and at the southern, or rather south¬ 
eastern extremity of Lake Michigan.* There is no doubt that 
this latter lake formerly extended much farther in that direc¬ 
tion, but its southern portion has gradually shoaled and at last 
been converted into solid land, in consequence of the preva¬ 
lence of the northwest winds. These blow over the lake a 
large part of the year, and create a southwardly set of the cur¬ 
rents, which wash up sand from the bed of the lake and throw 
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 
• 
very bounds of the ocean, exposed to the biting of the wind and the spray 
of the sea. The only shelter they require is, at first, some interruption to 
break the current of the wind, such as fences, houses, or other trees.” 
* The careful observations of Colonel J. D. Graham, of the United Statep 
Army, show a tide of about three inches in Lake Michigan. See a A Lunar 
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. 
