A glance at the map accompanying will show that this peak and its immediate neighbourhood is on the surface almost entirely composed of bostonite. The surface of the land is precipitous, and rises to Harbour Cone to form in appearance almost a typical denuded volcano and its bared solid pipe. the bostonite has a very great extent in this neighbourhood. It forms cliffs along the shore of the harbour to the east of Port Chalmers, and also tops the small hills surrounding the central peak. The central portion of the peak itself is composed of solid basalt extending down to the 900 ft. contour-line. Below this down to 750 ft. boulders of basalt cover the ground thickly, having been wedged off by frost and other natural agencies and rolled into their present position. On the northern slopes of the mountain an irregular area is occupied by a coarse breccia, very indurated, containing fragments of trachytoid phonolite and other rocks in large and small angular fragments. This is the Port Chalmers breccia, the greatest occurrence of which is on the peninsula on which Port Chalmers is built, which consists almost entirely of the breccia. The breccia also forms the upper portions of the small hill to the north-east of the main peak, as indicated on the map. The most important inclusion in the rock is large and small masses of alkaline syenite, showing that this rock must have enormous extent under the surface at an unknown depth, as the focus of the explosive eruption producing the breccia is somewhere near Port Chalmers.
The flanks of the mountain are pierced by numerous dykes. The position and nature of those occurring in the neighbourhood
of the mine are shown on the accompanying map. They are very varying in their nature, in this small valley four varieties being met with—two varieties of trachytoid phonolite, one of bostonite, and one of tinguaite. The large auriferous mass of alkaline syenite rises to within a few feet of the surface here.
Along the hill-slopes to the west there are two outcrops of Tertiary sandstone indicated on the map. One of these has been quarried, so that the dip and nature of the rock is plainly visible. It is surrounded above and below by bostonite. The sandstone dips about 5° west, and where visible is in no way contorted or faulted. The end of the drive down the shaft shown on the map is walled with a sandstone crumbling in the hands and glittering with scales of a golden-yellow mica.
In the valley in which the mine is situated a chip was obtained from a boulder, which under the microscope exhibited peculiar characters. This probably was derived from a dyke of the rock, the dyke now being covered with loam, as no trace of it in situ could be discovered. A description of the petrographical characters of the rock will be found under “Petrography.”
The first outflow in this area was undoubtedly the bostonite. The absence of any distinct flow-structures in it prevents any conclusions being arrived at as to its probable vent. It seems to have welled up and covered the neighbourhood with a deposit of enormous thickness. It flowed over a very highly uneven sandstone surface, as the outcropping of that rock at between 200 ft. and 300 ft., and its complete absence from the river-bed below, proves. It would seem that an original sea-cliff existed along that portion of the sandstone now outcropping. The top of this cliff now shows where the bostonite has weathered away.
Some time after the flow of bostonite the intrusion of the auiferous syenite took place. This intrusion is of large extent, since it occurs in fragments in the breccia thrown up at Port Chalmers as mentioned above. It probably forced up the sandstone in places, assuming dome-like prominences. This is probably the origin of the sandstone found in the drive in the mine, it being merely a portion of the main beds carried mechanically upwards. It is undoubtedly after the flow and consolidation of the bostonite that this intrusion took place, firstly because the bostonite has been altered along its junction with the syenite, secondly because this alteration is of small extent owing to the solid state of the bostonite.
Dykes were formed first of bostonite (see “Petrography”),
which was probably intruded into the hot bostonite. The heat of the bostonite kept this dyke in a molten condition long enough for the magma to act on the first-formed crystals, as described under “Petrography.” Then followed dykes of tinguaite and phonolite, in what order it is almost impossible to say. They were probably connected with the large eruptions of alkaline rocks on other parts of the Peninsula.
At a later stage followed an eruption of an entirely different nature. The basalt cap now topping the mountain is certainly the upper portions of a basalt-filled volcanic pipe, which communicated with the vent of a basalt-emitting volcano on the surface of the bostonite. After that enormous denudation went on and entirely denuded away any outflows of basalt which may have occurred. Some time during this period the explosive eruptions having their focus near Port Chalmers formed a deposit upon the partially denuded bostonite, which must have had a form then approximating to its present one, as a deposit of the breccia is found on the mountain-slopes. Since then more denudation has taken place. The breccia has, with the exception of the deposits shown on the plan, been entirely removed. A river has cut its passage through the flow, leaving the projecting hummocks now forming Quarantine Islands, which are entirely composed of bostonite. The hard pipe of basalt has resisted denudation while the bostonite all round it has been denuded away, thus giving to the mountain its present form. Thus, though the mountain has the typical form of a volcano and its projecting neck, it is far from probable that the materials composing its slopes have been ejected from its summit, but rather the white bostonite once formed a high plateau on the top of which was once a basalt-emitting volcano.
Thus, in this particular area, the order of outflow seems to have been—(1) bostonite; (2) intrusion of bostonite dykes; (3) intrusion of syenite mass, intrusion of tinguaite and phonolite dykes in unknown order; (4) outflow of basalt; (5) explosive eruptions producing breccia.
In giving the order of flow for the whole Peninsula, Dr. Marshall, in the work above referred to, writes—(1) trachite (bostonite); (2) basalt and nepheline basanites; (3) green trachytoid phonolite; (4) kenyte; (5) Port Chalmers breccia.
In the neighbourhood of Harbour Cone it seems probable that the phonolite and tinguaite dykes preceded the basalt, since the latter is remarkably fresh, while the former is very decomposed. The phonolite, however, lends itself to decomposition very readily, so that arguments based on relative decomposition are perhaps not reliable.