286 
sions, found only at Salt Lake Crater on Oahu, 
show the garnet-omphacite association charac- 
teristic of high pressure, and appear probably to 
be of mantle origin; though it is conceivable 
that the conditions responsible for the develop- 
ment of the high-pressure mineral facies may 
have had some other origin. 
Because of the very shallow position of the 
Mohorovicic discontinuity in the central Pa- 
cific, few persons still adhere to the belief that 
it may represent the basalt-edogite phase tran- 
sition (though a similar transition may take 
place from peridotite to olivine-garnet perido- 
tite containing sodic pyroxene at a deeper level 
within the mantle). However, it has recently 
been suggested, on the basis of high P-T lab- 
oratory studies of basalt in the presence of vola- 
tiles, that the Moho beneath the deep oceans 
may be a phase transition from basalt to glau- 
cophane-lawsonite rock (George Kennedy, ad- 
dress at Berkeley, California, October 7, 1964). 
The net conclusion from all available evi- 
dence seems to me to be that the mantle be- 
neath the central Pacific probably consists of 
peridotite approaching in composition the av- 
erage stony phase of stony meteorites, or the 
theoretical "pyrolite” of Lovering, undergoing 
a phase change at depth to olivine-garnet 
pyroxenite. 
PACIFIC SCIENCE, Vol XIX, July 1965 
In recent years a large amount of Important 
petrologic theory and petrogenic speculation 
has been based on Hawaiian rocks. We are 
accumulating a large mass of information on 
the volcanic rocks above sea level. The evidence 
is strong that, whatever the processes that give 
rise to the diversity of rock types in Hawaii, 
the primary magma or magmas that give rise 
to them are derived from the mantle. Yet it 
should be obvious from the foregoing that very 
little is known directly of the composition of 
the mantle in the Hawaiian region. A hole 
through the Moho in this region would give 
not only incontestable knowledge of the na- 
ture of the mantle, but also specific knowledge 
of Its local chemical and mineral composition 
that can be correlated with the composition of 
the extensively studied tholeiitic lavas found 
in Hawaii. Such correlative studies are essential 
if theories about the origin of magmas are to 
move out of the realm of speculation onto 
firmer ground. 
REFERENCE 
Engel, A. E. J., and C G. Engel. 1964. Igne- 
ous rocks of the East Pacific Rise. Science 
146:477-485. 
