The Geology of Dunedin. 191 



this may be due to the decomposition in situ of the alluvium, but 

 in the neighbourhood of the iron -mountains a sort of passage may 

 be seen between a conglomerate of fresh iron-ores and the more 

 general type of limonitic conglomerate (laterite ?). 



May 10th.— R. S. Hemes, M.A., Vice-President, 

 in the Chair. 



The following communications were read: — 



1. 'The Geology of Dunedin (New Zealand).' By Patrick 

 Marshall, M.A., D.Sc, F.G.S. 



The paper opens with an account of the physiography of Otago 

 Peninsula and the adjacent mainland, in which the origin of the 

 land-forms is briefly discussed. The author then passes on to a 

 detailed account of the petrography of the district. The age of the 

 oldest rocks seen, mica-schists, is not definitely known ; they have 

 been referred to the Archaean and to the Silurian systems. They 

 are followed by about 1000 feet of Tertiary sandstones and lime- 

 stones, which contain sufficient fossils to class them with the 

 Oamaru system of Oligocene age (Hutton) and with the Cretaceo- 

 Tertiary of the Geological Survey. Fine, plant-bearing shales 

 succeed unconformably ; and upon these again rests a pumice- or 

 light scoria-bed. These rocks are in their turn covered by the 

 igneous rocks next described. 



The rocks described include an ill-exposed, gold-bearing syenite ; 

 a diorite, with an uneven and weathered surface, underneath the 

 lavas; rhomb-porphyry containing anorthoclase, and partly in- 

 trusive, partly interbedded ; tinguaite, in the form of dykes ; 

 hypabyssal trachydolerite, containing nepheline and oegirinc, and 

 sometimes sodalite and analcite; a teschenite-dyke ; and trachyte, 

 notably lacking in ferromagnesian minerals, occurring in dykes and 

 lava-flows. A large number of trachytoid phonolites occur in the 

 form of interbedded sheets ; they are divided by the author into 

 five principal varieties, one of which bears leucite. Nephelinitoid 

 phonolite is rare. A considerable number of rocks are classified as 

 trachydolerites apparently occurring as lavas. The andesites are 

 characterized by hornblende and augite, with little or no olivine ; 

 basanite is restricted in its occurrence. Dolerites of two principal 

 types occur in dykes, one type being the commonest of all the rocks 

 in the area. There are many ]ava-flows and some dykes of basalt; 

 and one melilite-basalt is found in the extreme north of the district. 

 A note is appended on the economic application of these rocks. 



A considerable series of chemical analyses follows, showing that 

 the silica-percentage varies from 66 in the Portobello trachyte to 

 44*84 in one of the dolerites. The petrographical differences are 

 coincident with the chemical differences, and the rocks are generally 

 similar to rocks of the same classes in other parts of the world, 

 and more particularly to those of East Africa. A classification 

 is attempted, according to the methods proposed by Pirsson, 



