STRATIFICATION OF DUNES. Ly ¢l 
would, like any other moderate mechanical agitation, proba- 
bly produce the separation of a miscellaneous mass, like that 
described, into distinct layers. Again, the Pacific coast, like 
all others upon an open sea, is exposed to incessant concussion 
from the shock of the waves, which is repeated many thousand 
times a day. This concussion is often sensibly felt by the 
observer, and it seems not in the least improbable that the agita- 
tion may have tended to produce a stratified arrangement in 
the case at San Francisco, as well as in all coast dunes and 
other accumulations of loose mineral material in similar situa- 
tions. Kohl observes that the shore on the landward side of 
the files of dunes often trembles from the shock of the waves on 
the beach, * and Villeneuve established by careful experiment 
that at Dunkerque the ground is sensibly agitated by the same 
cause, in stormy weather, to a distance of more than a mile 
from the sea. 
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 inter- 
mixed 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. 
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 strati- 
* Inscln und Marschen, etc., ii., p. 34. 
