The Koolau Volcanic Plug— Adams and Furumoto 
305 
The velocities observed here should be com- 
pared with those obtained by Manghnani and 
Woollard (p. 291 in this issue) on ultrasonic 
measurements of Hawaiian rocks, and with seis- 
mic velocities obtained by Raitt (1956), Shor 
and Pollard ( 1964), and the Western Geophysi- 
cal Company (unpublished). 
Current refraction work by the U. S. Geo- 
logical Survey on the island of Hawaii sub- 
stantiates the results of Eaton (1962) with re- 
spect to velocity and depth (D. Plill, personal 
communication). The Koolau caldera is some- 
what unusual compared with those on the island 
of Hawaii because the gravity and magnetic 
anomalies are concentric to the geologic expres- 
sion of the caldera. On the island of Hawaii 
the gradients are lower, indicating a greater 
depth to the plug, and the anomalies are 
usually shifted southward, usually in a greater 
amount for the older calderas. Only at Kilauea 
do the anomalies appear to be nearly concen- 
tric with the surface caldera. 
ACKNOWLEDGMENTS 
This work was possible only because of the 
persevering efforts of the following workers in 
the field: William Ichinose, Kenneth Hiraki, 
Lafayette Maynard, Wayne Lu, David Schla- 
bach, Loren Kroenke, Monroe Woollard, and 
Noel Thompson. Interpretation was facilitated 
by discussions with George P. Woollard. Finan- 
cial support was provided, in part, by NSF 
Grants GP-2257 and GP-3473. 
REFERENCES 
Eaton, J. P. 1962. Crustal structure and vol- 
canism in Hawaii. In: The crust of the Pa- 
cific Basin. Am. Geoph Union Geoph. 
Monog. 6, pp. 13-29. 
Nettleton, L. L. 1940. Geophysical Prospect- 
ing for Oil. McGraw-Hill Book Co., New 
York. P. 272. 
Raitt, R. W. 1956. Seismic refraction studies 
of the Pacific Ocean Basin, Part I. Crustal 
thickness of the Central Equatorial Pacific. 
Bull. Geol. Soc. Am. 67: 1623-1640. 
Shor, G. G., and D. D. Pollard. 1964. Mo- 
hole site selection studies north of Maui. J. 
Geoph. Res. 69: 1627-1637. 
Woollard, G. P. 1951. A gravity reconnais- 
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Geoph. Un. 32:358-368. 
WORZEL, J. L., and M. Ewing. 1948. Explosive 
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