304 Transactions. 



Art. XXXIII. — Nephelinite Rocks in New Zealand. 



By Professor P. Marshall, D.Sc, F.G.S., Otago University. 

 [Read before the Otago Institute, 7th November, 1911.] 



It was not until 1891 that the mineral nepheline was recognized in any 

 New Zealand rocks. In that year it was recorded by Ulrich (3) as occur- 

 ring in a considerable number of rocks in the Dunedin volcanic region. 

 Statements had previously been made by Park and Hill that some of 

 the rocks on the south slopes of Ruapehu were phonolites. It has, 

 however, since been found that these statements were erroneous, and so 

 far as known all the rocks of Ruapehu are hypersthene-andesites. 



Though Ulrich described many types of nepheline-bearing rocks from 

 the Dunedin district, he by no means exhausted the locality. Other 

 types have since been described by Marshall, Boult, and Cotton ; but 

 there is still much room for research. Within the last few months 

 Mr. J. P. Smith has added greatly to our knowledge in bringing to light 

 the occurrence of interesting types of nephelinite, a rock that had not 

 previously been recorded from this neighbourhood. Among the speci- 

 mens deposited in the Otago Museum by Captain Hutton previous to 

 1879 was a rock with the label, " Dolerite, Lake Waihola." When this 

 rock was examined microscopically it was at once seen to be a nephe- 

 linite. Search for an outcrop of it on the east side of the lake was, 

 however, unsuccessful ; and Andrew's account of the basalt on the 

 south of the lake did not suggest that it occurred there. For some 

 time no opportunity presented itself for an examination of the western 

 shore of the lake. Recently, however, Mr. Smith was able to visit the 

 western side, and he at once found a large outcrop of various types of 

 this rock, which we were afterwards able to visit in company, through 

 the courtesy of Mr. W. Adam, on whose property the outcrop is situated. 

 Subsequently it was found that all the basaltoid rocks south of this lake 

 are related to the nephelinitoid type. A distinct dyke of a rock of inter- 

 mediate coarseness occurs at Clarendon, five miles south-west of Waihola. 



Later Mr. Smith found a still more interesting type of nephelinite 

 at Omimi, on the sea-coast, thirty-three miles north-east of the previous 

 locality, but still in the same volcanic district. Here the actual limits of the 

 doleritic form of the rock are not well defined, but, as at Waihola, the 

 doleritic type is associated with a basaltoid development, which contains a 

 large amount of allotriomorphic nepheline, and varies between nepheline- 

 basanite and nepheline-basalt. 



Another New Zealand locality for this type of nephelinite is in the 

 Auckland Domain. Here there is no exposure of the rock in the solid, 

 for all the specimens that have been obtained were derived from boulders 

 ejected during the explosive eruptions of the volcano of which the cricket- 

 ground now occupies the crater. This rock has been known for some 

 time, but the only description hitherto published of it is in a general 

 paper by Marshall (8). No analyses of any of these rocks have hitherto 

 been published. 



The occurrence of these rocks on the western side of the Waihola 

 Lake is of special interest. Here the rock is extremely coarse over 

 part of the outcrop (analysis A), and it has a structure that is appa- 

 rently granitoid in hand-specimens. Apatite was the first mineral to 



