(b). Intrusions in the Coal Point Sediments (? Miocene).
Three important intrusions were located in sediments of the Coal Point Series (? Miocene) not far from Kerr Point and North Cape. One of the largest is a sill of quartz-augite diorite (N.C. 5), 300 ft. or 400 ft. in depth, which intrudes into fine-grained sandstones near the base of the Coal Point Series as developed at the east margin of Tom Bowling Bay. Its lower surface is distinctly chilled and accords perfectly with the stratification of the laminated sandy mudstone beneath it. The mudstone exhibits no sign of metamorphism other than slight marginal induration at the western margin of the sill near the mouth of Tawakewake Creek.
In hand-specimen the diorite is a compact fairly finely crystalline rock which weathers deeply to a sandstone-like mass which can only be determined as igneous in relatively fresh examples. It contains about 45 per cent. of feldspar (acid labradorite), 15 per cent. of quartz and a very small quantity of micrographically intergrown quartz and orthoclase; the balance of the rock is mainly pale green augite with a moderate quantity of uralite, some greenish-brown hornblende, and magnetite in normal quantity.
A second member of this group of intrusions outcrops on the east coast a little north of Waikuku Flat. It is a mass of micrographic quartz diorite (N.C. 23) similar to the preceding one, but its relations to the intruded strata are not discernible and its thickness is greater than that of the sill just described. In its weathered condition at its outcrop it so closely resembles a sandstone as to have been described as such by earlier workers. In section the rock is dominantly feldspathic, the feldspar (acid labradorite) being in stout lath-shaped crystals which are accompanied by about 15 per cent. of quartz, and a lesser quantity of chlorite which is associated with occasional ragged remnants of brownish green hornblende and a little magnetite. There are, in addition, a fairly large proportion (10 per cent.) of micrographically intergrown quartz and feldspar which enwrap earlier minerals (see Fig. 17), and a few needles of apatite.
The third intrusion of this period intersects heavy basal conglomerates of the Coal Point Series outcropping on the coast 1½ miles west of North Cape. It proved to consist of a most interesting coarse-grained doleritic rock (N.C. 30) which includes alkaline varieties amongst its feldspars. These latter minerals constitute 50 per cent. of the rock, and are generally in lath-shaped crystals about 1 mm. in length; basic labradorite is the dominant variety, but there are also occasional crystals of albite, others of probable orthoclase and several
fairly large anhedral ones of an unidentified alkali feldspar closely crowded with vermiform growths of chlorite. It has a refractive index considerably lower than that of Canada balsam, and shews faint, highly irregular sub-microscopic twinning reminiscent of that of anorthoclase. An additional section was made from the only duplicate available, but failed to shew any trace of the mineral in question, and it is therefore believed to be present as xenocrysts. There is a possibility that this feldspar is albite, for lath-shaped crystals of this mineral were detected by their low refractive index and positive optical character, but the irregular outline of the (?) anorthoclase suggests that its origin, in any case, differs from that of the more tabular albite. The analysis of this rock (No. 2 of Table 1, p. 131) shews its relative basicity. The content of alkalis, though not high, is above the average and is clearly due to the presence of modal alkali feldspar.
Immediately west of Huka Creek, Tom Bowling Bay, there is a massive intrusion which outcrops at the shore-line for several hundred yards, and which, though intersecting the Older Volcanic Series, has been injected along the course of a fault which dislocates the basal tuffs of the Coal Point Series in this locality. This mass thus belongs to the same period of intrusion as the others of this section and is, moreover, a micrographic quartz-augite diorite (N.C. 59) closely similar to other intrusive rocks of this period. The micrographic material is in comparatively small amount, whilst the augite is accompanied by a very little brownish hornblende.
In view of the fact that all four of the intrusions just described definitely post-date the Lower Beds of the Coal Point Series, it may be affirmed with safety that they were injected in later Miocene times, during the period of igneous activity which gave rise to semi-basic lavas from which the andesitic conglomerates of the Coal Point Series were derived.