564 DUNES ON SHORES OF TIDELESS WATERS. 
in breadth, while at the mouths of the Nile they form a zone not 
less than ten miles wide. 
The base of some of the dunes in the Delta of the Nile is 
reached by the river during the annual inundation, and the in- 
filtration of the water, which contains lime, has converted the 
lower strata into a silicious limestone, or rather a calcareous 
sandstone, and thus afforded an opportunity of studying the 
structure of that rock in a locality where its origin and mode of 
ageregation and solidification are known. 
The tide, though a usual, is by no means a necessary condi- 
tion for the accumulations of sand out of which dunes are 
formed. The Baltic and the Mediterranean are almost tideless 
seas, but there are vast ranges of dunes on the Russian and 
Prussian coasts of the Baltic and at the mouths of the Nile and 
many other points on the shores of the Mediterranean. The 
vast shoals in the latter sea, known to the ancients as the Great- 
er and Lesser Syrtis, are of marine origin. They are still fill- 
ing up with sand, washed up from greater depths, or sometimes 
drifted from the coast in small quantities, and will probably be 
converted, at some future period, into dry land covered with 
sand-hills. There are also extensive ranges of dunes upon the 
eastern 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 
direction, but its southern portion has gradually shoaled and at 
last been converted into solid land, in consequence of the preva- 
lence of the north-west 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 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 some 
* 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 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 Asso- 
ciation for the Advancement of Science. 
