Geologic Implications of Magnetic Surveys — Malahoff and Woollard 
273 
units for susceptibility and 11 X 10~ 3 cgs units 
for natural remanent magnetization. 
For purposes of computation, the extrusive 
rocks on the islands of Maui and Kahoolawe 
were divided into two groups: those of West 
Maui and those of East Maui (the latter in- 
cluding Kahoolawe). However, no rock samples 
were actually collected on Kahoolawe, because 
it is a bombing range and a closed area. In 
assuming the same susceptibilities and inten- 
sities of natural remanent magnetization for 
East Maui and Kahoolawe, no error is likely, 
for, as seen from inspection of the total inten- 
sity magnetic map of Maui and Kahoolawe 
(Fig. 2), it appears that both Haleakala Vol- 
cano and Kahoolawe originated from extrusions 
from the same "primary” rift zone. 
On East Maui, the bulk of the lavas is repre- 
sented by the Honomanu basalts, which have 
an unusually low remanence value which aver- 
ages 1.0 X 10~ 3 cgs units with an average 
susceptibility of 2.6 X 1Q~ 3 cgs units. Rocks 
of Kula series, on the other hand, which 
have a maximum thickness of 2,000 ft at the 
summit of Haleakala, have an unusually high 
n.r.m. (natural remanent magnetization) of 
137.3 X 10 — 3 cgs units. Because the data avail- 
able are too sparse to determine what is a true 
representative n.r.m. -susceptibility value for the 
bulk of the rocks forming East Maui, the writers 
were forced to compromise and use an average 
value for all the rocks sampled in East Maui. 
Therefore, using Tarling’s values (Table 1), as 
well as the writers’ values, a mean natural 
remanent magnetization of 15.0 X lb -3 c g s 
units was employed in computing the topo- 
graphic effects on East Maui and Kahoolawe. 
Similarly, an average value using Tarling’s 
and the writers’ values of 12.1 X 10” 3 cgs 
units for the n.r.m. and 2.7 X 10 -3 cgs units 
for susceptibility was employed for the topo- 
graphic reduction over West Maui. 
Inasmuch as the writers did not collect any 
samples on Molokai and Lanai, Tarling’s values 
were used in the magnetic computations involv- 
ing these islands. An n.r.m. of 5.88 X 10 -3 cgs 
units and a susceptibility of 0.92 X 10 -3 cgs 
units were used for the island of Lanai. An 
n.r.m. of 19.43 X lb -3 and a susceptibility of 
2.13 X IQ -3 cgs units were used for East 
Molokai, and an n.r.m. of 13.22 X 10 -3 cgs 
units and a susceptibility of 1.16 X 10“ 3 cgs 
units were used for West Molokai. 
The topographic effects of the Waianae and 
Koolau volcanic series on the island of Oahu 
were reduced using the following values. An 
n.r.m. of 2.67 X 10~ 3 cgs units and a suscep- 
tibility of 2.19 X 10“ 3 cgs units based on 
Tarling’s measurements were used for the to- 
pography associated with the Waianae volcanic 
series. An n.r.m. of 4.47 X 10 -3 cgs units and 
a susceptibility of 2.68 X 10~ 3 cgs units were 
used for the topography associated with the 
Koolau volcanic series. However, intrusive 
rocks sampled by the writers in the Koolau 
caldera showed considerable variation in values. 
Specimens of fine-grained, dark, magnetite-rich 
intrusive rocks collected near the periphery of 
the caldera had an n.r.m. of approximately 
20.0 X 10 -3 cgs units and a susceptibility of 
3.9 X 10 -3 cgs units. On the other hand, fine- 
grained, dense intrusive rocks, rich in pyrite, 
collected near the center of the caldera had an 
approximate n.r.m. of 1.0 X 10~ 3 cgs units and 
a susceptibility of 0.5 X 10 -3 cgs units. Because 
of the scarcity of suitable outcrops, it is not 
known which of these intrusive-rock suites is 
representative of the bulk of intrusive rocks at 
depth. However, as both of the suites of intru- 
sive rocks sampled are normally polarized and 
the Koolau magnetic anomaly (Fig. 3) is in- 
versely polarized, a possible explanation for the 
inverse polarized anomaly on the basis of the 
data available may be that the magnetic anomaly 
results from rocks having lower susceptibility 
and n.r.m. value than the surrounding basalts, 
present at depths within the Koolau caldera. 
The magnetic properties of rocks on Kauai 
were averaged and only one set of values was 
used because there appears to be little difference 
in values between the Koloa basalts and the 
Napali basalts. For the topographic reductions 
on Kauai as well as on Niihau an n.r.m. value 
of 5.14 X 10 -3 cgs units was used. 
In order to solve many of the problems stated 
in this paper concerning the gross differences 
observed in the magnetic properties of rocks 
collected in the Hawaiian Islands, a thorough 
program of sampling both intrusive as well as 
extrusive rocks will be necessary. Sampling of 
both bathymetric and topographic features over 
the Hawaiian Ridge will be necessary also. Be- 
