Art. XXXVIII.—Description of Lava Caves at the “Three Kings,” near Auckland.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 2, 1869, Page 162

Art. XXXVIII.—Description of Lava Caves at the “Three Kings,” near Auckland. By James Stewart, C. E., Assoc. Inst. C. E. (With Illustrations.) [Read before the Auckland Institute, September 20, 1869.] It is well known that the various lava beds, near Auckland, are cavernous; caves are found of various dimensions, both in the older, and more recent lava streams, although in the former, the surface soil, washed in by floods, has partly, or wholly, filled many of them up. Many of these known caves are of considerable size, but by far the most extensive in the district, are those, the subject of this paper, situated in the great scoria bed lying to the north-west of the hills called the “Three Kings.” Some months ago, when a number of the members of this Institute met at these caves, for their exploration, it was seen that they presented some points of interest, and in their relative situation, a little intricacy. As these could not be investigated by a cursory examination, and as it was possible that the result might prove of some value, otherwise than by satisfying mere curiosity; I was induced, in conjunction with Mr. Kirk, to undertake a survey as much in detail as circumstances would permit. The result has shown the relative positions of the caves in this group to be not a little curious, as they branch off from, and underlie each other, to some extent. The survey occupied parts of two days, in all about ten hours. Bearings were taken underground, and marked out on the surface, and regular courses of levels were then run above and below ground, the sectional dimensions of the caves being roughly noted at numerous places. The accompanying plans and sections delineate the positions and levels; I have endeavoured to make the plan as distinct as possible, without the use of colour. (See Pl. X.) The number of distinct caves in this group, at present explored, is four. They form two sets, of two each. The two pairs have a remarkable resemblance to each other, in some distinctive features. The main pair is marked as the north-western, and the other as the south caves. In both cases, the branches return backwards, at low levels, and they just escape communication with each other. The main cave is generally straight, but of very irregular section, the roof has fallen in many places, and encumbered the floor with large blocks; at one place, near the mouth, it has broken through to the surface; it is clear that at this part the original thickness was not much over a foot. The main branch-cave is, for a great part of its length, in its original state, very little stone having fallen from the roof. At the extreme end, however, some has fallen, and much soil has been washed in. In section this cave is extremely irregular, but in most places very interesting, from the original form having been preserved, and affording, to my mind, convincing evidence as to the cause of formation. This branch underlies the main cave in a singular manner, having a thickness of rock, between, of eight or nine feet. Both caves have several small branches or chambers. The length of the main cave of this pair is 351 feet, and of the branch, from its entrance, 209 feet. The widths vary from thirty feet to six feet, and the height from seventeen feet to less than four feet. The south cave is much fallen in, and was not minutely measured. It may have been about 200 feet long, originally, but only about ninety feet (in three divisions) are still complete. At its present most northerly entrance, the branch returns at a low level, and is 112 feet long. This branch is by far the most perfect of the caves, having, excepting near its entrance, a regular and natural floor, and terminating in a fine chamber with a domed roof. The caves seem to be all well ventilated, the air, though extremely

Sketch Plan of caves at Three Kings near Auckland

humid, is quite fresh, perceptible currents of air are at all times existing in the more contracted passages, and during our last visit, when a strong breeze was blowing outside, the draughts in these places were so strong that candles could not be used without shelter. This was especially observable in the contracted parts of the north-western cave and branch, in which the current was flowing inwards, in both cases, but in directions diametrically–opposite; while in the branch of the southern pair, the current was outwards, but only near its outer extremity, being evidently fed by the other branch, which slightly overlaps it about forty feet from its entrance. No visible communication however was observed. The positions and directions of those strong air-currents are marked on the plan by arrows. Regarding the origin of these caves, it seems quite evident that the molten lava has found some outlet after the surface has solidified. Some very interesting traces of this process are visible, especially in the two branches. The floors are covered with curved corrugations, expressive of solidifying while in a state of slow motion, and in some cases the walls seem to have settled down a little, and squeezed the semi-plastic floor into long ridges and furrows. It seems probable that the whole group was formed from one molten stream, and not at different times from different levels. The upper caves, most likely, were formed first, and, after the floor had become firm, the still-fluid lava below seems to have found some passage downwards, somewhere near the abrupt descent or precipice in the main cave, thus forming the branch. On the face of this precipice, evidence can be traced of the descent of a portion of lava while in an almost solid state. Indeed, at this particular place, everything points to the probability of the lava having found its way into the hollows of some older lava stream. The occurrence of regular chambers, and minor branches, may be explained in the same manner, as the hotter and more fluid lava, collecting in pools, would be tapped and drained away. The roof, where in its original state, is also very suggestive of the above theory. It has all the appearance of some plastic material pulled asunder, having solidified in irregular guttæ. The percolation of water has, in some places, precipitated, on the surface of the roof, a mineral, generally white, but having, in several places, a red or green tinge. I am not aware of the nature of this mineral, but it is probable that lime, which enters in small quantities into the composition of our scoria, is the chief ingredient. Such is a brief and imperfect description of an interesting part of the Auckland scoria beds. The survey was attended with many difficulties, owing to the general roughness, and contracted dimensions of some of the passages, and the use of the level and staff in these, was anything but easy. On these grounds it is hoped that all imperfections will be excused.