Smith. — Alkaline and Nepheli-ne Rocks, Westland. 123 



therefore, only from the detrital rocks derived from the erosion 

 of the now bush-covered mountains that a knowledge of many 

 of the dyke rocks can be obtained. 



So far as is known, the Arahura and Kanieri formations of 

 the new geological survey are, with the exception of the Pounamu 

 series, remarkably free from intrusions ; but the Tuhua forma- 

 tion — an intrusive mass itself — is seamed in all directions with 

 narrow dykes. Already in the Hokitika sheet of the new survey 

 outcrops of the following dyke rocks have been located and 

 the rocks described : Pyroxene and hornblende camptonites, 

 pyroxene and hornblende porphyrites, diabases, and an augite 

 diorite. 



The Tuhua formation is a great granitic intrusion, with its 

 major axis roughly parallel to the axis of the main range. As 

 the flanks of the granite hills which expose this formation are 

 in many places covered with detritus to a height of several 

 hundred feet above sea-level, the outcrops are not continuous, 

 but appear as huge bosses and isolated groups of hills. The 

 Tuhua formation has been subjected to glaciation. The ice- 

 sheet at the period of maximum extension flowed around and 

 frequently over the summits of the granitic mountains. Enor- 

 mous erosion resulted, and upon the retreat of the ice-sheet 

 deep deposits of morainic matter covered the depression between 

 the main alpine range and the granite range, and between the 

 granite range and the ocean. The rivers emerging from the 

 retreating ice-cap immediately began the reassortment of the 

 glacial drift, and this work, with decreasing intensity, has con- 

 tinued on to the present time. 



The rocks herein described were collected from the fluviatile 

 gravels in the beds of the present streams and from the auriferous 

 gravels deposited at higher levels by streams no longer existing. 

 They were gathered from the beds of the New River and its 

 tributaries on the right bank, and from the higher-level gravels 

 between the New River and Arnold River basins, but not from 

 the Arnold basin itself. 



A rough estimate of the rock contents of the gravels was 

 made in three or four localities ; they contain about 80 per cent, 

 of greywackes, some 10 per cent, of slates, phyllites, argillites, 

 quartzites, sandstones, and conglomerates from the Arahura 

 and Kanieri series, and 10 per cent, of rocks from the Tuhua 

 series. Roughly, perhaps about 1 per cent, of the Tuhua rocks 

 may be classed as of hypabyssal origin, of which the larger 

 proportion are much-decomposed feldspar-porphyrites, and the 

 balance consist of the rocks herein described. Locally, these 

 rocks are known to the miners as ironstones. They are recog- 

 nised by their dark-green to black colour, and by their tendency 



