386 
PACIFIC SCIENCE, VoL XIX, July 1965 
bination of flow material and sediments with 
some porosity remaining. This picture more or 
less conforms to the results obtained in the pre- 
liminary Mohole drilling off the west coast of 
the United States. 
The density of the sediments in the bathy- 
metric deep adjacent to the Ridge was assumed 
to be 2.1 g/cc. This is in the range of densities 
normally chosen for such sediments, but admit- 
tedly it may be too low since it implies porosi- 
ties in the neighborhood of 40 per cent through- 
out the sediments. The thickness of the sedi- 
ments was chosen to match the seismic reflection 
results of Kroenke (p. 335 in this issue). On 
the Ridge itself the unsaturated density above 
sea level for the flows was assumed to be 2.3 
g/cc. Below sea level, the porous rocks were 
assumed to be water-saturated and a density of 
2.5 g/cc was used for the first 1.5 km below 
sea level. A density of 2.6 g/cc was then used 
for the next 1.5 km. Together these density 
layers were chosen to represent material which 
had erupted above or near sea level 
The density of 2.75 g/cc was chosen for the 
bulk of the Hawaiian Ridge which has a seis- 
mic velocity near 5.0 km/sec, under the as- 
sumption that the low seismic velocity results 
primarily from the presence of glass but that 
some intraflow porosity between pillow lava 
flows exists. It might be mentioned here that a 
velocity of about 5.0 km/sec is a usual velocity 
for submarine volcanic flows and has been 
found in many other areas, such as along the 
Mid-Atlantic Ridge and on the Tonga Ridge. 
The question might well be asked, what 
other density distributions are possible? Basi- 
cally the geologically acceptable possibilities in- 
volve changing the densities by about ±0.1 
g/cc in all or part of any layer, with attendant 
changes in densities of other layers or slight 
meometric changes of ± 1 km in boundaries 
between layers. There is probably no way short 
Fig. 3. Free air anomaly ( dashed lines) gravity map of the Hawaiian Islands (values in milligals). Ba- 
thymetry contours ( solid lines) in fathoms. 
