Bartrum. — A Con glome rate at Onerahi. 129 



similar to the very extensive limestone of the North Auckland area usually 

 spoken of as the " hydraulic limestone." The question of its age is still 

 unsettled. 



The band of conglomerate is lensoid, ranging up to about 5 ft. in depth, 

 and is exposed standing steeply on edge for about 15 yards. Its pebbles 

 and boulders are well rounded, and, though mainly small, vary upwards 

 in size to 3 in. or more in diameter. The material is chiefly greywacke 

 and shale, with other types of sedimentary rocks, but interesting igneous 

 rocks are also frequent. One other type was also found : it is a quartz-rich 

 granulite, which probably represents a granulated and partially recrystallized 

 quartzite. The rocks of igneous origin include : — 



1. Gneissic granite. 



2. Acid intrusives — 



(a.) Quartz porphyries ; 

 (6.) Granophyre. 



3. (?) Tufaceous acidic rock. 



4. Andesitic tuff. 



Detailed Petrography. 

 Granulated (?) Quartzite. (Plate XXVIII, fig. 1.) 



Macroscopically this rock resembles hornfels. In section it is seen to be 

 almost wholly quartz showing highly prominent shadow-extinction, with 

 a little untwinned feldspar and perthite. Granulation has been intense, 

 and the minerals form a mass of somewhat interlocking, coarse, partially 

 recrystallized grains with granulated borders. 



There are occasional grains of epidote, whilst minute shreds of muscovite 

 are present, mainly in the granulation-products. 



Gneissic Granite. (Plate XXVIIL fig. 2.) 



This is a very curious granitic rock showing gneissic structure distinctly 

 in hand -specimen. Quartz makes up about half the bulk of the rock, the 

 rest consisting of a very little acid plagioclase and of the coarse perthite 

 so prominently displayed by the photomicrograph. 



The only other minerals are rare sphene, a few tiny flakes of muscovite, 

 and some irregular patches representing alteration-products, probably of 

 biotite. 



The perthite is on a very coarse scale, and at first glance resembles 

 graphically intergrown quartz and orthoclase ; occasionally the crystals 

 may actually be such intergrowths. but in the majority of cases there is 

 very minute albite-lamination crossing the coarse perthitic striping of the 

 mineral. 



The gneissic structure visible macroscopically appears in section as 

 zones of shearing. 



Quartz Porphyries. 



These are conspicuously porphyritic rocks, somewhat dark in colour, 

 showing feldspar and occasionally quartz phenocrysts in hand-specimen. 



Nearly all the sections show a fairly finely holocrystalline granular 

 groundmass, in which are abundant large acid plagioclase phenocrysts. 

 Occasionally there are plentiful orthoclase and quartz crystals. Biotite 

 is present in greatly inferior amount ; it may be fresh or represented by 

 alteration pseudomorphs. The groundmass typically consists of quartz 

 and orthoclase in subequal amounts, but where quartz occurs as a 



5 — Trans. 



