INTERIOR STRUCTURE OF DUNES. 569 
Interior Structure of Dunes. 
The interior structure of the dunes, the arrangement of their 
particles, is not, as might be expected, that of an unorganized, 
confused heap, but they show a strong tendency to stratifica- 
tion. This is a point of much geological interest, because it 
indicates that sandstone may owe its stratified character to the 
action of other forces as well as of water. The origin and 
peculiar character of these layers are due to a variety of causes. 
For example, a south-west wind and current may deposit upon 
a dune a stratum of a given color and mineral composition, and 
this may be succeeded by a north-west wind and current, bring- 
ing with them particles of a different hue, constitution, and 
origin. Again, if we suppose a violent tempest to strew the 
beach with sand-grains very different in magnitude and specific 
gravity, and, after the sand is dry, to be succeeded by a gentle 
breeze, it is evident that only the lighter particles will be taken 
up and carried to the dunes. If, after some time, the wind 
freshens, heavier grains will be transported and deposited on 
the former, and a still stronger succeeding gale will roll up yet 
larger kernels. Each of these deposits will form a stratum. If 
we suppose the tempest to be followed, after the sand is dry, 
not by a gentle breeze, but by a wind powerful enough to lift 
at the same time particles of very various magnitudes and 
weights, the heaviest will often lodge on the dune while the 
lighter will be carried farther. This would produce a stratum 
of coarse sand, and the same effect might result from the 
blowing away of light particles out of a mixed layer, while 
the heavier remained undisturbed.* Still another cause of 
apparent stratification may be found in the occasional interpo- 
* The lower strata must be older than the superficial layers, and the parti- 
cles which compose them may in time become more disintegrated, and there- 
fore finer than those deposited later and above them. 
Hull ingeniously suggests that, besides other changes, fine sand intermixed 
with or deposited above a coarser stratum, as well as the minute particles 
