570 STRATIFICATION OF DUNES. 
sition of a thin layer of leaves or other vegetable remains 
between successive deposits, and this I imagine to be more 
frequent than has been generally supposed. Some geologists 
have thought that the sand strata of dunes are of annual forma- 
tion; * but the autumnal deposit of foliage from neighboring 
trees and shrubs furnishes a more probable explanation of the 
division of the sand-heaps into regular layers. 
A late distinguished American admiral communicated to 
me an interesting observation made by him at San I’rancisco, 
which has an important bearing on the arrangement of the 
particles of sand in dunes and other irregular accumulations of 
that substance. In laying out a navy-yard at that port, a large 
quantity of earthy material was removed from the dunes and 
other hillocks and carted to a low piece of ground which 
required filling up. Sand of various characters, fine and coarse 
gravel, and common earth were dropped promiscuously by 
the carts as accident or convenience dictated, and of course 
they were all confusedly intermixed. Some time after, when 
the new ground was consolidated, various excavations were 
made in it, and the different materials of which the filling was 
composed were found to be stratified with considerable regu- 
larity, according to their specific gravity. 
Two explanations of this remarkable fact suggest themselves 
to me, which, however, do not perhaps exclude others. San 
Francisco is subject to earthquakes, and though violent or even 
sensible shocks are not very frequent, it is highly probable that, 
as is shown to be the case in many other countries, by late seis- 
mological observations, there are, in the course of the year, a 
great number of slight shocks which escape unscientific obser- 
vation. A frequent repetition of slight tremblings of the earth 
resulting from the disintegration of the grains of the latter, may be carried 
by rain in the case of dunes, or by the ordinary action of sea-water in that 
of sand-banks, down through the interstices in the coarser layer, and thus the 
relative position of sand and gravel may be changed.— Oorsprong der Holland- 
sche Duinen, p. 105. 
*ScHOMANN, Geologische Wanderungen durch die Preussischen Ost-See 
Provinzen, 1869, p. 81. 
