THE EARTH, A FAILING STRUCTURE. 67 
One of the characteristics of a failing structure is that 
where failure begins, there later damage tends to concen- 
trate. The point of initial failure in the Quebec bridge was 
first located mainly by noting that two of the main mem- 
bers of the bridge had suffered much more distortion than 
the others. So, if the earth is a failing structure, we should 
expect to find indications that progressive and concentrated 
failure has occurred at some places rather than that the 
failure has been uniform in degree in all parts. Such indi- 
cations exist and have frequently been recognized and com- 
mented upon. It is recognized by geologists generally that 
there are regions of excessive deformation, as if some in- 
cipient weakness had concentrated failure there, and that 
there are other regions in which the deformations have been 
moderate in amount, as if those were regions of more than 
average strength in a failing structure. Where mountain- 
building commences, there it tends to continue for a long 
time, though not, perhaps, for an indefinite time. Ocean 
bottoms and great plains apparently tend to remain ocean 
bottoms and plains. 
Some of the reasons have been put before you for believ- 
ing the earth to be a failing structure, for believing that it 
is yielding in a non-elastic manner to the stresses produced 
by gravity and by other forces which are applied con- 
tinuously for long periods. Is there a contradiction between 
this conception of the earth as a failing structure and the 
known behavior of the earth in earthquakes and under the 
action of tidal forces? 
So far as I have been able to trace the connection, there 
is no contradiction whatever. 
The motions at the earth’s surface due to earthquakes, 
which are detected by seismographs, are vibrations of small 
amplitude, having periods of a few seconds, as a rule. The 
fact that these vibrations persist for a considerable time, for 
minutes or hours, and that they not infrequently travel all 
the way around the earth and at least part of a second trip, 
show that, under these particular applied forces, the earth 
