is present both interstitially and in veins less than one inch in width. The prehnite sometimes displays a radiating spherulitic structure between crossed nicols (7889, 7857), and in this respect resembles the prehnite from the Baltic described by P. Eskola (1934, p. 134). Many rocks (7849, 7862, 7884) contain abont equal amounts of uralite and prehnite; in others (7856, 7878, 7883) feldspar laths 3 mm. in length are essential constituents (5–30 per cent.). Although refractive index measurements were hindered by the presence of saussurite inclusions several determinations indicate that the feldspar is now albite (7856, 7883, 7878). Sphene is a common accessory mineral and its presence points to an original ilmenitic iron ore. Locally abundant are interstitial clear quartz (7877) and pale green chlorite (penninite) with anomalous blue interference tint (7876). Shearing of the basic rocks has occurred along the margins of the intrusions (8005). (d) Petrogenesis of altered dolerites and gabbors. The occurrence of unfractured feldspar crystals and relicts of pyroxene indicate that basic intrusions were dolerites and gabbros which have now been converted into uralite-prehnite rocks. Earlier workers (Benson, 1918, 722–733, 1926, p. 43; Grange, 1927; Turner, 1930, p. 198, 1933, p. 271) have shown that this uralitization and prehnitization is probably brought about by the metasomatic effect of aqueous solutions emanating from the basic magma and as a result of the serpentinization of lime-bearing minerals in the original peridotite. (4) Tectonic Significance and Date of Intrusion The serpentinites and altered dolerites and gabbros considered in the paper form the southern part of an arcuate line of ultrabasic and basic intrusions extending from D'Urville Island parallel to the main strike of the Southern Alps and then swinging in a westerly are from the Olivine Range. Since it may reasonably be assumed that these ultrabasic intrusions belong to a single orogenic period (Finlayson, 1909, pp. 364, 365; Benson, 1926, p. 44; Turner. 1933, p. 276), “the structural significance of the magnesian belt is too evident to be overlooked in this connection “(Finlayson, 1909, pp. 365). It is pertinent here to note the important conclusions of H. H. Hess (1937, 1948) concerning peridotite injection. “Peridotite intrusions in mountain belts are an important tool because of certain peculiarities of their histories and loci of intrusion. Peridotites of the ultramafic magma suite occur in all alpine mountain systems and nowhere else. They occur in two belts about 50 miles on either side of the original location of the tectogene axis and less commonly in the area between the two belts. They are intruded during the first great deformation of the belt presumably during buckling of the crust, and later deformations of the same belt are not accompanied by intrusion of peridotites. Thus location of the peridotite belt and dating of its intrusion locate the old tectogene axis and date the initiation of the deformation of that zone” (Hess, 1948, p. 432). It will be seen from this statement how important is the dating of the South Island peridotite intrusions, and the evidence bearing on their age will now be considered. In the Hokitika district, serpentinite pebbles have been found in the upper part of the “Miocene” Blue Bottom beds (Bell and Fraser, 1906, p. 87; Morgan, 1908, p. 121), and since the Blue Bottom beds are now considered to be early Pliocene in age (Finaly and Marwick,