42 
PACIFIC SCIENCE, VoL XXII, January 1968 
TABLE 1 
COMPRESSIONAL WAVE VELOCITIES IN BASALT 
(km/sec) 
SAMPLE 
DENSITY 
(g/cc) 
pressure (kb) 
0.1 
0.5 
1.0 
2.0 
4.0 
6.0 
8.0 
10.0 
Basalt 1 
2.91 
5.8 
6.03 
6.08 
6.13 
6.21 
6.25 
6.28 
6.33 
2.91 
5.9 
6.05 
6.11 
6.15 
6.23 
6.28 
6.32 
6.37 
2.88 
5.6 
5.76 
5.86 
5.97 
6.03 
6.10 
6.16 
6.20 
Mean 
2.90 
5.8 
5.95 
6.02 
6.08 
6.16 
6.21 
6.25 
6.30 
Basalt 2 
2.92 
5.8 
6.00 
6.03 
6.06 
6.11 
6.16 
6.20 
6.25 
2.91 
5.8 
6.04 
6.07 
6.09 
6.14 
6.20 
6.24 
6.28 
2.91 
5.9 
6.08 
6.14 
6.19 
6.22 
6.27 
6.36 
6.35 
Mean 
2.91 
5.8 
6.04 
6.08 
6.11 
6.16 
6.21 
6.27 
6.29 
Basalt 3 
2.95 
6.0 
6.14 
6.19 
6.25 
6.34 
6.36 
6.42 
6.46 
2.92 
5.9 
6.09 
6.16 
6.21 
6.27 
6.34 
6.36 
6.39 
2.94 
6.0 
6.11 
6.17 
6.25 
6.29 
6.34 
6.38 
6.42 
Mean 
2.94 
6.0 
6.11 
6.17 
6.24 
6.30 
6.35 
6.39 
6.42 
TABLE 
2 
Modal 
, Analyses 
1 of Basalts 
(Percentages by Volume) 
SERICITE + 
ROCK 
PLAGIOCLASE 
PYROXENE 
MAGNETITE 
CALCITE 
CHLORITE 
Basalt 1 
53.4 
26.5 
7.2 
3.2 
9.7 
Basalt 2 
54.2 
25.3 
6.9 
3.0 
7.6 
Basalt 3 
55.8 
30.2 
6.9 
2.6 
4.5 
basalts 1 and 3 correspond to cores of relatively 
low density. The low densities are the result of 
slight alteration of pyroxene to chlorite and 
sericitization of plagioclase. No preferred 
orientation of minerals was observed in either 
sample. Petrographic examination of basalt 
2 revealed a rough subparallel orientation of 
lath-shaped plagioclase crystals. Normals to 
the (010) twin planes of plagioclase concen- 
trate in the direction of highest velocity. This 
is consistent with relatively high compressional 
wave velocities normal to (010) in single 
crystals of feldspar reported by Christensen 
(19 66a ) . 
Manghnani and Woollard (1965) have cor- 
related elastic wave velocities in basalts at low 
pressures with glass content, olivine content, 
and volume percent of vesicles. The samples in 
the present study contain no microscopically 
visible glass or vesicles. Therefore, with the 
exception of a slight lowering of velocity due to 
alteration, the samples represent nearly maxi- 
mum velocities for olivine-free basalts with 
tholeiitic composition. As will be considered in 
detail below, the basalts have lower velocities 
than other varieties of basic rocks. It is some- 
what surprising that compressional wave veloci- 
ties in several granites reported by Birch (I960) 
are close to the velocities of the basalts at 
equivalent pressures. The basalt velocities are 
also equivalent to partially serpentinized peri- 
dotites containing approximately 50% serpentine 
(Christensen, 19 66b). 
Birch (1961) found a difference in velocity 
of about 5% between the means for diabase and 
gabbro. This difference presented a problem 
since the reported densities were about the same, 
and diabase and gabbro are generally considered 
to be approximately equivalent in chemical com- 
position. Birch (1961) and Christensen (1965) 
postulated that the discrepancy may actually be 
the result of differences in chemical composition 
of the two rock types. 
The mean compressional wave velocity at 10 
