Marshall. — Geology of the Central Kaipara. 449 



Port Albert. 



A black rock, dense, and with an irregular fracture. The only mineral 

 visible in section is feldspar in small sharp crystals. These are so frag- 

 mentary that it is impossible to identify them. The index of refraction is 

 higher than that of Canada balsam. The feldspar crystals are bordered 

 with dark margins of amorphous matter, as are the crystals in the glassy 

 basalts of Savaii and similar vitreous basic rocks. The rest of the rock is 

 brown glassy matter showing feathery crystallitic growths and incipient 

 spherulitic structure. The rock has all the appearance of a basic glass, 

 but the absence of olivine crystals suggests that it has the composition of 

 an andesite rather than a basalt. 



Conglomerates on the South Side of the Otamatea Funnel. 



A large number of these boulders have in hand-specimens distinctly the 

 appearance of plutonic rocks. The specimens vary somewhat in coarseness 

 and in relative abundance of the ferro-magnesian constituent. The feld- 

 spar has no regular outlines, and the arrangement of the grains suggests 

 a gneissic structure. There are many minute inclusions, and lamellar albite 

 twinning is general. The extinction angle is 20°, and the refractive index 

 1-560. It is therefore a basic variety of andesine. Hornblende is very 

 abundant in irregular ragged crystals. It includes much dusty ferruginous 

 matter, and sometimes distinct 'Crystals of magnetite. Pleochroism, green 

 to straw-colour. The appearance of this hornblende suggests that it is of 

 secondary origin. There is much magnetite and apatite. 



No rock that closely resembles this has yet been foimd in situ in the 

 North Island. It is quite different from the diorite of Mangonui and from 

 the, other plutonic rocks near Ahipara and Hokianga. 



Pukekaroro. 



This rock is light grey in colour, and is without any conspicuous crystals 

 in hand-specimens. In section it is largely composed of short small crystals 

 of feldspar with idiomorphic outline. The crystals are often polysynthetic- 

 ally twinned, and the angle indicates that they are andesine. In many 

 cases they are zoned. The finer matter of the rock consists of colourless 

 material which in some specimens has a most irregular form but in other 

 slices is regularly quadrilateral. In all cases it has small inclusions of dark 

 matter which could not be identified. I am indebted to Professor J. P. 

 hidings for pointing out to me that this mineral is quartz. When it has 

 the irregular form its arrangement suggests the micropoecilitic structure de- 

 scribed by Sollas in the dacites of the Coromandel Peninsula. The ferro- 

 magnesian mineral in this rock is completely chloritized, and the chlorite 

 has much dusty magnetite associated with it. So far as the shape of the 

 chloritic pseudomorph can be recognized, the original mineral appears to 

 have been biotite. The rock must therefore be classed as a dacite. 



The age of these igneous rocks cannot be very distinctly stated, and it 

 is even unlikely that all the volcanic rocks are of the same age. The diorite 

 is evidently derived from some relatively ancient rock -mass which has not 

 yet been discovered in situ. 



The Marahemu basalt constitutes a neck that traverses the hydraulic 

 limestone in one of its flinty horizons. As the rock is glassy on the selvages 

 and along the crevices, and at the same time the associated tuffs are not 

 oxidized, it becomes probable that the eruption was of a submarine nature. 



15— Trans. 



