Mugearites in the Dunedin District. By W. N. Benson By F. J. Turner [Read before Otago Branch, June 11, 1940; received by the Editor, June 18, 1940; issued separately, December, 1940.] 1. Field Occurrence and Petrology. By W. N. Benson Field Relations. In the course of a detailed geological survey of the Dunedin District made by one of us (W. N. B.) a small group of rocks of basaltic appearance attracted attention. They appeared different both texturally and mineralogically from the normal olivine-basalts with which they are associated, notably in the more sodic character of the plagioclase and dominance of olivine over augite and they possessed a peculiar mode of occurrence, partly intrusive, partly extrusive. They were known for some years before specimens were obtained sufficiently free from secondary minerals (notably carbonates) to yield a trustworthy analysis. Such, however, were collected both by Mr. M. Ongley for the Geological Survey (P. 5569)* Rocks denoted by a number following the letter P. were collected by Ongley for the Geological Survey, and were made available for our study by the courtesy of the Director of the Survey. Other rocks noted are in the collection of Otago University Department of Geology. and by Benson (246). An analysis of the latter made by Mr. F. T. Seelye (through the courtesy of the Directors of the Geological Survey and of the Dominion Laboratory), revealed the close similarity between its composition and that of the rock of Druim na Crìche near Mugeary, Skye, the type for the rock-species, mugearite, instituted by the late Dr. Harker (1904), and suggested the desirability for a further study here presented. The mugearites of the Dunedin district appear to be confined to an area of three square miles adjacent to the coast and about eight miles west-south-west of Dunedin, that is, in or on the ridge separating the Taieri Plain from the coast (see Figure 1). The total exposed surface of this rock is less than a quarter of a square mile, barely a fifth of one per cent. of the total area of volcanic rocks in the Dunedin district. The ridge in question is anticlinal, with a core of quartz-albite-sericite-chlorite-schist and a partial cover of sediments of Middle to Upper Cretaceous age according to current views (Ongley, 1939). They comprise a small extent of feldspathic sandstone and conglomerate resting on the planated surface of the schist on the northern flank of the anticline, overlapped near its crest by a