146 REPORT— 1891. 



silicate, that by molecular change and hydration gives rise to 

 the formation of these minerals, had been leached out too 

 quickly and completely to allow that process to take place; 

 for the white decomposed rock of the promotitory gives no 

 gelatinous silica on treatment with HCl. Or it may also be 

 that the nepheline has been converted into kaolin, the same as 

 recorded by Doelter of phonolites in Sardinia. 



The degree and manner in which the Dunedin phonolites 

 have been affected by decomposition is very remarkable. 

 Whilst, for instance, the white decomposed rock of the Porto- 

 bello promontory faces the sea in cliffs up to 80ft. in height, 

 and has not a clayey, but rather a kind of rough, sandstony 

 feel — • a rock, in fact, which, were it not for the small 

 undecomposed outcrops at the bottom of the cliffs, nobody 

 would ever suspect to have originated from those dark, 

 hard, and tough rocks before described ; — on the contrary, 

 the dense, dark phonolite of the Purakanui cliffs withstands 

 the action of the sea apparently unharmed, as far, at least, as 

 aspect goes, but is beyond its reach much decomposed into 

 brownish clayey matter. And this is the case also with the 

 rock-varieties of the inland localities, only the tops and steep 

 slopes of the mountains (Mount Cargill, Flagstaff Hill, Pine Hill) 

 forming in this respect exceptions. Owing to this strong 

 decomposition, and to extensive denudation having -mostly ob- 

 literated the original surface-boundaries, the attempt to define 

 its limits, in the case of the nepheline-bearing rock-formation 

 as a whole, would be, I think, a hopeless one; and much more 

 so m that of the special rock-varieties, irrespective of the fact 

 that the similarity in aspect of some of the varieties to other 

 kinds of volcanic rocks they come in contact with — viz., 

 andesite and basalt — requires for their identification both 

 chemical and microscopical examination nearly step by step. 



Eegarding the questions as to the geological age of the rocks 

 under notice, and of their relative age compared to the other 

 volcanic rocks occurring in the district — viz., andesite, trachyte, 

 and basalt — it can, as answer to the first question, with 

 certainty be stated that all the volcanic eruptions of the 

 Dunedin district took place within the Tertiary period. And, 

 concerning the question of their relative age, certain features 

 in the configuration of the country seem to indicate that the 

 nepheline rocks have broken through the andesites and are 

 therefore more recent, whilst, according to general rule, basalt 

 is the youngest in having broken through both. With the 

 trachyte the nepheline rocks have not as yet been found in 

 contact, and therefore no definite opinion can be formed on the 

 point in question. I am inclined to consider the trachyte 

 younger than the andesite, and as the immediate precursor of 

 the nepheline rocks. 



