REFLEXION OF ELASTIC WAVES 133 
is the other known factor and conclusions are drawn from differences 
in velocity, and (6) those in which velocities are known or assumed 
and depths derived. An example of the first group is the method of 
refraction shooting in which a disturbance is generated, usually by 
explosion of a charge of dynamite, and at substantially equidistant 
surrounding points the times of arrival of the resulting elastic waves 
are recorded. Knowing the quantities, time, and distance, we can 
derive the average velocity of propagation of the initial impulse which 
reaches these encircling points. Velocity of propagation being a char- 
acteristic property whose value is now known for many minerals, we 
have therefore a direct method of approximating the character of 
material traversed by the impulse. This method met with outstanding 
success in discovery of intruded salt plugs piercing the Oligocene and 
younger strata of south Texas and Louisiana. The latter group in- 
cludes the reflexion method in which a disturbance is generated at the 
surface and the time taken for the resulting wave front to travel from 
the origin down to a selected reflecting plane and back to the surface 
is measured; knowing the average velocity of propagation of the 
leading wave front it is obviously a simple matter to determine the 
depth to the reflecting plane. The best example of the brilliant ap- 
plication of this principle is in the method of determining the depth 
of water known as echo depth-sounding. In the simplest application of 
this method a disturbance is generated on one side of the hull of a 
ship and the travel time for an impetus to be transmitted downward 
through the water to the ocean bed and back to a detector on the 
other side of the hull is measured; a knowledge of the velocity of 
propagation of elastic waves in water permits the depth to be com- 
puted. This practical application antedated the advent of the method 
as a geological tool and must be regarded as the prior art which led 
to such application. 
Two great simplifications are present in mapping a water-rock 
contact by this method which do not pertain to the geological prob- 
lem. In the first place, the contact between two media such as water 
and rock is an excellent mirror for sonic waves, whereas the contacts 
of geological strata normally are but indifferent reflecting planes. 
Secondly, water being a fluid it transmits only longitudinal and sur- 
face waves, whereas any disturbance in a solid will set up a complete 
suite of longitudinal, transverse, and surface waves. 
VARIOUS WAVES RECORDED 
Let us now investigate the many impetii which strike a series of 
seismographs placed on the surface at varying distances from a source 
793 
