GEOLOGY: W. BOWIE 
173 
tion and with two different gravity formulas; otherwise they are iden- 
tical. The Bouguer reduction postulates a highly rigid earth and the 
free air reduction an earth with no rigidity. 
Relation Between the Gravity Anomalies and the Topography 
Mean anomalies 
With regard to sign 
MEAN ANOMALY 
Hayford, 
1912; 
depth 
113.7 km. 
Hayford, 
1916; 
depth 60 
km. 
Bouguer 
Coast stations 
Stations near coast 
Stations in interior, not in mountainous 
regions 
Stations in mountainous regions 
Below the general level , 
Above the general level 
All stations (except the two Seattle sta- 
tions) 
27 
46 
36 
20 
217 
-0.009 
-0.001 
-0.001 
-0.003 
+0.001 
-0.002 
-0.003 
+0.002 
-0.001 
0.000 
+0.016 
+0.001 
+0.017 
+0.004 
-0.028 
-0.107 
-0.110 
-0.036 
Without regard to sign 
Coast stations 
27 
0.018 
0.012 
0.021 
0.022 
46 
0.021 
0.020 
0.025 
0.023 
Stations in interior, not in mountainous 
88 
0.019 
0.019 
0.033 
0.020 
Stations in mountainous regions 
Below the general level 
36 
0.020 
0.018 
0.108 
0.024 
Above the general level 
20 
0.017 
0.022 
0.111 
0.059 
All stations (except the two Seattle sta- 
tions) 
217 
0.019 
0.019 
0.049 
0.025 
The table above shows that when the reductions are made by either 
of the Hayford methods the range of the mean anomalies with regard 
to sign is very nearly zero in most cases. The Bouguer and free air 
anomalies are much larger than the isostatic anomaHes. By anomaly 
is meant the difference between the observed and the computed values 
of gravity at a station. 
The table indicates strongly that the conditions under which the 
isostatic or Hayford reductions were made are very close to the truth. 
The evidence is that the depth 113.7 kilometers is closer to the truth 
than 60 kilometers, for in the former case the mean anomaly for the 
classes of topography indicated above varies from +- 0.001 to —0.009 
dyne, while with the latter the range is from +0.016 to —0.003. The 
