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Updated: May 2, 2025
Silica is the mineral with which the waters of this fountain are impregnated, and the substance which they deposit, as they slowly evaporate, is named siliceous sinter. Of this material is composed the mound, six or seven feet high, on which the spring is situated.
Amongst such eruptions of hot-water and steam we might expect the formation of siliceous sinter, and the deposition of sulphur and other minerals; nor will our expectations be disappointed. For here we have the wonderful terraces of siliceous sinter deposited by the waters entering Roto Mahana as they descend from the numerous hot-springs or pools near its margin.
The veinstone is pure quartz containing water in microscopical cavities, as in the quartz crystals of granite, but not combined as in the hydrous siliceous sinter deposited from hot springs. The lodes are not ribboned, but consist of quartz, jointed across from side to side, exactly like trappean dykes.
In May, 1887, the mountain of Tarawera, which rises to the north-east of Roto Mahana, and on the line of eruption above described, suddenly burst forth into violent activity, covering the country for miles around with clouds of ashes, and, pouring down torrents of mud, completely enveloped the beautiful terrace of sinter which had previously been one of the wonders of New Zealand.
Judging by their shape and tint I imagine that some of these incrustations are native crystals of the diamond and ruby, the sapphire, topaz, and emerald. In a deep recess or alcove on one side a spring of clear water gushed from the rock into a natural basin of sinter, enamelled inside and out with the precious opal.
The siliceous sinter is either quite white, of little specific gravity, and with a somewhat pearly fracture, passing into pinkish pearl quartz; or it is yellowish white, with a harsh fracture, and it then contains an earthy powder in small cavities.
Where, however, the temperature is high, some part of the deposit, even a little gold, may be laid down just about the spring in the deposits known as sinter, which are often formed at such places. In many cases the ore deposits are formed not only in the main channel of the fissure, but in all the crevices on either side of that way.
Indeed, it is quite at the boiling- point. The little mound, from the top of which the jet appears to rise, is composed of a substance named siliceous sinter, and is a deposit from the water of the fountain.
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