(4) The Pulpit Rock. (Plate 34.)
This is the last of the four large trachytoid intrusions to be mentioned. It stretches across the upper part of the French Farm Valley for over a quarter of a mile, and its summit lies at a height of about 1500 feet. The stream occupying the floor of the valley has cut deep into the rock dividing the exposed portion into two unequal
parts but not exposing the base. The southern part forms a bold landscape feature and rises precipitously for 250 feet above the ground fronting it; this part is known as the Pulpit Rock. The northern part is thinner and less striking; it exhibits a closely-spaced and parallel lamination, perhaps due to chilling by the underlying basalt when the mass was injected. The surface deposits have so obscured the contacts that a satisfactory conclusion cannot be arrived at concerning the form of the channel up which the magma came or what was the original shape and extent of the mass. This uncertainty as to its original form is accentuated by the erosion it has suffered during the formation of the caldera, and its exposure to the light of day may be due to that cause. It appears to be oriented on approximately N.N.W.-S.S.E. lines, and it probably extended in the former direction across the top of the ridge dividing French Farm Valley from the western branch of Okuti Valley; for, after an obscurity due to the covering of soil, trachyte in position appears for nearly a chain close to the old yards on the summit of the ridge (height 1,815 feet), and scattered fragments continue on the surface for more than five chains further into the upper part of Okuti Valley. This rock is a variant of that of the Pulpit Rock, and may be a marginal facies of it; it is a dark-coloured, very hard, fine-grained aegerine-augite trachyte.
The basalt flows above the main mass in the direction of Saddle Hill do not appear to have suffered any dislocation or warping from the intrusion of such a thick body of rock, and this suggests that it had flowed out at the surface as a bulbous dome and been covered up subsequently by material discharged as the cone was built up further. All the same it is doubtful how far away from it such a mass would cause warping in pre-existent flows. But if the exposure on the crest of the ridge near the yards does link up with the mass in the valley, then this explanation becomes unsatisfactory, especially as the exposure near the yards is interstratified with basalt flows at a higher stratigraphical level, indicating an upward transgression through them of the intrusive mass. The variation in facies can then be easily explained as due to the smaller size of the intrusion at higher levels or to more effective chilling of the margins.
However, the occurrence near the yards presents resemblances to another trachyte mass, which occurs on the crest of the ridge, height about 2,250 feet, about three-quarters of a mile to the south. This outcrops immediately above the Pulpit Rock, on the downhill side of the prominent escarpment of basalt and its underlying breccia which marks the northern flank of Saddle Hill; the dip of this basalt is between 5° and 7°. The trachyte outcrop extends along the crest of the ridge for about four chains. It reaches down the slope towards the Pulpit Rock for about a chain, when an undisturbed basalt in position cuts it off. The relation of the mass to the basalt of Saddle Hill is obscure; on the ridge to the north—that is, toward the yards, it is also cut off by basalt breccia and flows in position; while on the slope towards Okuti Valley the ground is soon covered by soil and debris, and the true extent of the mass in that direction cannot be determined, but basalt on both sides narrows the exposed width
to less than two chains. It does not appear to have any close connection with Pulpit Rock, though this absence of connection may be illusory. The form of the occurrence suggests that it is a small, independent intrusion extending down the slope of the Okuti Valley from just beyond the crest of the ridge, or perhaps an offshoot of the intrusion at Pulpit Rock, as suggested for the occurrence near the yards. It does look something like a flow, but if so, it is the solitary case known from the area. Although the connection with the Panama Rock is not obvious, I think it represents an off-shoot from that mass.
The exposed surface is coloured brown owing to the oxidation of the ferrous constituent, and is marked by denticulations like those of the trachytes referred to earlier. The rock is very hard, dark-coloured, and is a fine-grained aegerine-augite trachyte, without phenocrysts or with very small ones, and with a base composed of much augite in stumpy laths and alkali felspars in short, rarely lath shaped forms. This description applies to the rock exposed near the yards further north on the ridge. The two occurrences thus differ to some extent from the Pulpit Rock, but such difference may be due to marginal chilling or to the relative smallness of the intrusions or extrusions, if indeed they are actual flows; the latter suggestion I do not endorse.
It is possible that others of similar character may ultimately be located on the slopes of Saddle Hill.
Since writing the above, I have observed from the upper slope of Saddle Hill a mass resembling the Pulpit Rock in appearance and with similar orientation, outcropping in the upper part of Wainui Valley, but was unable to examine it closely. The large trachyte dyke, referred to on page 241 as occurring in this locality, meets it at its south-eastern end, and may be genetically connected with it.
On the northern side of Reynolds Gully, on the south-western slopes of Saddle Hill, there is a massive upstanding occurrence of basalt, only a few chains in length and oriented towards Onawe. At first sight it looks like an intrusion, but at the base of the northern face basaltic breccia shows underneath the exposed mass, so no doubt it is a remnant of a thick flow of basalt.
Analyses of the smaller trachyte intrusions, belonging either to the basement beds or to those of the cone, as well as one of a basic dyke near the neck at Onawe, are given in two articles by myself (Speight, 1923, p. 149; and 1940, pp. 71–2). The composition of these trachytes dykes is closely related to that of the massive intrusions.
The four main trachyte intrusions have much in common. They are all aegerine-augite trachytes with similar mineral and chemical composition, although they differ slightly in texture. All, except View Hill, have analogous minor surface features such as denticulations, and major features such as pronounced vertical partings. The flaky outcrops of the limited exposures of View Hill suggest that it might also have similar features if the surface had been similarly exposed to the weather. The intrusions are all short in length, of great relative width, and all but that at the Devils Gap dome-like in form. These resemblances point to a similar origin, and this has already been
Akaroa Caldera from the summit of Mount Bossu, view looking north; Wainui in the foreground; Duvauchelles Peak. View Hill, and Okains Peak in the distance.
Akaroa Caldera from the slopes of Rocky Peak, view looking south-east; Barrys Bay and Onawe in the middle distance; the harbour entrance in the far distance on the right.
Panorama of Little River Valley, taken from Te Oka Saddle, view ranging from west to north. Main valley in the middle distance; Western Valley beyond it in the distance; Puaha Valley behind the ridge in the middle; Okuti Valley in the foreground. The ridge in the left foreground is merely a bastion extending a short distance from the eastern wall of the valley. It will be seen that this landscape is entirely different from those shown in Plate 30.
Pulpit Rock, view looking south-east from Okuti Ridge near the yards; Saddle Hill to the right; the extension of the rock to the north-west hes under the face covered with serub.
Trachyte intrusion on the west side of Peraki Valley; Devils Gap at the top of the picture; trachyte bastion towards the floor of the valley below the Gap. View from the east side of Peraki Valley, looking south-west.
considered at some length in connection with Panama Rock, and perhaps the evidence of this well-exposed occurrence may furnish the clue to the origin of the others—viz., that they represent bulbous expansions, to use Stearns' term, of relatively short dykes, either extruded on the surface or they are laccolitic expansions at depth. In no case have I come across any evidence of alteration of rocks or faulting in their vicinity.
The formation of bulbous domes composed of soda-trachyte on the Island of Maui, Hawaii, is attributed by Stearns and Macdonald (op. cit. pp. 310–11) to differentiation arising partly from crystal settling of the heavier elements such as augite and olivine in the sub-surface magma chambers and partly from the transfer of the volatile constituents, particularly the alkalies, to the upper chambers feeding these domes The general similarity in the character of the igneous rocks and of the forms of the Hawaiian volcanoes to those of Banks Peninsula is interesting in this connection, and suggests that these intrusions may be attributed to similar causes
This explanation adopts and amplifies that given by Daly (1925, p. 79) when accounting for the numerous trachyte domes of Ascension Island and their association with its basalts. However, the forms of the Akaroa intrusions appear to resemble more closely those of the trachyte masses noted in the same author's account of Saint Helena (1927), as, for example, his drawing of Speery Islet (p. 53) and his account of Great Stone Top (p. 55) which he attributes (p. 91) to “emanation of the alkaline magma through a dyke fissure.” In the case of Panama Rock both the dome and the fissure are plainly visible, the latter indicated by the dyke.
The association of trachyte with basalt is so common, especially in the islands of the Pacific, that mention of particular cases is unneccessary. However, the hypothesis just referred to may serve to explain the formation of trachyte dykes as well as bulbous domes at various stages in the history of a basalt volcano. It is inconceivable that all the dykes were injected at one stage in the development of the volcano. If it be granted that they have been discharged from magma chambers at varying intervals, some coming out on the ground-surface and suffering erosion before they were covered up by subsequent eruptions, their infrequency at higher levels and their great number at lower levels will easily be explained. In the case of Akaroa I have not come across types intermediate between trachytes and basalts, but in the Lyttelton area the whole suite which would result from differentiation in a magma chamber are well represented—trachytes, trachy-andesites, andesites, alkaline basalts, and ordinary basalts occurring. Perhaps a more complete examination will show that these intermediate types occur at Akaroa.
Besides the intrusions just mentioned there are no doubt others awaiting location. It was not found possible to examine all localities, promising or otherwise, and I feel certain that other occurrences as interesting as the four major intrusions just referred to will be discovered eventually, and those already mentioned examined more thoroughly. They undoubtedly suggest complexities not hitherto suspected in what has up till now been regarded as a simple composite volcanic cone.