Stewart. — Aeration of the Auckland Lava-beds. 285 



indraught had been observed before. He found that it had, especially in 

 the Mount Eden district, and that sometimes the current was reversed, 

 or from the pit into the scoria, the smoke from the blasting-powder clearing 

 out of the pit with a downdraught. In a drainage-pit which has lately 

 been sunk in the Township of Grand Park, a mile south-east from the Epsom 

 depot, a band of loose rock was passed through, from which a decided 

 draught of air came into the pit, but in volume not to be compared 

 with that experienced at the Epsom depot. 



The facts as above stated seem sufficient to warrant the conclusion 

 that all the permeable mass of the volcanic formation in the County of Eden 

 is subject to constant aeration. From a sanitary point of view, the signi- 

 ficance of this fact cannot be overrated. It seems to afford a satisfactory 

 reason to account for the continued purity of the western and Onehunga 

 springs, notwithstanding the fact that Onehunga has been settled for about 

 sixty-two years, and Mount Eden district for about forty-five years. The 

 population of the volcanic lands of Onehunga, Epsom, Mount Eden, and 

 One Tree Hill at last census was 14,970, settled on an area in which there 

 is not a yard of surface stream in the ordinary sense of the term, and up to 

 the present no system of drainage excepting that of natural or artificial 

 drainage-pits. On these volcanic lands at least three-fourths of the rainfall 

 sinks into the ground, and certainly one-half passes into the vast mineral 

 sponge, which term correctly describes the volcanic formation of the isthmus. 

 The rainfall carries with it all sewage and liquid waste, and, if no purifying 

 reaction existed, the effluent at the springs would long ere now have been 

 quite unfit for domestic use. Without going into the records of chemical 

 and bacterial testing, the particulars of which are not at the writer's com- 

 mand, it is sufficient to observe that the water yielded by the Onehunga 

 springs maintains in all essential points the same high standard as from 

 the first. 



It must not, however, be deduced from this that it would be wise 

 to trust to a continuance of the purity of the springs, without a regular 

 system of drainage, at Onehunga, where a very considerable area of the crust 

 is comparatively shallow, and where an accidental exposure of any of the 

 larger fissures or cavities in the rock might lead to direct and rapid pollution. 

 It must have been from some such cause that nearly fifty years ago the 

 writer saw the main spring in Onehunga discharging in full volume red 

 muddy water. But so long as the aeration as observed in the higher parts 

 of the isthmus continues, the many millions of cubic yards of permeable 

 strata which form the gathering-ground of the springs can be considered 

 as nothing less than a gigantic aerobic filter, in which the soakage from such 

 as the Epsom depot anaerobic tanks cannot flow more than a few hundred 

 yards before it is thoroughly oxidized and rendered fit to mingle with, and 

 be carried by, the discharge of that vast mineral sponge at the two great 

 main springs and the numerous smaller ones that flow from the lava-beds 

 which border high water on both shores of the isthmus. 



