cating that a mass
of average hard rock a mile thick, domed to the curvature of the earth,
can support a layer of only about ten feet of its own material. The
structural geologist, through his study of folds, faults, and rock
flowage, comes to regard rocks essentially as failing structures.

Disturbances of equilibrium, resulting in rock movements under gravity,
may be caused by local loading, either natural or artificial. Natural
loading may be due to unusual rainfall, or raising of water level, or
increased barometric pressure. Artificial loading may come from
construction of heavy buildings or dams. Movement may also result from
excavation, which takes away lateral support--and such excavation again
may be caused by natural processes of erosion or by artificial processes
involved in construction. Movement may be caused by mere change in the
moisture content of rocks, or by alterations of their mineral and
chemical character, affecting their resistance to gravity. In still
other cases, earthquakes are the initiating cause of movement.

In unconsolidated rocks, a frequent cause of movement is the presence of
wet and slippery clay layers. The identification and draining of these
clay layers may eliminate this cause. In certain sands, on the other
hand, water may actually act as a cement and tend to increase the
strength of the rock. Planes of weakness in the rock, such as bedding,
joints, and cleavage, are also likely to localize movement.

Earth materials, and even fairly hard rocks, may creep under gravity at
an astonishingly low angle. The angle from the horizontal at which loose
material will stand on a horizontal base without sliding is called the
angle of rest or repose. It is often between 30 deg. and 35 deg., but there is
wide variation from this figure, depending on the shapes and sizes of
the particles and on other conditions. It has been suggested that even
the slight differences in elevation of continents and sea bottoms may,
during long geologic eras, have caused a creep of continental masses in
a seaward direction.

In problems relating to slides, the geologist is concerned in
determining the kinds of rocks, their space relations, their structures
and textures, their metamorphic changes, their water content and the
nature of the water movement, their strength, both under tension and
compression, and other factors.

In the digging of the Panama Canal, a geological staff was employed in
the study of the