37 



upper waters of the Wairau River in west Marlborough. It forms 

 rugged country of strong relief, which, according to Bell, Clarke, 

 and Marshall, represents a portion of an ancient dissected peneplain 

 covering a great part of the northern portion of the South Island. 

 The areas occupied by the magnesian rocks are generally detected 

 very readily in the field, even at considerable distance, by the peculiar 

 and stunted nature of their vegetation, which contrasts strikingly 

 with that flourishing on the near-by areas of sedimentary rock. 



In 1859 Dr. F. von Hochstetter visited Nelson, and, as a result, 

 his well-known publications early demonstrated the exceedingly 

 interesting nature of the ultrabasic igneous rocks of the " Mineral 

 Belt " to petrologists throughout the world. Dunite, a rock com- 

 posed almost wholly of somewhat granular olivine, was named by 

 Hochstetter after Dun Mountain (3,703 ft.), a conspicuous, brown, 

 grass-covered, rounded knob, prominent near the Town of Nelson, 

 and composed in large part of this rock. Even before the visit of 

 the eminent Austrian geologist the " Mineral Belt " had become 

 famous in the early " fifties " on account of its alluring but illusory 

 deposits of copper-ores and chromite, which were even then being 

 mined at various places near Nelson, and north-eastwards at Croiselles 

 and D'Urville Island. 



Little is known of the geology of the " Mineral Belt " other than 

 that portion near the Town of Nelson, which has been described 

 comparatively recently by Bell, Clarke, and Marshall in Geological 

 Survey Bulletin No. 12 (1911). For this reason the present description 

 applies chiefly to this latter, or the Dun Mountain area. 



Stratigraphically the ultrabasic rocks form sill-like masses intrusive 

 into argillites, greywackes, and coarser rocks of Permo-Carboniferous 

 age (or thereabouts), which are quite insignificantly metamorphosed 

 by the intrusion. In the same (Maitai) series there is a prominent 

 somewhat argillaceous limestone, which for nearly nine miles forms 

 the west wall of the magnesian intrusion. Westward of the lime- 

 stone the argillites appear to underlie with perfect conformity con- 

 glomerates and fossiliferous sandstones containing middle and upper 

 Triassic forms. The limestone and some of the closely adjacent 

 argillites of the Maitai series are sparingly fossiliferous, and Dr. C. T. 

 Trechmann has assigned a Permo-Carboniferous age to them. The 

 stratigraphical relations of these strata to the Triassic seem explicable 

 only on the supposition of overfolding or overthrusting, or a com- 

 bination of the two, and there is indeed strong evidence of faulting 

 in topography and in the actual in-tilting of Tertiary strata along 

 the west margin of the Triassic rocks, where the comparatively lofty 

 strike ridges in which they outcrop border the low-lying Waimea 

 Plain. 



In facies the rocks of the intrusive zone are highly interesting, 

 but have been very incompletely studied from areas not included 

 in the Dun Mountain Subdivision. Here serpentine, derived from 

 the olivine-enstatite rock harzburgite, forms the main portion of 

 the mass, and exhibits local gradation into fresh harzburgite. Near 

 Dun Mountain itself there is serpentine derived from the dunite 

 outcropping in that mountain. Dyke-like intrusions of doleritic 

 and dioritic rock are fairly numerous, chiefly as a western fringe 

 to the ultrabasic rocks near their contact with the enclosing 

 sediments. The most interesting dykes, however, are those of 



