ikewise involve geologic factors. Problems of wave action,
shore currents, shifting of shores, erosion, and sedimentation, which
are of great importance in such operations, have long occupied the
attention of the geologist. They belong especially in the branch of the
science known as physiography.

Geology in relation to underground water supplies is discussed in
Chapter V.

TUNNELS

The digging of tunnels for transportation purposes, for aqueducts, and
for sewage disposal requires careful analysis of geologic conditions in
regard to both the rocks and the underground water. Knowledge of these
conditions is necessary in planning the work, in inviting bids, and in
making bids. It is necessary during the progress of the work. Too often
in the past disastrous consequences, both physical and financial, have
resulted from lack of consideration of elemental geologic conditions.

The building of the great New York aqueducts and subways through highly
complex crystalline rocks has been under the closest geological advice
and supervision. The detailed study of the geology of Manhattan Island
through a long series of years has resulted in an understanding of the
rocks and their structures which has been of great practical use. In the
aqueduct construction the kinds of rock to be encountered in the
different sections, their water content, their hardness, their joints
and faults, were all platted and planned for, and actual excavation
proved the accuracy of the forecasts. An interesting phase of this work
was the tunneling under the Hudson at points where the pre-glacial rock
channel was buried to a depth of nearly a thousand feet by glacial and
river deposits,--this work requiring a close study of the physiographic
history of the river.

SLIDES

Slides of earth and rock materials, both of the creeping and sudden
types, have often been regarded as acts of Providence,--but studies of
the geologic factors have in many cases disclosed preventable causes. A
considerable geologic literature has sprung up with reference to rock
slides, which is of practical use in excavation work of many kinds.

The cause of such movements is gravity. The softer, unconsolidated rock
materials yield of course more readily than the harder ones, but even
strong rocks are often unable to withstand the pull of gravity. The
relative weakness of rock masses on a large scale was graphically shown
by Chamberlin and Salisbury,[66] in a calculation indi