Evening star., 1925-06-30

| There are thousands of earthquakes
in the United States every year. Most
of them are too slight to be recorded.
If 1t were not for the great number
of them. which act as a safety valve,
there would be one occasionally of
! such violence that 1t would be likely
TO shake all the works of man off the
globe.
So says DR.. William Bowie of the
Coast and Geodetic Survey, in con-
.nection with the California and Mon
tana QualIes of the past few days.
| DR. Bowie is among the leaders in
the present school of geological
| thought regarding the causes Of earth
| qualeS.
| In order to understand the mech
| anism of an earthquake, DIR. Bowie
said today, it is necessary to consider
| the globe as made up in two sections.
| First is a solid ball made up of heavy
! but plastic material which composes
the greater part of it's mass. Just
| what this material is is subject to
conjectures, but it is so formed that
| IL can stand any possible strain with
out breaking.
Crust Will Crack
Everywhere surrounding this is a
6O-mile average depth of brittle crust,
the earth upon which we live, which
will crack under a strain, rather than
give way in an elastic fashion.
Now. DR.. Bowie says. imagine this
outer crust cut up into 200 square
mile sections. Some of these sections
would contain high mountains, some
low valleys. some the bottom of the
ocean. But each would weigh almost
exactly the same. The mountains are
made of lighter material than the sea
bottom or the valleys. Thus the pres
sure upon every part of the, interior,
elastic ball remains about the same.
But, says DR.. Bowie. the continual
forces Of erosion, taking material
away from the mountains and de-
posting 1t in the valleys and on the
coastal plains, tend to make some Sec
tions temporarily heavier than others
When this happens an extra pressure
is placed upon that section of the
plastic ball over which the heavier
section is located. It bends under the
weight and the outer section sinks
into 1t.
Sinks With Snap.
But this outer section cannot sink
far without breaking. 1t breaks like
a stick. with a snap. and this is the
earthquake.
The snap may occur In the higher
section, which has lost some of it's
weight by erosion, or In the lower
section, which has increased in
weight, due to the deposits on 1t Of
material from the mountains.
The surface Of the earth, DR.. Bowie
explained. is somewhat like a pond
covered with ice which is in condition
to be broken up. Place q heavy
weight on one section of the pond
and the ice may snap. sending tremors
through a considerable area OF the
surface.
The Montana quakes, DIR. Bowie
said, probably can be explained on the
first hypothesis. The mountains have
surface of the ball underneath has
given to conform to the changed
pressure, and there is a snap.
Other Kind in Tennessee.
Examples Of the other kind Of
earthquake, those In regions where
the eroded material has been deposit
ed, now are occurring in west Ten
nessee, due to sediment deposited
from the Appalachian range. The
great Charleston, S. c., earthquake
was Of similar nature.
The Santa Barbara quake, DR..
Bowie said, is due to somewhat diff
ferent cause.-the still continuing for
nation Of the Sierra Nevada noun
tain range.
All mountains of the earth, accord
ing to DIR. Bowie, are comparatively
Young according to the geological
clock Sometimes sediment from
higher regions will pile up on a plain
until it is, for example, 8 miles
thick. A tremendous pressure Is bear
ing down on the surface Of the plastic
ball underneath. 1t gives, just like
tennis ball would give, under extra
pressure, but does not break. The
accumulated sediment sinks lower and