Geologic Implications of Magnetic Surveys — Malahoff and Woollard 
275 
Fig. 3 . Magnetic profile along line K-K' (see Fig. 29), Diamond Head-Kaneohe Bay, across the Koolau 
caldera, island of Oahu. 
cause of the lack of suitable samples, the writers 
adopted an average value of n.r.m. of 10.0 X 
10 _ 3 cgs units and a susceptibility of 1.0 X 
10~ 3 cgs units for the basaltic rocks forming 
bathymetric features. 
Although it can be argued that, because all 
of the Hawaiian Islands are composed pre- 
dominantly of tholeiite, average magnetic values 
could have been used also for all of the islands, 
rather than somewhat different values for each 
island, such a procedure cannot be justified 
when the data in hand indicate there are real 
differences in average values for each island. 
Even though the lithology may be identical, 
this does not guarantee that the n.r.m. values, 
which are related to the strength and direction 
of the earth’s field at the time of eruption, will 
be the same, inasmuch as it is known that the 
earth’s field is subject to secular change. 
The Magnetic Field Over Offshore Areas 
The Hawaiian Islands were the first portions 
of the Hawaiian Ridge to be surveyed in this 
investigation. Because of the apparent complex- 
ity of the magnetic field observed over the 
islands, and the lack of knowledge of the nature 
of the anomaly-free regional magnetic field, a 
companion marine magnetic survey was essen- 
tial. Although both the U. S. Navy Oceano- 
graphic Office of the Scripps Institution of 
Oceanography as well as the U. S. Coast and 
Geodetic Survey had made magnetic surveys in 
the region, none of these covered the essential 
area adjacent to the islands. The first measure- 
ments related to the present study were carried 
out to sufficient distance north of Maui to avoid 
probable magnetic anomalies over the extension 
of the Molokai Fracture Zone, which the Scripps 
Institution of Oceanography measurements 
