Art. XIV.—Notes on the Basin of Te Tarata, Rotomahana.
[Read before the Auckland Institute, July 6, 1868.]
On the 3rd of March last, in company with Colonel Haultain, Mr. H. Clarke and Mr. Traile, I visited the celebrated hot spring of Te Tarata at Rotomahana. As we crossed Lake Tarawera, in a canoe, large volumes of steam were seen issuing from the crater, but on reaching it, about an hour afterwards, it was much quieter, very little steam ascending, and the water only half filling the basin, and evidently sinking. In about an hour's time the basin was empty, and in half an hour more, the water had receded about 10 feet down the central pipe, where it remained during the rest of our stay. The wind was light and westerly.
This phenomenon appears to be of not very frequent occurrence, as Mrs. Spencer, of Tarawera, informed me that, although she had visited the spring some fifty times, she had only once seen it empty; a few notes, therefore, on the shape and dimensions of the basin will not be uninteresting.
After the water had descended into the pipe we were enabled with safety to go down into the basin, and approach close to the edge of the pipe, and walk all round it. Unfortunately I had no means of measuring it accurately, and the following dimensions are partly from pacings, and partly estimated. (See Section.)
The basin is situated about eighty feet above the level of the lake, and lies in a crateriform-hollow cut out of the hill behind it. This hill is composed of felspathic tufa, decomposed into yellow and red clays where acted upon by the steam and gasses exhaled from the spring. It is a slight admixture of these red clays with the siliccous sinter, that gives to the terraces of some of the springs their beautiful pink color.
The sinter-basin is irregularly circular, and about twenty-six yards in diameter, and five deep. The upper lip is smooth and flat, and from four to six feet broad. The sides and bottom of the basin are very irregular and rough, and apparently fissured, as steam escapes in two or three places on the western upper edge of the basin. The north and west sides are much steeper than the others, the easiest point of descent being on the east. The pipe is placed a little to the west of the centre of the basin, and is irregular in shape; the west and south sides being circular, while the north-east one is flat. The average diameter is about eight feet, and the sides are quite perpendicular and smooth.
The deposit from this spring is at first soft and granular, like very fine, fresh fallen snow; and the foot sinks in it to a depth of about a quarter of an inch. In time, it hardens and becomes more compact, probably, partly from the pressure of other layers, and partly from the infiltration of fresh siliceous matter. The microscopical structure of the freshly deposited sinter is extremely peculiar, and deserves a careful examination. It is, for the most part, composed of small elongated particles very variable in shape, but presenting, generally, the appearance of small sticks, and is altogether much more like organic than mineral matter. These sticks, of which I have figured a few (see Illustrations) are about 0·002 inches in length, and 0·0002 inches in breadth, and are mixed with larger angular grains of transparent silica, but without any crystals or crystalline particles.
Small deposits of sulphur were seen in two or three places round the upper edge of the basin, on which we also found many insects, such as beetles and dragon-flies, as well as some feathers of a lark, and the whole body of a hawk, incrusted with the siliceous sinter.
The water in the pools, on the terraces, was of very opaque light-blue color, and when we first arrived, I noticed, that the water in the basin, and the lower portion of the column of steam ascending from it, were also of the same hue.