Bartrum. — Geology of Great Barrier Island. 115 



Art. XIV. — Notes on the Geology of Great Barrier Island, New Zealand. 

 By J. A. Bartrum, Auckland University College. 



[Read before the Auckland Institute, 15th December, 1920; received by Editor, 31st 

 December, 1920 ; issued separately, 27th June, 1921.] 



Plates XXII-XXVII. 



In 1919 the writer spent a short holiday at the northern end of Great 

 Barrier Island, and found that the geology of that part of the island is 

 rather indifferently represented by Hutton's paper of 1869,* which still 

 remains the only important account of that area. In many respects 

 Hutton's work is admirable, for considerable difficulties attach even now 

 to a close study of some parts of the island, and years ago these must 

 have been even greater. Hutton's chief error is that he failed to recognize 

 a large area of rhyolitic rocks as such, and mapped them as " pinkish 

 slates." This is, however, entirely excusable, for these rocks are "very 

 finely banded, and in section resemble very finely granular sediments, 

 though upon examination with a first-class instrument they can be seen 

 to be in reality minutely microspherulitic rhyolites. 



Park (1897), who deals with the geology of the central portion of the 

 island in his paper on the geology and veins of the Hauraki goldfields, 

 makes a similar error in classing them as banded sinters similar to those 

 he asserts form the higher portions of a mountain-mass near Whanga- 

 parapara, with a remarkable series of breakaway cliffs which give it its 

 local name, Whitecliffs Range. (Plate XXIII, fig. 2.) The Maori name for 

 it is Te Ahumata. 



The writer's visit served to yield him little more than an approximate 

 idea of the geology : one or two large areas to the north are terra incognita 

 to him, and the appended map shows very crudely sketched boundaries 

 between the various rock formations. He managed, however, to spend 

 a day or so at Mine Bay, on the north-west coast, where Hutton maps so 

 many interesting dykes intrusive into the shales and greywackes which 

 form the basement of the island, and to make a moderately careful study 

 of many of these dykes. The number of them is so great at Mine Bay 

 itself, along the coast both north and south from there, and in the valley 

 of Mine Bay Creek, that a full collection was out of the question, and no 

 map could exhibit their location unless published on a very large scale. 



The writer made his headquarters at the house of Mr. Warren, of 

 Port Fitzroy, and cannot sufficiently thank Mr. Warren and all members 

 of his household for the assistance they gave him in numerous ways. 

 From there he made a number of excursions on foot and by boat, and 

 finally took a walk along the recognized foot route from near Cooper's 

 to the top of Mount Hobson, thence by a devious traverse to Awana Flat, 



* Full reference is appended in a list of literature cited to be found at the end of 

 this paper. 



