Wanaka Formation.

Distribution.—This formation extends in a broad belt, some 40 miles in width, through the centre of the Province. Commencing on the northern boundary, between Mt. Aspiring and Lake Hawea, with a breadth of 30 miles it gradually expands southward, embracing the lower half of Lake Wakatipu, the Dunstan district, and the valley of the Clutha River as far south as the Beaumont Ferry, when, turning to the eastward, it terminates before reaching the sea between Outram on the south, and the head of the north branch of the Waikouraiti River on the north, but appearing again on the beach at Brighton. Thus occupying an area of about 5000 square miles.

Rocks.—It is composed almost entirely of mica-schist, ranging from an argillaceous mica-schist, as at the Carrick Ranges, to a gneiss-like looking rock, as in the neighbourhood of Bendigo. Ordinarily, it is composed of very fine silvery-grey mica foliated with quartz, and it often has a waved or corrugated structure. Near Queenstown chlorite-schist is largely developed. At Mt. Alta, near Lake Wanaka, there is a quartzose-mica schist, composed of pinkish quartz and fine scales of mica in regular parallel folia),^‡ and in the Shotover a remarkable looking hornblendeschist is found, composed of irregularly foliated quartz and hornblende in about equal quantities; and as the hornblende often occurs in needle-shaped crystals lyiug across the plane of foliation, it gives the rock an appearance somewhat similar to that of graphite granite. Large crystals of iron-pyrites are found in this rock.

One of the chief characteristics of the rocks of this formation is the numerous laminae of pure white quartz, of considerable thickness generally found in them. This is no doubt the effect of extreme metamorphism, from which has resulted the eutire separation into distinct layers of all the superabundant silica that was distributed through the rock previous to metamorphism.

Corrugations.—The remarkable corrugation of much of the schist is not so characteristic of these rocks as the separation of the quartz into layers, for it is also occasionally found in the next (Kakanui) formation. These corrugations are generally supposed to be owing to the original rock, previous to metamorphism, having contained ripple marks, it being supposed that the foliæ have followed the curves of tho ripple mark in the same way as they follow the planes of lamination or of cleavage. But although this explanation may account for the phenomena in some places, it can hardly do so in Otago, partly because many of the corrugated rocks have evidently been clays which never carry ripple-mark, and partly because the corrugations, as seen in the beautiful sections in the Shotover, cover one another so accurately and regularly, though more than 20 feet iu thickness, that it is impossible to suppose they can owe their origin to ripple mark, which is always irregularly distributed through a rock. I think a better explanation can be found in the expansion by heat of soft rocks under great vertical pressure; the compression thus produced having been relieved by numerous small corrugations, instead of by fewer and larger contortions.

Position of Strata.—These rocks, although so old and so much metamorphosed, lie remarkably flat oyer their whole extent. They form, as has been already mentioned, a low broad anticlinal curve, rising to the north-west, and running through the province, but branching off at the Upper Taieri into two anticlinals, one running to the Waikouaiti Downs, and the other to Hindon, with a flat synclinal between them running from Sutton Stream to Blueskin Bay (see See. III). Thus on the southern and western side of the anticlinal, the rocks dip in the Shotover from 15° to 25°, N.W. to S.W; at Skipper’s, the dip is 30° to 50°, W; at Five Mile Creek, 40° S.W; at Queenstown, 25° to 45° S.S.W.; and at Kingston, it is 15° S.W. Further south the beds curve round more io the east, and the dip at Waipori is 25° S.S.E.; and at Outram, 35° S.S.E. On the north-east side of the anticlinal, the dip at the south end of Lake Wanaka is 40° N.E.; in the neighbourhood of Bendigo, from horizontal to 35° N.N.E.; and at Black’s, 10° N.E. Along the top of the anticlinal, the dip is variable, but generally to the south-east. For example, at the bridge in the Kawarau Gorge, it is 40° S.S.E.; at Clyde, 25° S.E.; at Alexandra, 20° S.E.; up the Conroy, 10° S.S.E. On the other hand the dip is very variable in the Carrick Ranges, and in the Dunstan Gorge it varies from 25° W. page 31to 28° S.W. Towards the east, where the main anticlinal splits up into two, the dip between the Lees and Deep Stream, is very slightly N.N.E.; in the Deep Stream it is 15° N.N.E, and in the Upper Sutton, slightly E.N.E. But at the lower end of Strath Taieri, the dip is 10° S.W.; and it again becomes north at Hydo and McRaes.

Foliation.—As the changes in the lithological character of these rocks take place at right angles to the plane of foliation, it follows that the foliation coincides with the original plane of bedding. This is further confirmed by the general geological structure of the district, for, as a general rule, the plane of foliation dips towards that portion of the next younger formation that lies nearest. The same might also have been inferred from the small angle of inclination that these schists have over a large extent of country. For if the foliation had taken place along planes of cleavage, it would have had the high angles of inclination which usually accompany that structure, due to the cleavage planes having been formed at right angles to pressure laterally applied.

Relation to underlying formation.—It has been already mentioned that no junction can be found in the Province of Otago between this and the Manipori formation; consequently it is impossible to say whether there is any unconformity between the two. This may perhaps be proved in Westland, as both the Manipori and Wanaka formations are stated to extend into it. ^*

Thickness.—As the base of this formation is not exposed it is impossible to ascertain its thickness, but it is certainly very great, tor between the Arrow River and Moke Creek it cannot be less than 50,000 feet. Its probable age will be better discussed after the two next formations have been described, for the rocks have undergone so much metamorphism that it is hopeless to expect that we can ever find fossils in them; although as plumbago has been found in the Dunstan and Wakatipu districts, it is evident that vegetable remains were once entombed in them.

Eruptive Rocks.—The almost entire absence of eruptive rocks in both this and the next formation is remarkable, and very difficult of explanation. I know but one small dyke of porphyrite in the Carrick Ranges.

Nomenclature.—As this formation has not yet received a name I have called it the Wanaka formation, for it surrounds Lake Wanaka on both sides. It is identical with the "contorted felspathic schist" of Dr. Hector.^†