32 
GEOLOGY OF THE FIRST DISTRICT. 
The south part of Coney island is a labyrinth of sand-dunes, formed by the wind, which 
present almost every imaginable shape that such a material can assume. These hillocks are 
from five to thirty feet high, with a few straggling tufts of beach-grass, and clumps of bushes 
half buried in the drifted sands. They owe their origin to a tuft of grass, a bush, or a drift 
log, serving as a nucleus. As the grass grows, the drift sand settles among its leaves and 
partly buries it, and the process is renewed for years, until a sand hill is formed. On the 
contrary, where there is nothing to bind the sand together, or shelter it from the wind, it drifts 
away, leaving deep hollows. Drifted snow-banks alford an apt illustration of the sand-dunes 
of the Great South beach of Long Island, extending more than one hundred miles ; and in a 
high wind we can there realize, in a small degree, the sand storms of the African and Ara¬ 
bian deserts. 
Land Slides. 
Many local changes on the surface of the earth, from the sliding of parts of hills or moun¬ 
tains into the valleys, have occurred, and examples of such phenomena are frequently attracting 
attention. Such facts are often made public when they have been attended by the destruc¬ 
tion of life, or property; but thousands of such occurrences have taken place, as geological 
observations attest, which were either unknown, or of which no record has been preserved. 
Land slides are the effect of one or several causes, acting conjointly: 
1st. The action of water on soft crumbling or disintegrating strata, so as to undermine 
cliffs and leave them without sufficient support. 
2d. Hydrostatic pressure of water in fissures. 
3d. Hydrostatic pressure, and water rendering inclined beds slippery. 
4th. Springs converting sands into quicksands, so that large masses flow from the cliffs 
and hills. 
5th. The action of frost expanding the water as it freezes. 
High cliffs of clay and sand on the sea shore and banks of rivers, which are undermined 
by water or by human labor, often crack at some distance from their edges, and stand upon 
comparatively narrow bases. If water filter into one of these fissures, its hydrostatic pres¬ 
sure is frequently sufficient to burst off the mass ; and if the surface upon which it falls be 
inclined, it slides and tumbles to a greater or less distance, according to the nature of the 
materials forming it, and the quantity of water gushing out at the time of the slide. 
The masses of earth or rock where the strata are inclined, are often raised by the hydro¬ 
static pressure of water in the fissures of the strata seams, so that they slide off to a lower 
level. If, in such inclined strata, one of the beds be clay, the water renders it slippery, and 
the superincumbent mass slides off, unless it be supported. On some clay beds where slides 
have occurred, the angle of inclination is almost inappreciable to the eye. 
Many examples of slides from this cause might be adduced, as having occurred on the 
north coast of Long Island. In consequence of the sea washing away the base of the; cliffs, 
large masses slip off, and sometimes slide into the sea. The washing away of the base of the 
