Geology of the Southern Waitakere Hills Region West of Auckland City. By E. J. Searle, Auckland University College. [Read before Auckland Institute, September 15, 1943; received by the Editor, September 27, 1943; issued separately, June, 1944.] Abstract.—A short account is given of the geology of a region west of Auckland composed mainly of the Tertiary volcanic Manukau Breccias of the southern half of Waitakere Range. These are underlain conformably on their east by sediments of the Tertiary Waitemata Series which are covered locally by Pleistocene clays. The writer deals also with the petrography and origin of the Manukau Breccias and of clastic dykes that they include. Introduction. This outline of the geology of the southern portion of the Waitakere Hills, west of Auckland City, is based on reconnaissance work carried out 10 years ago under the guidance of Professor J. A. Bartrum and submitted as a thesis for the Master's degree. No survey of the geology of the whole area had previously been attempted, although from the earliest days of New Zealand geology parts of the region, notably between Onehunga and Manukau North Head, have received attention, particularly from Hochstetter (1864) and Hutton (1870). Other writers on limited aspects of the geology have been Smith (1881), Cox (1881, 1884), Park (1886, 1889), Fox (1902), Mulgan (1902) and Bartrum (1923, 1926, 1937). General Description of Area. The Waitakere Range, which constitutes most of the region described, is a plateau composed essentially of fragmental volcanic rocks, which has a general elevation of about 1100 feet, but is surmounted by occasional peaks of which Te Torokawharu (1506 feet), north-west of Huia Bay, Manukau Harbour, is the highest according to Mr. A. D. Mead, Waterworks Engineer, Auckland City Council* Personal communication.. Vigorous short streams have carved characteristic precipice-walled deep gorges, causing very strong relief, and have very steep courses, often with falls as much as 80 feet in height. A canyon in the lower course of Piha Stream on the west coast is a remarkable cleft, maintaining a width of only 11 feet for a height of 200 feet above the stream bed before widening. The larger streams wander on wide alluvial flats between steep valley walls near their mouths; this clearly is a result of the recent submergence that is so well exemplified throughout most parts of Auckland Province. Dykes and inextensive thin flows vary the general fragmental material, but rarely affect the longitudinal profiles of the streams; thick pillow lavas furnish an exception, however, and give rise to falls at the head of the lake impounded by Nihotupu Dam. The east face of Waitakere Range descends very abruptly to lowlands along a line running approximately north from Puponga Point, a tongue-like promontory on the north coast of Manukau Harbour; this scarp appears not to be due to faulting but to sudden change in the resistance to erosion of the rocks concerned, for east