112 Transactions. 



of limestone (c) thickly crowded with decomposed rolled volcanic rocks up 

 to 1 ft. in diameter, while the limestone itself contains small fragments of 

 augite and olivine. The junction of this bed with the underlying tufaceous 

 bed is unconformable in the section, but is probably due to contemporaneous 

 erosion. Towards the top the limestone is free from the conspicuous 

 volcanic boulders, and becomes very hard. This limestone has been 

 proved phosphatic. It is overlain by glauconitic greensand — the typical 

 Hutchinson Quarry greensand — crowded with brachiopods belonging 

 chiefly to the two species *Pachymagas parki (Hutt.) and *Rhizothyris 

 rhizoida (Hutt.). 



As the gully is followed towards the Oamaru reservoir, outcrops are 

 conspicuous on the hillside, but the sequence is similar to that just described. 

 Past the farmhouse, however, the underlying massive volcanic rock crops 

 out, and this proves to be the same rock that occurs at Chelmers Street 

 quarry as a thick dyke which has overflowed to the north, and developed 

 pillow structure in the bed of Oamaru Creek near Grant's Creek. At the 

 latter locality it clearly underlies a limestone " conglomerate " and limestone 

 capped by the greensands, exactly as at Hutchinson Quarry. The section 

 at Grant's Stream is described in another paper in this volume (p. 121). 

 The brecciated pillow-lava is undoubtedly the same as the upper pillow-lava 

 that occurs above the fossiliferous beds near Oamaru Breakwater (see p. 109). 

 Park, McKay, and Hutton, in discussing the section at Oamaru Breakwater, 

 referred these fossiliferous beds to the Hutchinson Quarry horizon ; but 

 whatever interpretation is given to this term, these beds at the breakwater 

 are certainly below the volcanic rock which, at Hutchinson Quarry and the 

 locality near Grant's Creek, is overlain in ascending order by tufaceous 

 beds, from 10 ft. to 20 ft. of limestone, and the typical Hutchinson Quarry 

 greensand. 



The well-rounded appearance of the volcanic boulders in the limestone 

 in the neighbourhood of Oamaru Creek, and the confused intermingling 

 of limestone, greensand, and rolled igneous rocks above the brecciated 

 pillow-lava furnish evidence of denudation of the underlying volcanic 

 rocks, which had formed small islands in the Tertiary sea. Formation 

 of islands by submarine eruption is not an uncommon occurrence even in 

 quite recent times, and has been noted by competent observers, who have 

 always recorded the rapidity of their disappearance. This is probably the 

 explanation of the various stratigraphical breaks that have been already 

 referred to. 



Lister (1891, pp. 596-606), in a paper on the geology of the Tonga Islands, 

 arranges the islands in three main divisions — (a) purely volcanic islands ; 

 (b) islands formed of volcanic materials laid out beneath the sea, since 

 elevated , with or without a covering of reef-limestones ; (c) islands formed 

 entirely of reef-limestone. Some of the islands of class (b) exhibit suggestive 

 resemblances to the upper Ototaran rocks. These resemblances may be 

 summarized under the following heads : — 



(1.) They are built up of layers of tuff capped by calcareous rocks ; 



(2.) The fragments of some of the breccias are cemented by a calcareous 

 matrix ; 



(3.) Rounded boulders of volcanic rocks occur in layers embedded in a 

 calcareous matrix ; 



(4.) Some of the tuffs are penetrated by volcanic dykes which do not 

 penetrate the overlying limestone ; 



(5.) Fragments of basic plutonic rock occur on one of the islands ; 



