478 
FORM OF DUNES. 
stratification may be found in the occasional interposition of a 
thin layer of leaves or other vegetable remains between succes¬ 
sive deposits, and this I imagine to be more frequent than has 
been generally supposed. 
The eddies of strong winds between the hillocks must also 
occasion disturbances and re-arrangements of the sand layers, 
and it seems possible that the irregular thickness and the 
strange contortions of the strata of the sandstone at Petra 
may be due to some such cause. A curious observation of 
Professor Forchhammer suggests an explanation of another 
peculiarity in the structure of the sandstone of Mount Seir. 
He describes dunes in Jutland, composed of yellow quartzose 
sand intermixed with black titanian iron. When the wind 
blows over the surface of the dunes, it furrows the sand with 
alternate ridges and depressions, ripples, in short, like those of 
water. The swells, the dividing ridges of the system of sand 
ripples, are composed of the light grains of quartz, while the 
heavier iron rolls into the depressions between, and thus the 
whole surface of the dune appears as if covered with a fine 
black network. 
Form of Dunes . 
The sea side of dunes, being more exposed to the caprices 
of the wind, is more irregular in form than the lee or land side, 
where the arrangement of the particles is affected by fewer 
disturbing and conflicting influences. Hence, the stratification 
of the windward slope is somewhat confused, while the sand in 
the lee side is found to be disposed in more regular beds, in¬ 
clining landward, and with the largest particles lowest, 
where their greater weight would naturally carry them. The 
lee side of the dunes, being thus formed of sand deposited 
according to the laws of gravity, is very uniform in its slope, 
which, according to Forchhammer, varies little from an angle 
of 30° with the horizon, while the more exposed and irregular 
weather side lies at an inclination of from 5° to 10°. When, 
however, the outer tier of dunes is formed so near the water¬ 
line as to be exposed to the immediate action of the waves, it 
