HORIZONTALITY OF STRATA, 
41 
This tendency in newly-formed strata to assume a horizon¬ 
tal position arises principally from the motion of the water, 
which forces along particles of sand or mud at the bottom, 
and causes them to settle in hollows or depressions where 
they are less exposed to the force of a current than when 
they are resting on-elevated points. The velocity of the 
current and the motion of the superficial waves diminish 
from the surface downward, and are least in those depres¬ 
sions where the water is deepest. 
A good illustration of the principle here alluded to may 
be sometimes seen in the neighborhood of a volcano, when a 
section, whether natural or artificial, has laid open to view a 
succession of various-colored layers of sand and ashes, which 
have fallen in showers upon uneven ground. Thus let A B 
(Fig. 1) be two ridges, with an in¬ 
tervening valley. These original 
inequalities of the surface have 
been gradually effaced by beds 
of sand and ashes c, e, the sur¬ 
face at e being quite level. It will be seen that, although the 
materials of the first layers have accommodated themselves 
in a great degree to the shape of the ground A B, yet each 
bed is thickest at the bottom. At first a great many parti¬ 
cles would be carried by their own gravity down the steep 
sides of A and B, and others would afterwards be blown 
by the wind as they fell off the ridges, and would settle in 
the hollow, which would thus become more and more effaced 
as the strata accumulated from c to e, Now, w^ater in mo¬ 
tion can exert this levelling power on similar materials more 
easily than air, for almost all stones lose in water more than 
a third of the weight which they have in air, the specific 
gravity of rocks being in general as 2^ when compared to 
that of water, which is estimated at 1. But the buoyancy 
of sand or mud would be still greater in the sea, as the den¬ 
sity of salt-water exceeds that "of fresh. 
Yet, however uniform and horizontal may be the surface 
of new deposits in general, there are still many disturbing 
causes, such as eddies in the water, and currents moving first 
in one and then in another direction, which frequently cause 
Fisr. 2. 
Section of strata of sandstone, grit, and conglomerate. 
