Gilbert. — Geology of Waikato Heads District. 113 



Waitangi Bay Basalt. — A couple of miles to the north of Waiuku there 

 occurs a basaltic lava-flow which can be traced along the bed of the Waitangi 

 Stream. Elsewhere it is covered deeply in a ferruginous clay resulting from 

 the decomposition of basalt, so that its extent could not be ascertained. 

 At one time the rock was quarried for road-metal at Waitangi Bay, where 

 the stream enters Waiuku Creek. Here it is coarsely columnar. Examined 

 under the microscope this rock is seen to be quite similar to the basalts 

 common round Auckland City. It consists of a holocrystalline, pilotaxitic 

 groundmass of long feldspar laths and small granular interstitial augite 

 and olivine, enclosing numerous large phenocrysts of olivine and pale to 

 colourless augite. The feldspar is mainly a basic labradorite, some of the 

 twinned crystals being fairly large laths. Subsidiary iron-ore (magnetite) 

 is scattered about in short streaks. 



The Pakau Basalt. — About seven miles up the south bank of the Waikato 

 River is a lava-flow across which several streams, including the Pakau, flow 

 in relatively shallow valleys after leaving the deep, gorge-like valleys their 

 headwaters have cut in the Mesozoic rocks of the Waikato fault-scarp. 



At a point about a mile to the east of the Pakau Stream, where another 

 stream forms a waterfall over the edge of the basaltic lava-flow, the Mesozoic 

 sandstones and shales are seen in direct contact unconformably underlying 

 the lava-sheet. This lava-sheet and the associated basaltic breccia to the 

 east are believed to be of the same age as the basaltic flows and breccias 

 spread over a wide area to the north of the Waikato River and eastward 

 of Waiuku, and to be more recent than the Kawa flow. 



At the Opuatia Bridge and in the environs of Puke-o-tahinga the lava 

 rests directly on the Notocene calcareous sea-muds. Behind the Onewhero 

 store and Post-office is a circular basin one mile in diameter, originally a 

 crater and subsequently a lake, which has been insilted to the level of its 

 present floor and now forms good farm land. It was drained by a stream 

 through a breach in the north-eastern margin of the crater. This stream 

 exposes the original lava-flow beneath the tuffs of the lip and for a mile 

 farther on, till finally, where its waters tumble into a deep gully, the lava 

 can be seen to occupy a trough representing an old stream-valley in the 

 Notocene calcareous sea-muds. 



The Pakau basalt, microscopically, consists of numerous large phenocrysts 

 of olivine with less important colourless augite, in a pilotaxitic groundmass. 

 The groundmass is made up of twinned feldspar microlites of basic labradorite 

 with abundant augite and olivine granules and subsidiary iron-ore scattered 

 about in large and small grains. 



Algal Limestone. — South end of Matuatua Beach. This limestone occurs 

 at the base of the cliffs less than a mile south of the Huruwai Stream, and 

 near Te Orairoa Point. Examined in microscopic section it is seen to 

 consist mainly of algal concretions, fragments of echinoid shells and shell- 

 plates and spines, corals, Foraminifera, and a good many polyzoans. (See 

 Plate XX, fig. 2.) 



Glauconitic Limestone. — From same locality. Associated with the coarse- 

 looking algal limestone, and probably above it, is a glauconitic type, com- 

 posed of numerous grains of glauconite with tests of Foraminifera and a 

 few angular grains of quartz. Recrystallized calcite forms a finely granular 

 mosaic filling some of the interstices between the organic fragments. 



Marly Limestone. — North Kawa Head. This limestone consists chiefly 

 of the tests of Globigerina, of which the chambers are frequently detached, 

 and other Foraminifera. Small granular calcite often fills the foiamimferal 

 chambers. 



Globigerina Limestone. — Koruahine Point, south end of Waiwiri Beach. 

 Globigerina shells almost entirely constitute this rock, which may therefore 



