120 Transactions. 



Structure. 



The structure of the oldermass in the northern portion of New Zealand 

 is still imperfectly known, so that a few definite observations of strike and 

 dip may prove of value. In the vicinity of the intrusive dykes intense 

 shattering has disguised the structure of the sediments. Plate XXV, fig. 3, 

 illustrates folding in these beds at Harataoaga Bay. 



Observations. — (1.) Immediately south of Miner's Head a conglomerate 

 band strikes N. 35° W., with a dip of 70° to the north-east," (2.) In 

 the valley of Mine Bay Creek two conglomerate bands gave respectively 

 (a) strike north-west and south-east, dip 60° to the south-west ; (b) strike 

 N. 60° W., dip 70° to the south-south-west. (3.) Towards the head of 

 Mine Bay Creek : strike N. 5° W., dip 30° to the east. 



Conglomerate Bands in Basement Sediments. 



Three outcrops of conglomerates were found — the first at the foreshore 

 near the adit crosscut of the old copper-mine at Miner's Head, on the north- 

 west coast of the island ; the other two not far distant in branches of a small 

 tributary to Mine Bay Creek, which enters from the south about half a mile 

 up-stream from the foreshore. The first varies in width from about 8 in. 

 to a little over 1 ft., and contains large well-rounded beach-boulders ranging 

 in size up to 10 in. in diameter. The material of the boulders is typical 

 coarse granite with conspicuous white mica, a biotite granite with 

 equally conspicuous biotite, and plentiful hard shales and other sedimentary 

 types. 



The outcrops in Mine Bay Creek basin show a much more substantial 

 depth than the first mentioned ; both probably belong to the same band, 

 which has a width of about 7 ft. The majority of the boulders are much 

 smaller and less assorted than in the other band, and there is an abundance 

 of arkositic matrix. A bi-mica granite, in boulders as large as 18 in. in 

 diameter, forms the bulk of the constituent boulders, but shales too are 

 plentiful, whilst granulites (some with garnet, some without), pegmatites, 

 and occasional andesite are also represented. 



Sections were cut from a number of the boulders, but microscopic 

 examination did not add greatly to the knowledge gained by macroscopical 

 examination. One fact worth mention is that the biotite of some of the 

 boulders from the band near the copper-mine adit contains small zircon 

 crystals around which are intense pleochroic haloes. 



The pegmatites are fine-grained, composed almost wholly of graphically 

 intergrown orthoclase and quartz, with frequent small flakes of biotite. 

 The photomicrographs, figs. 1, 2, and 3 of Plate XXVII, adequately 

 exemplify a pegmatite and two types of granulite, one with garnet and 

 the other lacking it. 



Significance of the Material of the Conglomerates. 



The presence of rocks such as granulites in the basement shales and 

 greywackes of Great Barrier Island indicates the existence near that area 

 of a land-mass which had been subjected to intense pressure before the 

 deposition of those sediments, a question already considered in some detail 

 by the writer in a recent paper (Bartrum, 1920). The coarse, well-rounded 

 nature of the boulders of the conglomerates, and their freshness, particularly 

 in the band near Miner's Head, indicate that they were deposited near the 

 shore-line of a land-mass. They suggest a temporary movement of elevation 



