Ultramafic Mass in New Zealand — Malahoff 
lar to the residual gravity anomaly contours and 
are shown in Figure 4. 
Profile A-B (Fig. .5): Alpine Greywacke- 
Walrau Fault-Wairau Ultramafic Mass. In 
order to eliminate local irregularities, the aver- 
age gravity values over a 2-mile-wide belt were 
taken normal to the anomaly contours as shown 
by profile A-B. Two of the observed values, 
marked by question marks in Figure 5, fall well 
below the others and have been neglected. A 
model with a theoretical curve which fits the 
observed curve to within 0.1 mgal is shaded. 
This model represents a simplified geological 
cross section across the Wairau ultramafic mass. 
A northeast dip of 67° for the Wairau Fault 
provides the best gravimetric solution and is 
probably correct to within 15°. 
Profile C-D (Fig. 6) : Brook Street V olcanics 
-Waimea Fault-Wairau Fault. Gravel and mo- 
raine obscure the contact between the Brook 
Street volcanics and the Rotoroa igneous com- 
plex. The western contact of the volcanics is 
at the Waimea Fault and gravimetric studies 
147 
show that the axis of maximum thickness of the 
volcanics lies 3 miles west of the fault outcrop. 
The calculated gravity profile for the geologi- 
cal model (shaded) fits the observed curve to 
within 0.5 mgal along the margins of the curve. 
The peak of the calculated profile falls below 
the peak of the observed profile by 3 mgal. 
The differences can be resolved only by assum- 
ing that the density of the Brook Street vol- 
canics is not uniform throughout and it is 
likely that denser rocks underlie the surface 
rocks. 
Profile E-F (Fig. 7): Rotoroa Igneous Com- 
plex-Brook Street V olcanics-Maitai Series— 
Wairau Ultramafic Mass. The profile is per- 
pendicular to the regional strike of the rocks 
and extends east from the Brook Street volcanics 
to the Maitai series and then northeast to the 
Wairau ultramafic mass. Observed values are 
irregular over the Wairau ultramafic mass and 
values have been averaged over a width of 2 
miles along the profile. 
The calculated gravity curve for the assumed 
