Park. — On some Thames Andesites. 435 



country now occupied by the andesitic ranges of Waitakerei. 

 The Parnell grit may well have had its origin there, and have 

 been brought down by rivers and currents operating at a 

 time when the physical features of the district were altogether 

 different. But the whole question, either of locating precisely 

 these old vents or of estimating with certainty their probable 

 number, is one which offers but a feeble chance of ever being 

 satisfactorily solved. 



Note. — The Waitemata series can be traced further north 

 than Whangaparaoa, good sections showing along the cliffs 

 at Waiwera and round the Mahurangi Harbour, where the 

 volcanic grits again appear. North of this the sedimentary 

 rocks change considerably in character, the numerous layers 

 which distinguish the sandstones on the Auckland isthmus 

 giving place to thicker bands of a more highly indurated 

 sedimentary rock darker in colour and closer in texture. 

 The grits at Waiwera and Mahurangi are very similar both 

 in appearance and texture to those already described, and, like 

 these, contain numerous angular and subangular fragments 

 of augite-andesite, some of which reach upwards of 1 ft. in 

 diameter. Their mode of occurrence, however, does not 

 throw any additional light on the questions raised in this 

 paper. 



Art. XL. — Notes on some Andesites from Thames Goldfield. 



By Professor James Park, F.G.S., Director, Otago Univer- 

 sity School of Mines. 



[Read before the Otago Institute, 12th November, 1901.] 



HoRNBLENDE-ANDESITE. 



From the Mata Stream southward the deeply eroded 

 surface of the crumbling Palaeozoic slaty shales, which form 

 the basement rocks of the Hauraki Peninsula, are covered 

 with a great pile of andesite lavas, tuffs, and breccias. From 

 the Mata northward the coast-line is occupied by the slaty 

 shales for a distance of eight or nine miles without interrup- 

 tion. Between the Mata and Waikawau Streams the slaty 

 shales are intruded by seven massive dykes of igneous rock 

 which are well exposed in the deep road- cuttings winding 

 around the indentations of the rocky shore-line. The general 

 trend of the dykes is east and west, but, so far as I could 

 discover, they do not appear to reach the valley of the 

 Waikawau, which runs parallel with the coast-line for some 



