Bartrum. — Geology of Great Barrier Island. 117 



from which place he visited Oroville, in the Kaitoke Valley, and then 

 returned by the road and tracks along the east coast from Kaitoke Creek 

 to Harataonga Bay, whence he took the bush track, crossing the higher 

 country some distance from the coast, to Whangapoua. 



In spite of the cursory nature of the writer's examination of a large 

 portion of the area mapped, it appeared to him desirable to make such 

 provisional alterations and additions to Hutton's account of its geology 

 as are now possible, instead of waiting perhaps a long time until an 

 opportunity presented itself for making a more thorough geological survey. 



Scheme op Paper. 



The aim of this paper on the geology of Great Barrier Island may be 

 summarized as follows : — 



(1.) To present a statement of the physiography and stratigraphy of 

 the island, and more particularly of its northern half : 



(2.) To record the discovery in the basement (? Mesozoic) rocks of 

 some interesting conglomerate bands containing granite, peg- 

 matite, granulite, and other boulders : 



(3.) To describe a little more fully than Hutton (1869) the rocks 

 intrusive into the basement : 



(4.) To discuss briefly the origin of the copper lode at Miner's Head. 



Physiography of Great Barrier Island. 



Great Barrier Island is a rugged, elevated, much-dissected, probably 

 one-cycle mountain-mass, about twenty-four miles in length, and varying 

 up to thirteen miles in width. It is built largely of moderately resistant 

 rocks, amongst which well-compacted andesitic conglomerates and breccias 

 and rhyolite lavas figure most prominently. Each of these two rock-types 

 builds its own characteristic terrain, recognizable with ease even at con- 

 siderable distance. The andesitic fragmentals often build the hill land- 

 scape best described as turreted, with successions of frowning bluffs 

 breaking the monotony of gentler slopes. The rhyolites lend themselves 

 to the evolution of the weirdest pinnacled crags and sheer precipices, 

 which, with alluring whiteness, give a fascinating picturesqueness to the 

 landscape carved from them. (See Plate XXIV.) 



The area of shales and greywackes at the north of the island lacks much 

 of the ruggedness of the more southerly portion, but is none the less steep 

 and topographically fine-textured. On the north-west it descends abruptly 

 to the sea in stupendous lofty precipices. (See Plate XXIII, fig. 1.) 



Like its prototype the Cape Colville (or Coromandel, or Hauraki) 

 Peninsula, of which it is undoubtedly the former continuation, Great 

 Barrier Island represents the remnant of a maturely dissected, mountainous, 

 heterogeneous land-mass with insequent drainage, which was depressed 

 with reference to sea-level in the not-far-distant geologic past, so that the 

 sea entered far into the deep, comparatively narrow trenches carved in 

 the earlier mass. 



More particularly on the western coast, islets and reefs thickly fringe 

 the shore-line, representing extensions of this earlier land-mass which have 

 not yet been cut down by wave-attack. (See Plate XXII, fig. 1.) Youthful, 

 precipitous, lofty cliffs form this highly irregular immature coast, except 

 locally where bays such as Katherine, Blind, and Tryphena Bays exhibit 



