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PACIFIC SCIENCE, Vol. IX, July, 1955 
thought to have built the shield of the Koolau 
Range. In the third episode a number of pyro- 
clastic cones and craters, such as Diamond 
Head, and several intra-valley flows were 
erupted, constituting the Honolulu Volcanic 
Series of Stearns. 
Fig. 12. Envelopes of the 500- 1,000- 1,500- 2,000- 
and 2,500-foot contour lines on the lee side of the 
Koolau Range. The lines A — A 1 , B — B 1 and C — C 1 
show the locations of the profiles of Figure 11. Small 
circles give the locations of Wahiawa, Aiea, and the 
view in Figure 18. 
One unexplained peculiarity of the last epi- 
sode is that it seemed to be restricted to the 
part of the Koolau Range southeast of a line 
from Pearl Harbor to Kaneohe. (Several 
young structures lie on the south end of the 
Waianae Range, and may be of about the 
same age.) 
Was anything happening northwest of that 
line? 
The hypothesis here offered is that there 
was a large amount of eruptive action north- 
west of the line, more or less at the time that 
Diamond Head (Fig. 13) and the rest were 
active. At least this was after the Koolaus 
had been considerably eroded. This activity 
involved the quiet effusion of a large amount 
of very fluid lava, which flowed out to form 
very gently sloping surfaces. This hypothesis 
would account for the extensive, rather 
smooth, lava-built surface known to many as 
the Wahiawa Plateau. It would also explain 
the profiles of the ridge crests in the middle 
and northwestern parts of the Koolau Range. 
The vents that supplied these lavas would 
have been located more or less parallel to the 
Koolau Crestline, but down the leeward side 
a way. If their altitudes were fairly low, like 
the altitudes of many of the vents of the 
Honolulu Volcanic Series, they would readily 
build a plateau that would overlap the lower 
slopes of the northwest end of the main 
Koolau Range, as it then was, filling in the 
valleys and partly burying ridges. The lavas, 
of course, also overlapped the lower slopes 
of the Waianae Range. 
The southwest rift of Mauna Loa, on the 
island of Hawaii, is marked by cones, cracks, 
and fissures from which great volumes of lava 
Fig. 13. Index map for Figures 14 and 16, the 
maps of lateral lava shields. 
have been poured out to mantle the slopes 
of the mountain. The volume of lavas far 
exceeds the volume of pyroclastic materials 
erupted from the rift. At the south end, this 
rift on Mauna Loa is marked by a great fault 
at Ka Lae, with the shoreline of the down- 
thrown, western side offset about two miles 
inland as compared to the upthrown, eastern 
side. 
A rather similar fault offsets the shore line 
of Oahu about two-thirds of a mile at Waimea 
Bay (Fig. 12), at the northwest end of the 
Koolau Range. The similarity of the Waimea 
Fault to the Ka Lae Fault suggests that a rift 
extends eastward from Waimea Bay, and that 
it may well have determined the location of 
