Ultramafic Mass in New Zealand — Malahoff 
of the meter are negligible. The error in the 
Bouguer anomaly at any gravity station is not 
likely to exceed 1 mgal and is probably about 
0.5 mgal. 
Rock Densities 
The density and magnetic susceptibility of 
representative rocks of the Tophouse district 
is given in Table 5. 
In the Tophouse district, Maitai and Te 
Anau rocks are faulted against rocks of the 
Alpine facies whose measured densities range 
from 2.67 to 2.74 gm/cc and average 2.7 
gm/cc. For the purposes of computation, the 
standard rock density for the Tophouse district 
is assumed to be 2.7 gm/cc, and differences 
from 2.7 are measured as the density contrast. 
Density differences between the standard rock 
and the overlying Maitai and Te Anau rocks 
are inferred to have produced the variations in 
the Bouguer anomalies (Fig. 3). 
As the result of compaction and cementation, 
the porosities of most Maitai and Te Anau 
rocks are small. Only serpentinite and some 
tuffaceous rocks in the Maitai series have porosi- 
ties greater than 5%. Fresh samples of Maitai 
and Te Anau rocks and of Alpine greywacke 
have porosities less than 2% and therefore 
particle densities have been adopted as the pre- 
141 
ferred density. Table 6 gives the particle den- 
sities of the Tophouse rocks and the density 
contrasts between these rocks and the local 
standard density of 2.7 gm/cc. 
Gravity Anomalies of the Tophouse District 
1. the bouguer anomalies: Dominant 
features of the Bouguer anomaly map (Fig. 3) 
are the northeast lineation of the anomaly con- 
tours and the decrease in gravity values south- 
east toward the regional low of the northern 
end of South Island. The uniform gravity trend 
on the southeast side of the Wairau Fault is a 
reflection of deep-seated variations in the thick- 
ness or composition of Alpine greywacke. 
Northwest of the Wairau Fault, the Brook 
Street high and the ultramafic high are super- 
imposed upon the uniform trend. The anomalies 
are due to the fact that the rocks representing 
the Brook Street volcanics and the Wairau ul- 
tramafic mass are denser than the average base- 
ment rock. 
2. THE REGIONAL BOUGUER ANOMALIES: 
These were used to remove the regional field 
from the Bouguer field and were plotted from 
values of gravity stations located on basement 
rock with a density of 2.67-2.7 gm/cc for the 
northern part of South Island (Fig. 3). 
3. the residual anomalies: Local grav- 
Fig. 3. Bouguer gravity anomaly pattern for the Tophouse district. Bold lines indicate regional Bouguer 
gravity trend. Contour interval at 5 mgal. 
