Bartrum. — Intrusive Igneous Mocks from West port District. 269 



(6.) Dolerite. 



A dolerite* lias been described from near Lyell by Hutton. The writer 

 has found no reference other than this to dolerites in the Westport district. 

 Hutton's type is totally different from that herein noted, and, as has already 

 been mentioned, the Lyell rock is possibly a camptonite. 



The dolerite now described is found in boulders deposited, along with 

 much greywacke debris, by small streams crossing the Karamea-Mokihinui 

 Road not* far from the ford of the left branch of the Six-mile Creek. In 

 hand-specimen it is a dark-green basic rock with fine laths of feldspar pro- 

 minently displayed on partly weathered surfaces. Seen under the micro- 

 scope, the dolerite is even-grained, holocrystalline, and fairly coarse, and 

 shows most interesting and perfect ophitic structure. The main constituent 

 is andesine in typically long laths, between and around which is abundant 

 augite, its boundaries thus sharply defined by the dominating laths. Iron- 

 ore, chiefly ilmenite, is in noteworthy amount ; it precedes the augite and 

 follows the feldspar. Some of it is secondary. A few zircon prisms, pyrite, 

 secondary calcite, chlorite, and uralite, with a little biotite, were also 

 observed. 



The augite is widely altered, but frequent cores and crystals show that 

 it is a pale-pink titaniferous variety. Uralitization, the first stage of the 

 alteration, is effected in two steps — first to a fibrous brownish-yellow, 

 faintly pleochroic, and strongly birefringent amphibole, and then to a 

 bluish-green pleochroic product. The final stage is the change of the 

 uralite to chlorite, with a few minute flakes of biotite. In a few cases 

 there is direct chloritization of the augite, with accompanying separation 

 of a little iron-ore. A little of the biotite in the rock seems to be a product, 

 along with iron-ore, of magmatic resorption of the augite. 



Relations of the Rocks. 



The granites have already been briefly dealt with in connection with the 

 general geology of the district ; from these the gneissose granites cannot be 

 separated . 



The diorites, syenite-porphyry, and probably the dolerite are directly 

 connected with the granite intrusion, and probably not long posterior to 

 it, for they were subjected to the same erosion as the granites before the 

 deposition of the coal-beds. 



The lamprophyres must probably be correlated with the similar rocks de- 

 scribed by Bell and Fraserf from the Hokitika district, and furnish reliable 

 data for the determination of the age of the series, since in the Blackwater 

 Valley they intrude the basal beds of the bituminous-coal measures. 



Descriptions of similar rocks from Mount Tapuaenuka district, Marl- 

 borough, and notices of some from the Reefton gravels (per Dr. J. Hender- 

 son), have appeared lately in a paper by Dr. J. A. ThomsonJ " On the 

 Igneous Intrusions of Mount Tapuaenuka, Marlborough." 



Acknowledgments. 

 In conclusion, the writer wishes to acknowledge gratefully the kindly 

 help that he has received from Dr. J. A. Thomson, particularly in the deter- 

 mination of some of the minerals, and also to thank Mr. P. G. Morgan, 

 Director of the Geological Survey, for his courtesy in allowing this paper 

 to be presented. 



* " Note on the Geology of the Country about Lyell," 1889. 

 t Bull. No. 1 (n.s.), N.Z. Geol. Surv., pp. 82-84, 1906. 

 % Trans. N.Z. Inst., vol. 45, pp. 308-15, 1913. 



