These rocks have also been found by Mr. E. O. Macpherson and the writer, in the Red Hills and in the south-west branch of the Wairoa River (Gordon Survey District) lying to the south-west of the Dun Mountain, and in the upper Takaka Valley, West Nelson, during the survey of the Motueka Subdivision. Among the erratic boulders collected by Mr. H. Hamilton, of the Dominion Museum, on Macquarie Island, during the Australasian Antarctic Expedition, in 1913, are gabbros containing prehnite.

A complex sill of basic and ultrabasic rock intrudes the Maitai (Permain) and Te Anau (Devonian?) argillites greywackes, and con-glomerates in the Dun Mountain Subdivision (Geological Survey Bulletin, No. 12), in the Wairoa River, and in the Red Hills at the head of the branch of the Motueka River, flowing south from Mount Rufus. Northward, they have been traced as far as D'Urville Island. These localities are on the northern end of the main moun-tain-axis of the South Island.

At Dun Mountain, and in the Red Hills, there is a central area of dunite and a border of serpentinized harzburgites, wehrlites, pyroxenites, and dunites traversed by numerous dykes and veins of altered gabbro and diorite, and veins of the grossularite diopside rock. In the upper Takaka Valley there is an area of about six square miles formed of bastite serpentine and a little serpentinized dunite, with a wide border of gabbro and diorite. The central area is traversed by a granite dyke and a few thin dykes of garnetised gabbro.

The prehnite rodingite of Marshall is made up of pyroxene and an aggregate in which prehnite and zoisite can be distinguished. It occurs in fine-and coarse-grained gabbros. The change from a slightly saussuritized gabbro to one in which the feldspar is entirely replaced by the prehnite-zoisite aggregate can be clearly traced under the microscope. The material here called saussurite is a dense dirty-brown irresolvable mixture. Ordinary saussurite is an aggregate consisting largely of zoisite, with more or less albite, and occasionally garnet, together with tremolite, chlorite, and rarely scapolite, etc.

The least altered gabbros are, as a rule, fine-grained. One (SpeC. No. 134) from the head of the branch of the Motueka, flowing south from Mount Rufus, contains colourless augite, feldspar, serpentine, and ilmenite. The feldspar (oligoclase) is fractured and bent, and the ilmenite is partly altered to leucoxene. Some of the plagioclase in a gabbro (Spec. 104) from the Wairoa River, six miles above the junction with the south-east branch, is completely altered to “saus-surite.” A specimen (No. 283, No. P. 1574 of table of analyses) from a branch of the Motueka River, three miles west of Ellis Trig. station has the augite altered to greenish-brown hornblende of irregular outline. The feldspar is fresh, and titanite or sphene occurs as an accessory mineral.

A more altered gabbro (Spec. No. 32) comes from a small branch of the Motueka River four miles west-north-west of Red Hill Trig. Station. The plagioclase is altered to “saussurite” with patches of the prehnite aggregate. The augite has changed to green hornblende and ilmenite is partly replaced by leucoxene. In a further stage the prehnite-zoisite aggregate entirely replaces the feldspar. This is

well seen in section D 13a from a boulder in the Roding River, where the outline of the feldspar crystals is still visible. The other minerals are diallage and enstatite, now altered in part to bastite. In sections 123 and 128 from the Wairoa River, two miles and a-half north-north-west of Mount Ellis, the feldspar cleavage can be made out in the fine-grained aggregate. The ferromagnesian minerals are inter-locking diallage and brown hornblende. No. Q 2071 is an analysis of the aggregate from the Wairoa River (Spec. No. 106). It agrees closely in composition (Q 3310a) with that in the altered gabbro from Macquarie Island. Here the ferromagnesian minerals are diallage and hornblende as in section 123. Q 3310 is an analysis of the rock. A gabbro (Spec. 12) from the Motueka Valley, one mile and a quarter south-south-east of Mount Glennie, of which No. Q 3308 is an analysis consists of diallage altered to tremolite and antigorite, the remainder being an aggregate in which prehnite can be distinguished. The aggregate shows traces of the feldspar cleavage.

In the rock (Spec. D 20) from the track up Hacket Creek, one mile below Goat Creek junction (Aniseed Valley) zoisite and prehnite can be distinguished. The diallage is serpentinized in part. Section 113 from the Wairoa River contains diallage with the cleavage planes twisted and in part altered to tremolite and an aggregate of prehnite and zoisite. In other sections, for example those of the rock from the track two chains below the Champion Mine (Aniseed Valley), only zoisite can be recognised as an alteration product of the feldspar.

The garnetized gabbros vary widely in texture, the pyroxene crystals ranging in length from a fraction of a cm. to about 8 cms. The coarse rocks, which are probably veins, are much more common. The grossularite has a light green colour, and under the microscope is either colourless or light brown.

Direct evidence that the grossularite forms from feldspar is want-ing. There is, however, the fact that in a hand specimen (No. 106) of the coarse rock from the Wairoa River the prehnite aggregate is associated with garnet (S. G. 3.438). But there is good evidence in several of the sections that the grossularite is secondary. In a band 2 inches wide in the serpentine on the west side of the branch of the Motueka River, flowing south from Mount Rufus, the grossularite enters the diallage occasionally as narrow veins. Sections of thin dykes of altered gabbro at the head of the Wairoa River, east of Mount Glennie (Spec. 129), and also from lower downstream, show in several parts the garnet eating into and isolating small areas of the diallage. Q 3306 is an analysis of a grossularite-diallage rock outcropping near the upper Motueka River, one mile and a half south of Mount Harvey (Gordon Survey District), in which the diallage is the more abundant material. The rock from Coad's Point (D 1A, fig. 1) on the Dun Mountain tramline shows the grossularite replacing and veining the diallage. The garnet this specimen contains crystals of zoisite and diopside. Grossularite veins are seen in section 259 (Fig. 2) from the Asbestos Claim, upper Takaka River. The pyroxene of D 5 from Champion Creek is now altered to chlorite and serpentine, and garnet has penetrated along the cracks. As specimen from the same locality as D 1A shows roundish areas of serpentine, probably formed from pyroxene, a colourless

cavernous mineral with vitreous lustre and a light brown mineral. Under the microscope the colourless mineral is seen to be zoisite and the brown to be grossularite, crowded in parts with long thin needles of diopside. Veins of grossularite cut across the serpentine.

Radical chemical changes are involved in the alterations described above. To produce grossularite and prehnite from the anorthite of feldspar, lime and silica, and, in the case of the prehnite, aggregate water must be added if the alumina is to be kept constant. Accord-ing to Graham (1917, p. 166) when diallage is serpentinized the following reaction takes place:—3Ca Mg Si₂ O₆+2H₂ O-> H₄ Mg₃ Si₂ O₉+3 Ca SiO₃+SiO_2.

Thus it appears that the alteration of diallage will supply the lime and silica needed for the conversion of feldspar to garnet and prehnite. During this change the albite in the feldspar will be set free. Secondary albite crystals 0.5 cm. in length line a vug in gabbro close to Hacket Creek, one mile below the point where it is joined by Goat Creek. Alongside is a vein, 3 feet wide, of secondary albite. In other cases the albite is altered to pectolite (see Weinschenk, 1912, p. 300). This mineral (analysis No. P 1599) has been found in small veins cutting altered gabbro in the Wairoa River.

Murgoci and Benson have described gabbros now containing prehnite and grossularite. Benson (1918, p. 722) summarises thus the former's work (1900): “Murgoci found included in the Paringu (Roumania) serpentine, masses of diopside, diallage, with grossu-larite, vesuvianite, fassaite, clinozoisite, lotrite (a form of prehnite), clinochlore, apatite, ilmenite, rutile and sphene. He concluded that the more coarsely granulitic masses, with an appearance like that of saussurite-gabbro, were indeed an altered from of gabbro, but that some hornstones of similar mineral composition were altered inclu-sions of chloritic calc-schist.” Benson (1914, p. 682) considered that the prehnite and grossularite of the gabbros of the Great Serpentine Belt of New South Wales were secondary after feldspar. Having examined some of Dr. Marshall's material from the Dun Mountain Subdivision he concluded (1914, p. 687) that there also these minerals were secondary, but could not “suggest how they had become so altered.”

The dense white rock, previously mentioned (fine-grained “rod-ingite” of Marshall), occurs as veins or lenticular masses, and is often much slickensided. It is made up of grossularite and needles and tabular crystals of diopside. The rock from the saddle between the Maitai and Roding Rivers, and at Champion Mine, Anisced Valley, contains also irregular areas of a mineral with a sulphur-yellow colour which under the microscope are seen to be vesuvianite. Two “boulders” of serpentine in the Asbestos Claim of Upper Takaka Valley, show the manner in which this garnet-bearing rock has been formed. The pyroxenites were divided into cuboidal masses, along which solutions penetrated and gradually serpentinized the masses to the centre. They now appear as rounded “boulders,” much like those left by spheroidal weathering. The cores of two boulders are formed of irregular masses of a white rock. Thin sections of the white rock taken from the contact with the serpentine show serpentinized diallage cut by veins of grossularite containing diopside. Evidently

in penetrating to the centre the solutions have gathered lime, mag-nesia and a little alumina from the pyroxenes to form these secondary minerals high in calcium. A somewhat similar origin has been ascribed by Graham (1917, pp. 174–76) to grossularite, diopside, and vesuvianite veins in serpentine in Quebec.

The writer is indebted to the Director of the Geological Survey for suggestions and facilities in the preparation of this paper. Dr. Benson kindly lent the writer his slides of the altered gabbros of the Great Serpentine Belt of New South Wales, and examined many of the slides that have been described above. He agreed in the main with the writer's interpretations, and pointed out some of the errors he had made. Through the courtesy of Professor Easterfield, of the Cawthron Institute, the writer was able to look at the sections made by the late Mr. W. F. Worley. The excellent microphoto-graphs reproduced in Figs. 1 and 2 were made by Mr. W. C. Davies, of the Cawthron Institute.

P 1574. Gabbro, branch of Motueka River, three miles west of Ellis Trig. Station, Gordon Survey District. Analyst, F. T. Seelye, Dominion Laboratory.

Q 2071. Prehnite-zoisite aggregate in altered gabbro, south-west branch of Wairoa River, Gordon Survey District. Analyst, F. T. Seelye.

Q 2069. Diallage in gabbro, south-west branch of Wairoa River. Analyst, F. T. Seelye.

Q 3308. Gabbro with prehnite-zoisite aggregate, Motueka Valley, one mile and a quarter south-south-east of Mount Glennie, Gordon Survey District. Analyst F. T. Seelye.

Q 3310a. Prehnite-zoisite aggregate in altered gabbro, Macquarie. Island. Analyst, F. T. Seelye.

Q 3310b. Diallage in altered gabbro, Macquarie Island. Analyst, F. T. Seelye.

Q 3311. Gabbro with prehnite-zoisite aggregate, Macquarie Island. Analyst, F. T. Seelye.

Q 3306. Garnetised gabbro, one mile and a half south of Mount Harvey, Upper Motueka Valley, Gordon Survey Dis-trict. Analyst, F. T. Seelye.

2209. Grossularite, Roding River, Dun Mountain Subdivision (N. Z. Geol. Surv. Bull. No. 12, p. 32, 1911). Analysed in Dominion Laboratory.

2209 and 2212. “Rodingite” calculated from ⅔ grossularite (2209) and ⅓ diallage (2212) from Long Gully, Lee River, Dun Mountain Subdivision (N. Z. Geol. Surv. Bull. No. 12, p. 32, 1911).

P 1572. Dense white grossularite-diopside rock, half a mile south-south-east of Cairn at 2602 feet, 1 ½ m. south of Mount Glennie, Motueka River Valley, Gordon Survey District. Analyst, F. T. Seelye.

Q 3307. Dense white grossularite-diopside rock, near head of Motueka River, Gordon Survey District. Analyst F. T. Seelye.

P 1599. Pectolite, mouth of branch of Wairoa River, 130 chains north-north-east of Mount Glennie. Gordon Survey District. Analyst, F. T. Seelye.