The Bouguer map of New Zealand exhibits two major negative gravity anomalies. The Rangitikei-Waiapu Anomaly, which intersects tne axial ranges of the North Island, indicates a crustal downwarp which is not in isostatic equilibrium. This anomaly is closely parallel to the zone of intense seismicity, to the Taupo-White Island volcanic belt, to the Kaimanawa-Huiarau-Raukumara Ranges, to the thick Upper Pliocene sediments, and to the Hikurangi Trench; and it appears to be an expression of an active tectonic belt related to the Tonga-Kermadec-Hikurangi Trenches. The Rimutaka-Ruahine axial ranges are apparently not underlain by mountain roots. The Alpine Anomaly in the South Island is considered to be the gravitational expression of the crustal roots of the Southern Alps and the mountains southwest of Blenheim; the magnitude of the anomaly suggests that this region is' approximately in isostatic equilibrium.

remained the basis for recording gravity results within New Zealand. Subsequent adjustment of the network, both to make it internally consistent and to bring it into agreement with the first order world network based on Potsdam, has resulted in a revised set of values of gravity, which constitute the New Zealand Potsdam System (1959), and which differ by an average of + 5'0 mgal from the New Zealand Provisional System values. Values of gravity and of free air and Bouguer anomalies are given for the 437 network stations. 1960) ROBERTSON AND REILLY -N.Z. GRAVITY NETWORK 4 " .J Ag = t (A,g + A2g)and a measure of the accuracy of the mean ! A 2g _. A1g I were obtained. Most of these measurements go to form networks of polygons, or closed circuits (figs. 3 and 4), whose sides include two or more stations.

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The variation of ground temperature with depth was measured between 1954 and 1961 at numerous locations in the Wairakei thermal area of New Zealand. Analysis of these data clearly shows that conduction is the dominant mechanism of heat transfer until the difference between the temperatures at the surface and at a depth of 1 metre reaches a value of about 25 deg c. For higher ground temperatures, heat transport by convection of a mixture of air and water vapour becomes progr,essively more important and the conductive effect becomes insignificant. These conclusions are used to present a simplified picture of the nature of the mechanisms in the transfer of heat to the surface in areas of steaming ground at Wairakei. The temperature data are also used to study the geothermal microclimate, to determine the thermal conductivity of the soil, and to derive relations between the temperatures at various

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