44 T. L. ALLEN 
their results showed a gradual thinning of the sedimentary beds and 
a gradual decrease in the velocity values. The seismograms shown in 
Plate 2 are the ones from which profile B is plotted, Plate 3 shows the 
seismograms from which profile C is plotted, and Plate 4 shows those 
from which profile D is plotted. 

PLATE 2.—Seismograms for profile B. 
STUDY OF RECORDS 
A study of the three sets of seismograms reveals four distinct 
record characteristics. The first five seismograms of profile B, Plate 2, 
break sharply and plot on the surface velocity curve; the next four 
seismograms do not break sharply but have a small wave of short 
period for the first impulse followed by a wave with a sharp break 
and large amplitude. The small impulse and the sharp break were 
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