7

C—2c

The Whangarei Formation, which includes the coal-measures of North Auckland, is found only in the vicinity of Pahi, where greensands containing fossils of Waiarekan aspect unconformably overlie the argillaceous limestone. The massive sandstones and fine and coarse grit beds, which make up the bulk of the sandstone highland area of Pakiri and Tauhoa survey districts, were called the Tamahua (Tamahunga) greensands by Cox and Park. This series contains a fossiliferous grit at its base on the east coast near Cape Rodney, and pockets or lenses of basaltic agglomerate at various higher horizons on the Kaipara side. Coaly partings are not uncommon, but coal-seams of economic value are wanting. The beds at Pakaurangi Point an; highly fossiliferous, and probably belong to the Hutchinsonian stage of the Oamaruian System. Above the Pakaurangi sandstones are thick beds of basaltic and pumicoous tuffs, which pass upward into fresh-water sandstones, containing lenses of coal of no commercial value and lignitized and silicified tree-trunks. These beds are probably of Pliocene age, and are well developed on the west side of Hukatere Peninsula, where they are partly covered by sheets of lava. Pleistocene deposits are represented by a few gravel terraces and by the consolidated sand-dunes of Pakiri on the east coast and of the Kaipara Heads on the west coast. The igneous rocks of the district fall into three categories : (1.) Sheared plutonic rocks are found as pebbles at various horizons in the Miocene sandstones and grits, but such rocks have not been found in situ. (2.) Gabbroid masses, largely altered to serpentine, occur fairly frequently. Those masses appear as gently rounded domes, usually surrounded by argillaceous limestone, of the Onerahi Series, but no evidence is obtainable in the vicinity as to whether the original gabbroid rock underlies or intrudes this series. (3.) Volcanic breccias containing fragments of Tertiary sedimentary rocks occur in Otamatea Survey District. The vulcanism which gave rise to these breccias also gave rise to the tuffs and ash-beds of Hukatere Peninsula, and at a later stage to basaltic lavas. The breccias may perhaps be correlated with the agglomerates of the Whangarei Heads district, and the tuffs and lavaflows with the Kerikeri Series of the Whangarei - Bay of Islands Subdivision. Economic Geology. The geological survey of North Auckland embraces a stock-taking of its natural resources with a view to develop such as are of economic value. Traces of metals such as copper have long been known to exist in the district under review, and money has been wasted in endeavours to discover workable deposits. The copper occurs in the native state as segregations 1 in. or more in length locally disseminated in the Otamatea beds. Traces of coal in the form of coaly partings in the younger or Cape Rodney sandstones have led to the belief that payable coal exists in the district. The occurrences, however, whether of coal or of copper, are of no economic importance. Vast deposits of limestone suitable for cement-making, and in a less degree for agricultural purposes, exist in the district. They are worked at two points only—namely, at Warkworth and at Mark's Bluff, near Pahi. At Warkworth, Wilson's (N.Z.) Portland Cement Company has its works near the spot from which " building-lime " was shipped to Auckland as long ago as 1849 ; at Mark's Bluff the Northern Wairoa Lime Company is pulverizing the limestone for agricultural use, although the lime content is not as high as that of portions of the same formation which occur in just as accessible a position on the eastern side of Otamatea estuary. Outcrops of rock suitable for macadamizing roads, means of access to them, and their position relative to main roads, have been studied. Four classes of rock are noteworthy —namely, the greywackes (which occur abundantly in some localities), the occasional masses of serpentinized gabbro, the widespread volcanic agglomerates, and the gritty sandstones. The gritty sandstones are the poorest, but, since tfiey ocour in quantity in localities where better material is wanting, their free application to newly graded roads may be advisable. Other natural products have little value at present. No freestone occurs in the district, but the younger sandstones occasionally display structures which allow them to be used as building-stones. Clays for making earthenware utensils, and sands suitable for glass-making, occur in places. Kauriresin deposits, which arc so abundant farther north, are no longer an economic factor in the present area. The soils of the district belong mainly to five categories, which accord with the rocks from which they are derived. The table on page 6 gives the names, and to some extent describes the various rock formations which have been geologically mapped. Of these the Otamatea sandstones form rough inferior country which is prone to slip, and is therefore largely neglected. The Onerahi claystones and limestones give rise to good pasture-lands. These were the lands taken up by the early settlers, and they are now the best in the district. The Cape Rodney sandstones cover rough hilly country. They form, however, a fertile soil, which supports rich pastures. The soils covering the Pakaurangi sandstones and tuffs are poor, and at present are largely covered by manuka scrub. The alluvial soils, which cover small areas only, are in greatest demand. Since the productivity of the soil and its textural and chemical constitution depend so largely upon the underlying rock formations, a geological map of the district will be of great value in connection with the soil survey that is mooted. 3. Kaitangata Subdivision. (By M. Ongley.) During tin; period from 13th October, 1923, to 20th March, 1924, the Kaitangata, Taratu, Waronui, and near-by coalfields and adjoining country were examined by the writer, who was assisted by Mr. R. C. Penseler, B.Sc., of Otago University. Altogether some 543 square miles of country, extending from the Taieri River to Romahapa, and from the coast thirty-three miles inland, were examined and mapped, and about a fifth of this was found to be possibly coal-bearing. The; adjoining Tuapeka district was examined in 1915 by Dr. P. Marshall, and his report, "The Geology of the Tuapeka District," published in 1918 as Geological Survey Bulletin No. 19, has facilitated the work in the Kaitangata Subdivision.