VELOCITY IN GRANITE AND LIMESTONE 117 
variation in velocity of even o.10 per cent. This effect may, therefore, 
also be neglected. 
If, therefore, the samples used in making the laboratory determin- 
ations are truly representative of the bulk of the material in which 
the velocity is determined in the field, the results obtained in each 
case should be directly comparable, and the lack of agreement must, 
therefore, be interpreted as being due to an actual difference in the 
elastic constants of the samples used and the average value of those 
of the rock in situ. In the present investigation only two samples of 
the granite were used in making the laboratory determinations so that 
the “sampling error’’ may have been large. There is a further possi- © 
bility that the elastic constants of the small samples may differ from 
those characteristic of the material before being cut out. 
Another probable cause of the observed difference is a possible 
variation in the moisture content of the granite 7m situ and that of the 
samples used in the laboratory work. Recent work of the authors has 
shown that the presence of a small amount of moisture may seriously 
affect the elastic constants of rocks. This effect has also recently been 
reported by W. A. Zisman.’ 
Preliminary work by the writers indicates that the presence of 
moisture, in general, lowers the value of Young’s modulus. The lower 
value of Young’s modulus observed in the field determinations may, 
therefore, be due, in part at least, to larger percentage of moisture in 
the surface granite in the field as compared with that in the laboratory 
samples. Although no effort was made to dry out the samples used in 
the laboratory work, they were actually in an air-dry condition. 
It is interesting to note that this explanation serves also as a pos- 
sible explanation for the unexpectedly large curvature of the time- 
distance graph obtained from the field data. The increase of velocity 
with depth may prove to be partially due to a decrease in the moisture 
content of the rock below the surface. 
Work is now in progress to test the validity of the explanations 
here advanced and it is hoped that they may be reported later. 
ARBUCKLE LIMESTONE 
Two profiles were laid out in the Arbuckle limestone with dis- 
tances of 100-2,500 feet from the shot point. Detectors were placed 
every 200 feet along these spreads. A section of the limestone outcrop 
where the angle of dip of the bedding planes was approximately 90° 
was selected for the work. The lines of the two profiles were at right 
7 “Elastic Constants of Rocks and Their Relation to Seismic Wave Speeds,” 
Physical Review, Vol. 43, p. 501. 
