a—9

24

(1.) Clay-shale ; Chalky Limestone ; Glauconitic Sandstone. —The north-western portion of the subdivision is composed mainly of the rocks belonging to this group of the series. Clay-shale occurs in the Mangatahu Valley and in the head-water valley of the Waitangirua, in both cases overlain by chalky limestone. It is a laminated rock, finely textured, dark grey in colour, containing about 4 per cent, of carbonate of lime (CaCO 3 ). Chalky limestone occurs in the valleys of the Mangatu River, Mangamaia, the Makerewau and the Mangapapa streams, and also forms spurs and ridges in the vicinity of these watercourses. It is exposed in the Mangatu Valley for a distance of seven miles below the northern boundary of the subdivision ; in Slip Stream and in the Waipaoa River ; in the valley of the Mangataikapua Stream ; and as an inlier in the valley of the Makahakaha Ngarara Stream near Hokoroa Homestead. The rock is of whitish appearance, often tinged pale green or brown by ferruginous minerals. Veinlets of calcite occur frequently, varying from an inch in thickness to mere threadlike streaks. Glauconitic sandstone occurs overlying the chalky limestone. The largest continuous outcrop covers an area of about thirteen square miles, extending from the middle course of the Mangatahu Stream south and south-west down the valleys of the Mangamaia and Makerewau streams. Glauconitic sandstone forms the Pulpit Rocks on the left or eastern bank of the Mangatu River opposite the mouth of the Mangamaia Stream. From here inliers of the same strata occur at intervals in a general westerly direction as far as Waitangi Hill. The glauconitic sandstone is a moderately well-compacted rock, varying in texture. Its colour is olive green on unweathered surfaces and dun-coloured on weathered surfaces. At the Pulpit Rocks it contains veinlets of calcite, small druses of rock crystal, and chalcedony. (2.) Claystone and Argillaceous Sandstone, with Calcareous Concretions. —Claystone and argillaceous sandstone, with calcareous concretions, occur mainly in the valleys of the Waingaromia, the Waihora, and the Waikohu rivers. In general, the rocks show no stratification, and their relative position in the series can only be judged approximately, since actual contacts with the first group of the series are indistinctly shown. The claystone and argillaceous sandstone are finely textured grey-black rocks, which frit and crumble on exposure. The concretions are of very hard, fine-grained, calcareous claystone, and occur either as spheroidal and tubular masses or as interrupted bands. These concretions are said to have yielded specimens of the fossil Inoceramus* ; none, however, was found during the present survey. (3.) Claystone and Argillaceous Sandstone ; Limestone ; Conglomerate ; Fossiliferous Argillaceous Sandstone ; Coarse Sandstone. —The claystone and argillaceous sandstone of this group are lithologically similar to those of tho last; the distinction made is purely a structural one. These formations occur either singly, showing stratification, or together, interbedded, with or without one or more of the following : finely textured sandstone bands, shelly conglomerate bands, concretionary bands, limestone bands. The concretionary bands referred to are highly calcareous sandstone, containing a variable quantity of fossil shells. The limestone bands occur but sparingly. They are hard crystalline rocks, containing grains of sand or of glauconite. The limestone of this group is that referred to by Mr. McKayf as the coralline limestone at the base of what he terms the lower Tertiary Series. It occurs on Sub-Trigonometrical Station A (Mangatu Survey District) ; on Trigonometrical Station I (Waikohu Survey District) ; and near the head of a right branch of the Tawa Stream. It is of a light-pink colour, is finely textured, very compact, and contains an abundance of fossil shells. It occurs overlying the claystone and argillaceous sandstone of this group of the Whatatutu Series, but nowhere in the subdivision has a rock formation directly overlying it been observed, so that its actual relative position is uncertain. Bands of conglomerate! consisting of pebbles and boulders cemented together by an arenaceous or an argillaceous medium were located in four different localities, in all cases interbedded with claystone and argillaceous sandstone. The largest outcrop is in Conglomerate Stream, a small right branch of the Waipaoa River entering opposite Waipaoa Homestead. Exposures in this stream show intermittently from a point on the ridge about 20 chains to the east of Wheturau Hill down almost to the mouth of the stream. The best exposure lies between the ridge by which the Mangatu-Waipaoa Stock Track crosses the stream, and the waterfall over which the stream flows before entering the river. Here occur, resting on the claystone and argillaceous sandstone, bands of extremely fine conglomerate, followed by bands containing boulders up to 2 ft. in diameter, with occasional interstratified bands of argillaceous sandstone. The total thickness of the coarse bands is over 20 ft. On the ridge large blocks of coarse material occur, while just below it are bands with pebbles averaging 6 in. in diameter. A second outcrop of this formation occurs in the Waikohu Valley on the boundary between Belmont and Waihuka Stations, a mile and a half (air-line) west of Poututu Accommodation-house. This outcrop consists mainly of very fine material, with, as far as was observed, only one or two bands containing inclusions as large as 6 in. A third outcrop with moderately coarse bands occurs in the Urukokomoku Stream, about four miles (air-line) above its mouth. A fourth outcrop occurs in the waterfall of Atarau Stream, a left branch of the Mangaorango. The material contained in these bands consists of water-worn pebbles and boulders of igneous origin, fragments of concretionary boulders, and occasional small pieces of quartz. The igneous boulders consist of diorite, gneiss, gabbro, &c, and, as no rocks of similar character have been described as occurring in situ anywhere in the North Island of New Zealand, their source is at present doubtful. An argillaceous sandstone containing an abundance of fossil shells covers a large area in the southwestern portion of the subdivision. Owing to the fact that it occurs for the moSt part in the troughs of synclines, it has been placed in the upper part of the Whatatutu Series.

♦Mines Report, 1901, C.-10, p. 23. " Material for macadamising Roads."

t Mines Report, 1901, C.-10, p. 24. J See reference under