272 
That there may be a relation between the 
composition of the mantle and the overlying 
crust is suggested by several lines of evidence. 
For example, the seismic measurements con- 
ducted by the Lamont Geological Observatory 
during the International Geophysical Year over 
the Scotian Sea show that the mean velocity 
and thickness of the crustal layer as well as the 
velocity of the underlying mantle all decrease 
as the nose of the Scotian arc is approached. 
If "blanket” data are considered, and all meas- 
urements in the Atlantic Ocean area are consid- 
ered, it is found that there is a positive corre- 
lation between the mean velocity of the crust 
and the mean velocity of the mantle where 
there are positive isostatic gravity anomalies, 
and a slight negative correlation where there 
are negative isostatic anomalies. There is no 
systematic relationship between the two quan- 
tities where isostasy prevails. On the continents 
the same relations are observed, except that the 
correlation slopes are much higher and about 
equal in magnitude in areas of positive and 
negative isostatic anomalies. In the Pacific 
Ocean no appreciable difference in relations as 
a function of gravity is found, and both the 
negative and positive isostatic gravity anomaly 
areas show a slight negative dependence of the 
velocity of the crust on the velocity of the 
mantle. 
These regional differences between the Pa- 
cific and Atlantic oceans are also brought out 
by the relation between crustal thickness and 
Bouguer and free-air gravity anomalies. Again 
using "blanket” data, it is found that, for 
oceanic depths 5. 0-6.0 km in both oceans, the 
anomalies in the Atlantic Ocean are about 20 
mgal more negative than in the Pacific Ocean 
and the crust is about 1 km thinner. As the 
depth of water is the same, this implies either 
a difference in mean crustal density or a dif- 
ference in mantle density, or a combination of 
the two. As indicated earlier, the median man- 
tle velocity in the Pacific Ocean is significantly 
higher than in the Atlantic Ocean, which would 
imply a higher density for the mantle and, 
hence, greater density contrast with the crust. 
To maintain the same bottom elevation under 
hydrostatic equilibrium conditions with a 
thicker crust, the density of the crust in the 
PACIFIC SCIENCE, Vol. XIX, July 1965 
Pacific Ocean must be greater than in the At- 
lantic Ocean. When one examines mean crustal 
velocity values, it is found that the values sup- 
port this hypothesis. This positive correlation 
between crustal and mantle velocity values 
strongly suggests that there is a genetic rela- 
tionship between the crust and mantle. That the 
mantle and the basal layer of the crust might 
represent polymorphic phase transformations of 
the same material, with the depth of the Mo- 
horovicic discontinuity being a function of 
pressure and temperature relations at depth, 
has been suggested by Kennedy (1959). Hess 
(1955) has suggested serpentinization as a 
reversible process that would also provide a 
genetic relationship between the crust and 
mantle. 
The most convincing arguments as to the 
reality of such a phenomenon are the anom- 
alous relations of crustal thickness in areas of 
crustal subsidence and uplift. For example, the 
value of crustal thickness determined seismically 
in Texas, where the crystalline rock basement 
has been down-warped approximately 7.5 km 
since late Paleozoic, is about 33 km, which is 
normal for the surface elevation of 50 m. That 
the seismic measurement is not substantially in 
error is indicated by the local gravity anom- 
alies, which show there is essentially complete 
compensation for the thick column of low 
density sediments having a theoretical gravity 
effect of about 80 mgal after allowing for com- 
paction and variations in lithology. As the sedi- 
ments are either terrestrial in origin or repre- 
sent shallow water facies, it is probable that the 
crustal thickness has remained essentially con- 
stant, and the progressive down-warping of the 
surface has been accompanied by a correspond- 
ing upward migration of the mantle at the ex- 
pense of the basal crustal layer to maintain iso- 
static equilibrium. 
Another line of evidence bearing on this 
problem is the absence of a pronounced crustal 
root beneath areas of eustatic uplift. On the 
Mexican Plateau, for example, the seismic 
crustal measurement at Durango shows the 
same sub-sea level elevation for the mantle 
(-41.2 km) as is found at Calgary, Alberta 
in front of the Rocky Mountain block. Although 
Calgary is essentially in isostatic equilibrium, 
