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PACIFIC SCIENCE, Vol. XXI, January 1967 
of the maximum thickness of the volcanics 
below sea level. The Brook Street volcanics are 
flanked to the west by the Rotoroa igneous com- 
plex and to the east by the Maitai series. South 
of Tophouse Hotel, however, the axis of the 
Brook Street high swings around to strike 
parallel to the Wairau Fault. The change in the 
axial trend of the gravity anomaly from south 
to southwest is attributed to the bending and 
dragging of the Brook Street volcanics by trans- 
current movement along the right lateral 
Wairau Fault. 
The ultramafic high reaches a maximum posi- 
tive value of 40 mgal. Along the margin, the 
gravity gradient is steep and it is inferred that 
the contacts surrounding the Red Hill ultra- 
mafic mass are also steep. Small gravity lows 
near the crest of the anomaly are attributed to 
shallow inclusions of serpentinite. The serpen- 
tinite along Maitland Fault is not reflected by 
a gravity low and is therefore considered to be 
underlain at a depth of a few hundred feet by 
peridotite. 
The small negative anomaly of 4 mgal at the 
eastern bay of Lake Rotoiti is inferred to be 
caused by about 600 ft of glacial outwash gravel 
and silt. Another small negative anomaly with 
a maximum negative value of about 7 mgal is 
situated along the Wairau Valley 1 mile east of 
the Tophouse saddle and is inferred to be 
caused by about 1,000 ft of alluvium. No nega- 
tive anomalies were observed over the Tophouse 
saddle, where the alluvium is about 300 ft 
thick. 
Structure of the Maitai and Te Anau Rocks 
1. gravity anomaly profiles: Two regu- 
lar geological features are represented by the 
gravity contours. One is the contact between the 
Rotoroa igneous complex and the Brook Street 
volcanics at depth, and the other is the contact 
between the peridotite of the Wairau ultramafic 
mass and Alpine greywacke at the Wairau Fault. 
These two structures have been studied in detail. 
The three gravity profiles are drawn perpendicu- 
