HuTTON. — The Geological History of Neiu Zealand. 163 



rently unconformably, by slate rocks containing graptolites of 

 Ordovician age, and this is the only stratigraphical evidence 

 we have as to the age of the schists ; but we find in the rocks 

 themselves other evidence of their great age. 



In Central and Eastern Otago, away from the main range, 

 these schists are not contorted, but lie at low angles — usually 

 from 15° to 45°- — so that the schistose structure cannot have 

 been caused by lateral pressure. Neither can it be due to con- 

 tact with large masses of igneous rocks, for there is a remark- 

 able absence of those rocks throughout the whole area, the 

 only eruptive rocks as yet described being the chlorite schists 

 near Queenstown. We are therefore driven to the conclusion 

 that the schistose structure is an original one, caused by the 

 interior heat of the earth at a time when it was much greater 

 than at present ; and so we are constrained to class the rocks 

 of the Wanaka system as pre-Cambrian.''' An additfonal 

 argument may perhaps be found in the large quantities of 

 graphite and graphite-schist which occur occasionally in some 

 of the older strata, for the occurrence of graphite is charac- 

 teristic of Laurentian and Huronian rocks. The Wanaka 

 system contains the gold-bearing rocks of Otago. 



Takaka System. 



The rocks of the next overlying system, called the " Ta- 

 kaka system,"! are found chiefly in the valley of the Aorere 

 Eiver, in Collingwood County ; but also in two detached 

 localities — -one in the basin of the Baton Eiver (a branch of 

 the Motueka), the other near Reefton. Their united thick- 

 ness has been estimated by Sir James Hector at between 

 15,000 ft. and 18,000 ft., but they cover a comparatively small 

 portion of the country. 



The lower, or Aorere, series consists chiefly of blue slates 

 with beds of feldspathic and quartzose schist, the former con- 

 taining graptolites belonging to the genera Didymograptus, 

 Tctragraptus, Dichograptus, and Phyllograptus, and is no 

 doubt of Ordovician age. The Baton Eiver and Eeefton series 

 consist of slates and sandstones with calcareous beds, some- 

 times pure limestone. These calcareous rocks contain Trilo- 

 bites and a number of Brachiopods, as well as a few MoUusca 

 and corals, which appear to be of Siluro-devonian age. In 

 Collingwood County gold reefs are found in rocks belonging 

 to the Aorere series. 



Maitai System. 



The next rock-system consists of a large mass of sand- 

 stones and uncleaved argillites, with occasional beds of lime- 



* Trans. N.Z. Inst., vol. xxiv., p. 361. 



t Quart. Journ. Geol. Soc. Lond., vol. 41, p. 194 (1885). 



