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Volume 32, 1899
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Art. XXIV.—Castle Rock, Coromandel.

[Read before the Auckland Institute, 28th August, 1899.]

The subject of these few notes will be readily recalled to mind by those who have had occasion to travel within sight of the northern portion of the Cape Colville Peninsula. The abrupt manner in which this peak rises from the main range, and its extremely castellated appearance, tend to make it the most salient feature in an otherwise featureless landscape. Situated some five miles south-east of Coromandel Township, and in a low saddle on the main range, here 1,250 ft. in height, it reaches an altitude of 1,724 ft., a height insignificant in itself, but sufficiently striking when the last 400 ft. of ascent in sheer on three sides.

To the casual observer the appearance of the peak probably conveys a suggestion of Titanic agencies that, in another quarter of the globe, have once again attempted to pile Ossa on Pelion. And, indeed, it must have been a mighty convulsion of nature that heralded its appearance; but at the same time it is to be remembered that the present form of the rock is not that which it assumed at its birth, but that into which it has been moulded by the action, during the ages, of the sculpturing-chisels of nature—the rain and the winds.

Though notable in itself, Castle Rock is much more interesting as being the most prominent feature of an extrusion of igneous matter that extended for miles across the country, from north-west to south-east, breaking indiscriminately through Palæozoic slates and slaty shales and Tertiary andesitic lavas and tuffs. Its most northern extremity is at Kiko-whakarere Bay, to the north-west of Coromandel, from whence it crosses into the township, being met with in the Kathleen Crown and Blagrove's Freehold Mines. Here also is a lava-flow that probably issued from the fissure, flowing to the west, and covering the older andesites in the Kathleen Mine to a depth of 158 ft. Further to the south-east the dyke is obscured by thick Pleistocene alluvial deposits, but reappears on the low foot-hill north of the Tiki Creek at an altitude of 550 ft. In the bed of the Tiki Creek the characteristic grey hornblende trachyte appears about half a mile beyond the sawmill, and then, still pursuing its south-east course, crosses the Pukewhau Track at an elevation of 770 ft., and is here about 2 chains wide. It next reappears on the main range as a knoll 1,250 ft. above sea-level, and a mile further to

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the south-east culminates in Castle Rock, 1,724 ft. in height. From the top of this peak the southern prolongation may be traced for several miles. Exactly how far south the dyke reaches I cannot say, the only satisfaction derived from an attempt to solve this question being that of spending a miserably wet night in the bush. This much is, however, certain: that the dyke extends in a fairly straight line for nine miles, and may possibly reach to Sugar-loaf Hill, in the Waiwawa Creek, a distance of fourteen miles from Kikowhakarere Bay.

The life-history of Castle Rock, or Motutere, to use the more euphonious native name, may be here briefly sketched. It owes its birth to the same agency that covered the whole of the Hauraki Peninsula area with its immense deposit of igneous rocks—viz., the folding of the Palæozoic slates in a direction parallel to the protaxis of New Zealand. From the concomitant line of weakness welled forth immense flows of lava similar to, but lesser in degree than, those of the Hawaiian Islands at the present day. After the deposition of the Upper Eocene andesites a period of quiescence ensued, to be broken in Miocene times by volcanic outbursts at Coromandel, and on the east coast from Port Charles to Whangapoua. That these outbursts were very different in character from preceding eruptions is clear from the nature of the ejecta, and from the numerous dykes that everywhere radiate from the foci of eruption. For both the breccias and the dykes of this period indicate a superabundance of steam, in the former case by the propulsion of débris from a crater, and in the latter case by the rending and fissuring of the adjacent rock.

In the case of the Castle Rock dyke, the centre of eruption was probably to the west of Kikowhakarere Bay, for at the south end of this bay occur true lava-flows of an identical rock. Moreover, the coarser breccias of Beeson's Island, a little to the south, are petrologically identical with the rock of the dyke and of the lava-flow, thus pointing to a common origin.

In few places along its course does the dyke show greater resistance to the weathering agents than the enclosing rock. At Motutere itself the rock is seen to be horizontally columnar, the columns being about 10 ft. in length and 1 ft. to 2 ft. in diameter. Additional evidence in favour of slow local cooling is furnished by the superior hardness and porphyritic nature of the rock at this place.

In hand specimens the rock is grey, with large porphyritic hornblende and feldspar crystals. The specific gravity is about 2.6. Under the microscope the base is seen to be feldsitic, apparently the result of devitrification. The feldspars are both plagioclase and orthoclase, the former

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perhaps predominating. Length, up to 2 mm. Carlsbad twins are common among the orthoclase feldspars. Plagioclase feldspars twinned polysynthetically, but not numerously. Amphiboles, light-green and porphyritic, reaching to 10 mm. in length. Many show strong resorption borders. Magnetite common. Free quartz sparingly present. The following is the determination by chemical analysis:—

[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]

SiO2 58.50 per cent.
FeO 17.67 "
Al2O3 5.64 "
CaO+ 3.86 "
MgO 1.09 "
K2O 7.22 "
Na2O 4.17 "
Loss on ignition, &c. 1.60 "
99.75 "

From the above characteristics, I have named the rock a hornblende trachyte, but it must be confessed that it stands petrologically on the border-line between trachytes and andesites.

Before concluding these notes I take the opportunity of drawing attention to a point that, so far as I am aware, has escaped the observation of geologists who have dealt with the igneous rocks of the Hauraki Peninsula. Though the Thames andesites have often been compared with those of Transylvania, and of the Comstock region of North America, yet the closer parallel drawn between the rocks of the two above-mentioned localities, and also of North Germany, has not yet been applied to the succession of igneous rocks on the Hauraki Peninsula. So struck was Von Richthofen by the almost invariable order of succession of volcanic rocks that he enunciated the following as a natural law: The first-ejected lavas in a district are the andesites. Succeeding these are trachytes; and in the final stages of eruptive action the acid rhyolite lavas and the basalts are poured forth. Now applying this law (to which, indeed, there are many exceptions) to the rocks of the Hauraki Peninsula, we find that the first deposits were the auriferous andesites, mainly developed on the western slopes of the range. In Miocene times we have the trachytic breccias of Beeson's Island and of Port Charles, &c., typically developed in the localities named, and we have also in this period the trachyte dykes of Castle Rock and Torehine, and finally, to complete the parallel, we have in the rhyolites of Tairua and the southeastern portion of the peninsula the youngest of the volcanic rocks occurring in the area under discussion.