Outline geology of the Inner Islands of the Hauraki Gulf

[First paragraphs...]
The geology of Waiheke, Ponui, Motuihe, Motutapu, Rangitoto, and associated islands (Fig. 1) can not be considered separately from that of the adjacent Waitemata Harbour and Auckland Isthmus, for their lithology, structure, and geologic history are intimately connected.
The basement rocks over the whole region consist mainly of indurated,. sparsely fossiliferous, bedded sandstones and. argillite of probable Jurassic age (Waiheke Formation). Less frequent red and brown cherts and jaspilites are found on Waiheke, Pakihi and Motutapu, on the first two being associated with quantities, of manganese ores which were once quite intensively mined. Also, at a number of points on Waiheke grey and reddish lavas are found interbedded with the sediments.