:j12 T ransactionx. 



Cteological History. 



A tentative aecount of the later geological history of west Nelson is 

 here put forward. In Mesozoic times what is now New Zealand formed 

 the littoral of the primary coast of the Pacific Ocean. The vast accumula- 

 tion of sediment which seems to precede the formation of fold-mountains 

 was deposited. Then a fold-movement was initiated, probably contempo- 

 raneous with the sinking of the land now occupied by the Tasman Sea. 

 This folding ridged the land into a series of folds running north-north-east 

 and south-south-west to north-west and south-east, or, say, on an average 

 north and south.* Then, probably in late Mesozoic or early Cainozoic 

 times, the alpine orogenic folding commenced, which culminated in the 

 production of the great alpine overthrust. This crosses the older north-and- 

 south folds obliquely, running as it does north-east and south-west parallel 

 to the alpine folds. The pressure forced molten magmasf into the rocks 

 along the lines of weakness. Of these lines of weakness the principal is the 

 alpine overthrust, and granites occur just to the west of this line in west 

 Otago and from Mount Bonar in Westland to Lake Rotoiti in Nelson. 

 Besides the granites immediately abutting on the alpine overthrust, the 

 intense pressure from the east forced up magmas along lines of weakness 

 faither to the west. Thus great laccolites occur along the axes of the older 

 north-and-south folding. In west Nelson the longer diameter of the granites 

 of the Murchison peneplain are continued north-north-east and north to 

 Separation Point, and the granites of Victoria peneplain trend north and 

 south, and their northern end is very close to and perhaps continuous with 

 the granites flanking the west of the Lyell and Mount Arthur peneplains. 

 These north-north-east and south-south-west lines of weakness continued 

 northward pass through Taranaki and the Auckland Peninsula, while the 

 alpine-overthrust line passes near Taupo and White Island. It is probable 

 that even at this period the great fracture-lines had been initiated,! the 

 breaks running north and south or north-east and south-west, according 

 as the influence of the north-and-south folding or the pressure from the 

 south-east predominated. At already pointed out, the faults of the alpine 

 peneplain strike nearly either north and south or east and west, while the 

 overthrust strikes north-east and south-west. It is suggested that the 

 north-and-south faults follow lines of weakness developed by the old north- 

 and-soiith folding, while the east-and-west faults represent the breaking- 

 back of the fissures to the line of greatest tension. 



The land seems to have been above sea-level till Tertiary times, when 

 depression permitted the inroads of the sea into the rift-valleys which had 

 already been formed. Deposits accumulated in these rift-valleys. In 

 ascending order, they are conglomerates, sandstones, and shales with coal- 

 seams ; then mudstones and dark strata, analogous to those at present 

 forming on the shores of Blind and Golden Bays, West Wanganui Inlet, 

 Otago Harbour, and, indeed, any land-locked arm of the sea silting up. 

 Further depression permitted the formation of sandstones, calcareous sand- 

 stones, arenaceous limestones, and finally pure shelly limestones. When 

 these last were foi'med the land-surface of what is now west Nelson was 



* Cf. Morgan : N.Z. Geol. Surv. Bull., No. 6 (n.s.), p. 78. Dobson (quoted by 

 Hochstetter, "New Zealand," p. 485) .states that the strike of the beds is north-east, 

 while the general trend of the range is north-east. 



t The granites of west Nelson are not at all the same age. 



i The numerous lodes of west Nelson occur near the great faults. 



