44 BASIC AND ULTRABASIC IGNEOUS ROCKS— BENSON. 



southwesterly continuation of the same zone of schists in the adjacent survey-district. The 

 sections published indicate that in one region it seems to invade a broken anticline, slightly 

 overturned toward the west. The upper limit of the possible age of these beds is fixed by the 

 occurrence of pebbles of serpentine in the " Miocene" beds overlying the ancient rocks (Morgan 

 '08). The occurrences were especially mentioned by Suess (IV, p. 566) when discussing the 

 origin and significance of the green-rocks. Their relationship to the adjacent masses of gneissic 

 diorites is not yet quite clear. Some authorities (Morgan '22) have considered that the latter 

 are probably pre-Ordovician, but there is a strong petrological similarity between them and the 

 gneissic diorites of the southwestern portion of the island, with which are associated a series of 

 ultrabasic intrusions. During the last decade there ultrabasic and accompanying basic in- 

 trusions have been generally considered to have been injected during the late Mesozoic orogeny, 

 but more recently Park ('21) has declared his belief that they were injected during the early 

 part of the Triassic, accompanying the orogeny he believes to have occurred then, and he 

 considers that all the other occurrences in the South Island are probably of the same age. The 

 following are the known masses of basic and ultrabasic rocks in this southwestern development : 



North of Big Bay and Milford Sound a large mass of peridotite occurs as a lenticular patch 

 within the gneisses, forming the Olivine Range more than 8 miles in length and over 2 miles in 

 width (Park '86). In the vertically-dipping schists at the Cow Saddle, northwest of Lake 

 Wakatipu, Marshall ('06) found a laccolitic complex of which the eastern half consisted of 

 lherzohte, with a small nucleus of dunite near its western side. This was immediately followed 

 passing to the west, by pyroxenite, gabbro, and diorite in succession. Unfortunately, he was 

 prevented from examining in detail this interesting mass which thus bears a very considerable 

 resemblance to the complexes in the Urals. Not far from this are the masses of mica-norite in 

 the Bryneira and Darran ranges, the former also associated with gabbro and serpentine. These 

 occur in the northern portion of the great batholith of diorite which forms the southwestern 

 portion of the island or "Fiordland." This mass is gneissic on its western margin where it 

 contains bands of amphibolite, and invades more or less concordantly, a series of schists which 

 pass westward into Ordovician slates etc. On the east it is less gneissic or is even massive, and 

 invades Permian or Lower Triassic greywhackes. It is in turn veined by aplite or pegmatite 

 often garnetiferous, and by massive granite. (Speight '10, Benson '21, Park '21.) Gneissic 

 norites occur on the western margin at Bligh Sound, and at the entrance to Milford Sound 

 (Anita Bay). Marshall ('05) observed among the schists a mass of peridotite containing much 

 magnesite, and at times completely crushed. Southeast of the main batholiths are smaller 

 outlying masses of norite such as that at Orepuki (Farquharson '10) and at the Bluff at the 

 extreme south of the island. The last mass is associated with amphibolite (Wild '12), while 

 that of Orepuki invades greywackes of probably Permian age. The southernmost occurrence of 

 serpentine known in New Zealand is the small sill near Cromwell which invades mica-schist 

 (Park '08). Concerning the age of this schist opinions have varied between those which claim 

 for it an Archaean, or Paleozoic age, and that which would place it among Mesozoic formations. 



Southward from here we may note the occurrence of gabbro in the Auckland (Speight '09) 

 and Campbell Islands (Marshall '09). In South Victoria Land the dominant feature of the 

 igneous geology is the occurrence of an immense series of sills of quartz-dolerite very like those 

 of Tasmania (David '14, Benson '16), and cutting obliquely across the almost horizontally- 

 bedded sandstones of Permo-Carboniferous and possibly Devonian age. The intrusion was not 

 apparently associated with faulting, and the supply-channels for the sills have not been noted 

 (Ferrar '07, David '14). Some gabbros were found, but not in, situ, and were almost certainly 

 derived from the folded complex beneath the sandstone. 



