C—9

20

and east-north-east direction, and which is broken through by the lower part of the Waitara River. The crests of the ridges show a gradually increasing height as the country is traversed from west to east. The highest points in the subdivision are the trigonometrical stations of Tuahu (1,075 ft.). Taramouku (1,055 ft.), and Urenui (1,030 ft.). These elevations, however, are only slightly higher than many other points on the ridges to which they belong. All but the very smallest streams flow at grade almost to their source, and in the eastern part of the subdivision, even where, near their head-waters, they enter narrow little gorges with precipitous walls cut in the soft " papa " (claystone, &c), their course is still excessively sinuous, and but rarely interrupted by even the smallest waterfalls. In the Waitara Survey District the sea-beach of black sand is generally bounded by vertical and in places overhanging cliffs, usually less than 100 ft. in height, though they show a slight increase in altitude to the north. The cliffs are interrupted by the alluvial plains at the mouths of the larger streams such as the Mimi, Urenui, Waitara, and Onairo. The smaller streams, which enter the sea directly, do so, as a rule, over waterfalls. Outlying reefs and islets are markedly absent from the coast of the Waitara Survey District, but in a few places portions of the cliffs have become almost or quite separated from the mainland by narrow rifts fretted out by the waves. A small lake not more than 20 ft. deep occurs in the south-western corner of the Waitara Survey District. It occupies what is probably an area of subsidence. Other subsided areafe show the remains of similar ponds now converted into raupo swamps. The present topography of the New Plymouth Subdivision may be understood when it is recognized that at the close of the period during which the great series of sediments generally regarded as of Miocene age accumulated, there ensued elevation which raised the sediments 1,000 ft. or more above sea-level. After a considerable period of dissection, the new land-surface was buried, except perhaps in the extreme east of the subdivision, under debris from the great volcano, or group of volcanoes, that for convenience will be referred to in this report as the Taranaki Volcano. Of this volcano the present Mount Egmont is but a remnant. The formation of the Taranaki Volcano profoundly influenced the drainage of the country. Before the period of volcanic activity the rivers flowed in the main from east to west. The great mass of volcanic material formed a barrier diverting the westerly flowing streams to the north and the south, and produced also a number of new streams that flowed eastward off the flanks of the mountain. A glance at the map will show that such rivers as the Waitara in the north, and probably the Wanganui in the south, have been deflected in the manner just described. The effusion of volcanic debris was either preceded or followed by tin appreciable subsidence, though following this event there has probably been some elevation. The coast-line of the Waitara Survey District shows clearly the effects of this last elevation, followed by marine denudation. The uniform character of the rocks, which therefore entails a nearly uniform rate of erosion, explains the almost total absence of outlying reefs and islets already mentioned. Summary of Geology.—The oldest rocks in the area under consideration are a. succession of claystones and sandy claystones, with less frequent sandstones and conglomerates. These rocks, from paleeontological evidence, appear to be Miocene in age, and will be spoken of as the Miocene Rocks in this report. Lying in marked unconformity on the Miocene Rocks are rudely stratified layers of basic or semibasic volcanic debris. These are probably to be assigned to the earlier phases of activity of the Taranaki Volcano. Near the sea-coast a small development of marine sands overlies, apparently unconformably, the Miocene Rocks, and may be said to be Pleistocene in age, in order to distinguish it from the recent alluvial and marine accumulations of sand, mud, and gravel that occupy the sea-coast and the floodplains of the streams. The writer has not been able to recognize the unconformity near the mouth of the Onairo Stream that according to Hector marks the division of the Miocene rocks mentioned above into older and newer Tertiary. Miocene Rocks. —Distribution. —Almost throughout the area under consideration the Miocene Rocks are covered by volcanic debris, but over about eighteen square miles in the south-east corner of the area this covering is so thin as to be negligible from a geological standpoint. In general it may be said that the Miocene Rocks are found outcropping in almost every gully as far west as the Waitara River. On the sea-coast the Miocene beds are not found west of a spot near a small stream about three-quarters of a mile east of trigonometrical station IX (Waihi), where they disappear beneath the Volcanic Debris. Petrology. —The Miocene Rocks are in the main soft calcareous claystones, frequently rather sandy, and rarely passing into rather incoherent sandstones. These sandstones are often abundantly micaceous, and occasionally contain small coaly partings. This latter feature is especially noticeable near the quarry on the Okoke Road not far from its junction with the Mokau Road. The claystones and sandstones are frequently fossiliferous. In places bands of conglomerate varying in thickness from a few inches up to ten or twelve feet are found. Like the claystones and sandstones, the conglomerates are often fossiliferous. The pebbles of which they are composed are in the main of quartz, quartzite, and jasperoid rock, but fragments of grauwacke, and possibly diorite, are also rather common. The pebbles vary in size from small fragments to boulders 6 in. in diameter. All are well rounded and water-worn. The material of which the conglomerates are composed is evidently derived from the old sedimentaries and intrusives lying to the east of the area under review, which make up the structural axis of the North Island, and which were usually referred to the Maitai Series of supposed Carboniferous age by the staff of the former Geological Survey.