284 Transactions. 



Art. XXXIV. — On the Aeration of the Auckland Lava-beds. 

 By James Stewart, M.Inst.C.E. 



\^Read before the Auckland Institute, 22nd November, 1909.] 



About forty years ago the writer had the honour of reading before this 

 Institute a paper describing certain lava caves at Three Kings, near 

 Auckland.* In that paper, inter alia, reference was made to the fact that 

 in the main pair of caves, upper and lower, currents of air were at all times 

 observed blowing inwards from the outer air, and on one occasion, when 

 a stifi breeze was blowing outside, the draughts were so strong in the more 

 contracted places that the candles used for marking the main points of the 

 traverse hues guttered and wasted away in a very short time. These cui-- 

 rents were observed through the whole ramifications of the caves, and even 

 at the extreme ends or in side chambers the flame of a candle showed de- 

 cided draughts into the more open beds of the lava. Only in one place — 

 viz., where the innermost end of one cave was found to overlap the position 

 of another — was any outward draught observed. As the entrance to the 

 main pair of caves at Three Kings is of considerable area — about 80 

 square feet, in fact — the velocity indicated that a very large volume of air 

 passed inwards ; and, although to the writer and his friend the late Mr. 

 T. Kirk, F.L.S., who was one of the exploring party, the phenomenon was 

 unexpected, no great importance was attached to it, as it was natural to 

 suppose that the circulation would be quite local, and quickly difiused 

 upwards through the more open patches of the rocky crust. 



Two years ago, however, the significance of this terrestrial circulation 

 of air was very forcibly called to the writer's notice. In the work of drain- 

 age of the Epsom depot of the Auckland Electric Tramways, situated 265 ft. 

 above sea-level, he had occasion to extend the depth of a drainage-pit, 

 sunk some years previously after consultation with the then Health Officer 

 of the district. It was foimd, however, to be better to construct a new 

 pit alongside the first one, which terminated in rock not sufficiently fissured 

 to carry off the greatly increased amount of drainage from the septic 

 tanks, &c., of the depot and adjacent show-grounds. The new pit passed 

 through 19 ft. of rich volcanic soil, then through 7 ft. of slightly fissured but 

 very hard bluestone lava. At the depth of 26 ft. a very open stratum of 

 yellowish-brown scoria gravel of large size was struck, and opened out for 

 about 2 ft. in depth. From this gravel a strong draught of pure fresh air 

 was blowing into the pit. So strong was the current that a match could 

 not be struck and lighted in it, and the workmen stated that a candle could 

 not be kept alight in the strongest part of the draught. The indraught, 

 as observed and noted by the writer,, was strongest from the south-west, 

 or from the direction of the Thi-ee Kings Hills, about a mile distant, 

 but it extended with varying force more or less round fully one-half of the 

 circumference of the pit. This inblast of air steadily continued until the 

 pit was finished and put into use. 



As these drainage-pits have become very common of late years in the 

 scoria lands, the writer set about inquiries as to whether or not such an 



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* Trans. N.Z. Inst., vol. ii, p. 162. 



