308 Transactions 



Art. XXXVII. — On the Igneous Intrusions of Mount TapuaenuJca, 



Marlborough. 



By James Allan Thomson, M.A., D.Sc, F.G.S., Palaeontologist to the 

 Geological Survey of New Zealand. 



(By permission of the Director of the Geological Survey.) 

 [Read before the Wellington Philosophical Society, 23rd October, 1912.] 



INTRODUCTION. 



Mount Tapuaenuka,* the highest point of the Inward Kaikoura Range, 

 has long been known to be intersected by numerous igneous intrusions. 

 The aim of this paper is to give a preliminary petrographical account of 

 some of these. The material studied consists of a series of rock-specimens 

 obtained from boulders in the gorge of the River Dee, Middle Clarence Valley, 

 by Mr. C. A. Cotton and the writer in the early part of 1912. The Dee is 

 a mountain-torrent rismg in enormous screes on the abrupt slopes of Tapuae- 

 nuka, from which it carries in flood-time boulders of very large size. In 

 its bed there is little to be found but crystalline rocks of varying textures 

 and colours, and there is no reason to doubt that these rocks come from 

 within the watershed of the torrent. Care was taken to confine the col- 

 lection to the upper part of the gorge, above the intersection of the " Post- 

 Miocene conglomerate," in order that post ible exotic rocks should be excluded. 



Our knowledge of the structure of the Inward Kaikouras is due almost 

 entirely to the labours of A. McKay, who traversed the Middle Clarence 

 Valley in 1884-85, and the Awatere Valley in 1888-89. The three lengt,hy 

 reports in M^hich he embodied his results are full of details of stratigraphical 

 and structural geology of the highest interest, and will probably long remain 

 the chief source of information on the geology of Marlborough. f Only 

 those parts which relate to the intrusions need be noticed here. 



The Inward Kaikoura Range is composed of sandstones, grits, grey- 

 wackes, argillites, and jasperoid slates of uncertain age, but Jurassic or 

 Lower Cretaceous at the latest. McKay placed them in the Maitai series, 

 of Carboniferous age, with a saving clause that the only evidence obtained 

 indicates a Secondary age. On each side the range is bounded by Upper 

 Cretaceous and Tertiary rocks, let down along tremendous faults. On the 

 Clarence Valley side the Lower Tertiary rocks are covered uncomformably 

 by the " Post-Miocene conglomerate," which is interposed between them 

 and the fault-plane. 



Volcanic rocks of Cretaceous age occur in the Clarence Valley, opposite 

 the Bluff River and in the Gore River, but below Tapuaenuka they are 

 absent, and dykes through the Cretaceous and Tertiary rocks are rare. A 

 greater development of Cretaceous volcanic rocks is found in the Awatere 

 Valley, and these are seamed by dense dykes, w^hich pass uninterruptedly 

 into the greywacke series, and run almost to the top of the range. 



* Also spelt Tapuaenuku. 



t " On the Geology of the Eastern Part of Marlborough Provincial District," 

 Rep. Geol. Expl., vol. 17, pp. 27-136; 1886. 



" On the Geology of Marlborough and the Aniuri District of Nelson," Rep. Geol. 

 Expl, vol. 20, pp. 85-185; 1890. 



"On the Geology of Marlborough and South-east Nelson, Part II," Rep. Geol. 

 Expl., vol. 21, pp. 1-28; 1892. 



