Art. LIX.—Notes on the Geology of the Bluff District.
[Read before the Southland Institute, 21st January, 1886.]
Now that the Bluff is becoming a sea-side resort to the inhabitants of the district, as well as a place of call, and often of a little detention, to strangers passing to and from the Australian Colonies, it may be interesting to some to know a little of its geology. The first thing that will attract the attention of the visitor who has an eye to the rocks will be the ragged slaty strata standing on edge, and striking N.W. and S.E., exposed between low and high water, and in some places considerably above, and flanking the hill from the pilot station to the jetty. These, according to Dr. Hector, belong to the Devonian period, and to the great series of palæozoic rocks that form the backbone of New Zealand, or, in other words, the axis of the great mountain system extending from Auckland to Stewart Island. On closer examination, these argillaceous slates are seen to be, at least near the jetty, interstratified with bands of syenite, or granite, from a few inches to several feet thick, becoming more granitoid towards the base of the hill, which is a solid mass of syenite. This would almost lead one to suppose that the hill itself had once been a mass of slate formations of similar age, and that it has been granitized by metamorphic action, and that, at the present junction with the slates, the bands of interstratified granite are only the more silicious layers which have become granitized; while the more argillaceous layers have withstood the dying-out metamorphic action.
This appearance Captain Hutton says he is “positive is fallacious,” and holds the opinion that the whole range from the New River Heads to Ruapuke is an immense dyke of intrusive syenite. I am not aware that Dr. Hector gives an opinion on this point; but from the fact that in his Geological Map it is coloured as metamorphic, instead of true granite or volcanic, it would appear that he inclines to this view. It may also be observed here that Mount Anglem, and the northern half of Stewart Island, is also coloured in his map as metamorphic, while the southern half of the island is coloured as true granite, the same as the West Coast. There can be very little doubt, however, that Captain Hutton's view is the correct one, and that both Mount Anglem and the Bluff Range are of volcanic, or at least of eruptive, origin. Conclusive proofs of this are seen in following round the beach from the Bluff to the Greenhills railway-station. For a mile or two the interstratified bands of syenite or diorite are parallel with the slate and the base of the
hill; but as soon as the range trends to the north, the outlying dykes strike still parallel to the base of the hill, but almost at right angles to the strike of the slates, cutting them transversely, and crumpling and contorting them in every conceivable way, just as a wedge driven into wood transverse to the fibre would bend and crumple the parted ends towards the right or left. Many instances of this are seen where the slate beds are cut at different angles by dykes of syenite, or diorite, from 4 to 20 feet thick, and bent by the intrusion of the latter sidewise in curves, in some instances almost semicircular.
In the débris of these dykes I found two large crystals, apparently of amphibole; one a thick hexagonal prism about 7 inches long, and as much or more in circumference, dark greenish-grey, and rough on the faces; the other about 4 inches long, also hexagonal, but instead of ending in a pyramid its ends consisted of only two planes meeting at an angle of about 60°, but I did not measure it, intending to examine it more carefully at home. This, unfortunately, I was precluded from doing. As they were somewhat heavy to carry, and I had a day's walk before me, I put them aside, meaning to get them on my way home. On my return the tide had risen higher than my calculations, and had taken temporary charge of my crystals; and notwithstanding that I have twice sought for them since, I have not been able to pick them up again.
In these dykes also blade-like crystallizations are not rare, of large size, and sometimes ending in an imperfectly-shaped four-sided pyramid. Small rough crystals are also frequently observable of undoubted hornblende or augite. All these could not have been the result of metamorphism, and prove conclusively that the range itself, of which these are but the outliers, must be a truly instrusive mass.
The age of the syenite, or, in other words, the period of intrusion, is the next question: and this can only be inferred from the characters of the rock itself. In many respects these are quite peculiar. It is heavily charged with sulphides and bisulphides, and so full of magnetite that a piece of the size of the hand will, in many cases, deflect the compass-needle 8° or 10°. The whole mountain is an immense magnet; and, in walking over it, the needle is constantly varying both in declination and dip. Iron is therefore present in far larger quantity than is usual in ordinary granitic rocks. Copper is present in every specimen I have tested, and often in considerable quantity. It is not at all improbable that a workable lode may yet be found of this metal at some of the points of junction with the slate. Manganese occurs plentifully in the detritus on the shore, from the wearing down of the rock by the sea. Black ironsand, auriferous and platiniferous, occurs under the same conditions so plentifully that it has been profitably washed for
gold. Molybdenum, tungsten, tin, antimony, and arsenic are also found in small quantities in many parts of the range. The great quantity of iron and sulphur alone would argue a younger age than that of ordinary intrusive granite. The argillaceous slate through which it has burst is, as has been said, according to Hector, of Devonian age, and, according to Hutton, of carboniferous; and corresponds in this respect with the formation of Longwood, the Takitimos, Lake Te Anau, and the south of Lake Wakatipu. As there are no fossils, this can only be inferred from the lithological character of the strata. There is, however, little room to doubt that it is at least not younger than the Devonian. The outburst must, therefore, have taken place posterior to this period, but probably at no great distance of time, geologically speaking.
It is probable that the basin of the Southland Plain, now filled up by younger formations, was formed at the time of this outburst, and that the elevation of the Bluff Range was at the expense of Southland, by the extrusion of material in a plastic condition from under the surrounding district. There is evidence to show that strata may become plastic at no great elevation of temperature in many parts of the Hokonui District, in formations ranging, according to Hector, between the Permian and the Cretaceous. In many places, where there is not the least sign of any volcanic agency, patches occur, often not more than an acre or two in extent, of true trap rock full of small round boulders, and rock of a basaltic character, which must have resulted from the ordinary strata becoming plastic through chemical agency, and presenting all the appearances of an incipient volcano on the smallest possible scale.
The Cannon Ball sandstone of the Bastion and the Otapiri (so named by the officers of the Geological Department) seems to have originated in a similar way. If this be so, it is quite conceivable that the ancient strata under the Southland Plain, underlying the great Silurian and Devonian period, had, from chemical agencies, become plastic on a large scale, and, under pressure from the slow evolution of gases, had ruptured the overlying strata that imprisoned the sulphurous semi-fluid mass at the weakest places—in this case, along the line of the Bluff and New River Ranges—and by exuding a great viscid drop-like irregular excrescence, formed, on solidifying and recombination of material, the present range of granitic hills, extending from Ruapuke to the New River Heads.
Whether this hypothesis will fit in with all the facts, or not, must be left to the judgment of observers. It may at least serve till a better is found. Between the Greenhills quarries and the Mokomoko, the slates flanking the syenite are dark-blue, and of a fine compact texture, intersected here and there by veins and dykes of white quartz, which, being in the vicinity of much
finely-crystallized iron pyrites, are likely to be auriferous in parts. Indeed, alluvial gold can be obtained in small quantities almost everywhere along the flank of the range. The beds are here, as at the jetty, quite perpendicular, and even more contorted and crumpled. As they recede from the range they flatten out to a gradually-decreasing N.E. dip, and change in character to a massive indurated sandstone of a greenish colour, often studded with splendid crystals of iron pyrites that stand exposure for a long time without rusting.
It is in these blue compact slates that fossils are likeliest to be found. I have often seen what I took to be fragments of shells, but have never been able to prove conclusively that such was the case; but from the abundance of lime in the composition of these blue slates it is almost certain that shells were embedded with them, and may still be found where the conditions of preservation are most favourable. They are not so much metamorphosed as might have been expected from their close proximity to the granite, and in some places fossils are quite likely yet to be found. The Bluff Harbour itself seems to have been at no distant date a freshwater lake. The floor of the harbour is a soft bluish-green very friable sandstone, scarcely more than hard-pressed sand with a little clay in it, and highly micaceous. There is not even a spectroscopic trace of lime in it, which must have been the case had it been a marine deposit. There is a good deal of sulphur, as sulphide of iron, which sea-water would have decomposed. There is also timber, quite fresh, and apparently in situ, from the roots being dredged up by the dredging-machine, with the embedding clay still adhering to the curly gnarled roots as naturally as if the tree had been pulled out of the ground on which it grew. The timber dredged up was evidently that of rata (Metrosideros lucida), which is still abundant in the vicinity.