1898-99.] Prof. Knott on Earthquake Vibrations. 585 
then than now. We have recognised within the last decade that 
the earth’s crust is everywhere in a constant state of tremor, slight, 
no doubt, but measurable. In former ages these tremors existed 
also, and probably in a much more pronounced form. The geologist 
can to some extent build up the volcanic conditions of the past ; 
but the earthquake seems to leave behind it in the geological 
record no effect that can be chronologically interpreted. Faults, 
thrust planes, synclines and anticlines, all tell a story of seismic 
action ; but with what rapidity these changes and distortions were 
effected we know not. Geologists tell us that there is no evidence 
of increased volcanic activity in the past; but that does not 
settle the question of seismic activity. Distribution of volcanic 
rocks, and faults and contortions of strata near the surface , throw 
practically no light on the seismic phenomena of to-day, nor can 
we expect them to throw much more on the seismic activity of the 
past. It is at all events not improbable that in former times, and 
perhaps not so very long ago, the continual shakings, tremblings, 
and tremors had an average intensity and a distribution far exceed- 
ing what they have now. Under the influence of these, materials 
would yield more easily and more quickly to the stresses acting 
on them, just as a steel rod becomes fatigued and deteriorates 
under intermittent loadings. Is it not conceivable that under such 
conditions contiguous strata might slip the one over the other 
without any apparent unconformability ? And is it not likewise 
conceivable that strata may have suffered extension, or the reverse, 
while they were being bent ? In the production of these strains 
or distortions under the influence of appropriate stresses, the 
mechanical vibration that accompanies seismic phenomena must 
certainly have played an important rdle. 
VOL XXIT. 
8/11/99 
