Bartrum. — Geology of Great Barrier Island. . 121 



of the area of deposition, followed by a continuance of depression. There 

 is, however, no evidence to show the exact location of the land-mass, but 

 we are undoubtedly beginning to know a little more of it than previously. 

 It certainly lies buried beneath the unmetamorphosed sediments of the 

 Whangarei district, for andesitic rocks intrusive into these sediments at 

 Parua Bay contain very abundant xenoiites of hornblende-schists and 

 hornblende-epidote-schists. 



The period of pressure causing this acute metamorphism of the rocks 

 of this pre-Mesozoic land probably was coeval with that causing the 

 granulation of granites in the central portion of the North Island (Park, 

 1893), and of dioritic rocks at Albany, near Auckland (Bartrum, 1920), 

 now found in Tertiary conglomerates in those districts. It is a fair inference 

 that this land was extensive both north and south of the city of Auckland. 



The Andesitic Volcanic Rocks. 



The andesitic mass resting upon the basement of eroded sediments, 

 and occupying the main portion of the island south of the northern area 

 of sediments, consists to a great extent of coarse frao;mentals, breccias in 

 the main, though conglomerates are also abundant. Many of the rocks 

 here loosely called breccias are perhaps more strictly agglomerates, but the 

 writer had not an opportunity in the field of making the distinction. Lavas 

 of limited extent are frequently intercalated in the mass, but tuffs are scarce. 

 Hutton (1869) records the presence of seams of black laminated shale in a 

 coarse soft tufaceous sandstone forming the base of the series at Onewhero, 

 in Maori (Katherine) Bay, a locality not visited by the writer. 



Sollas and McKay (1905, vol. 1, p. 146) describe a hyalopilitic pyroxene- 

 andesite belonging to this series of rocks, and Park (1897), though he does 

 not definitely state that the propylites, or altered andesites, of the central 

 portion of the island, which carry gold-silver veins, belong to the series, 

 leads one to infer that he believes such to be the case, and records types 

 that " are augitic and generally contain hypersthene, which often occurs 

 in excess of the augite." 



A number of sections were cut from flows in many diverse locali- 

 ties, and of some of the fragmental material. All indicate a remarkable 

 uniformity of facies. Hypersthene-andesites are very common, augite in 

 these being greatly subordinate to the hypersthene, or even absent. In one 

 slide the hypersthene has deep resorption borders of iron-ore, which is not 

 at all a common phenomenon ; this is well illustrated by the photomicro- 

 graph, fig. 4 of Plate XXVI. The other varieties of andesite can be classed 

 as pyroxene types, with both augite and hypersthene prominent. 



In the majority of the sections there is surprising uniformity in general 

 appearance. The clear-cut phenocrvsts, embracing always plentiful feldspar 

 in addition to pyroxene, are spread in a very constant minutely crystalline 

 groundmass consisting mainly of tiny feldspar laths with a little pyroxene 

 and iron-ore. Sometimes it is so fine as to be practically irresolvable, and 

 in such cases it is perhaps to be considered hyalopilitic. 



A boulder from a conglomerate at Port Fitzroy furnishes a good example 

 of intersertal structure : numerous small crystals of plagioclase, with other 

 coarser crystals of the same mineral and pyroxene, are interspersed closely 

 in a glass crowded with minute prisms of pyroxene and a few small crystals 

 of magnetite. Fig. 5 of Plate XXVI illustrates a typical poition of a 

 section of pyroxene-andesite. 



