170 R. ABBAY ON THE BUILDING-UP OF THE 



15. On the Building-up of the White Sinter Terraces of Eoto- 

 Mahana. By the Bev. B. Abb at, M.A., F.G.S. (Read Decem- 

 ber 5, 1877.) 



It is proposed in this paper to describe certain peculiarities in the 

 structure of the " White Terrace " of Roto-Mahana, in the Northern 

 Island of New Zealand, which appeared to the writer, when visiting 

 the spot in 1875, to be of considerable interest, as affording an ex- 

 planation of the manner in which this most remarkable sinter for- 

 mation has been built up. The geyser which has deposited the 

 silica of which the terrace is formed, is situated on the side of a 

 little hill of rotten rhyolitic rock, at an elevation of about 100 ft. 

 above the warm lake (Roto-Mahana) into which the foot of the 

 terrace projects. The rim of the geyser is between 300 and 400 ft. 

 in circumference ; and the banks that surround it are steep on all 

 sides except where the water finds an exit. Opposite this opening 

 the wall of earth is almost perpendicular, and some 50 ft. in height. 

 The geyser-basin is about 15 ft. deep, with a flat rim from 4 to 6 ft. 

 broad, the whole being composed of a beautifully white fiocculent- 

 like deposit of silica, that yields slightly, like frozen snow, to the 

 foot. The pipe is about 8 ft. in diameter, and is situated slightly 

 on the exit side of the basin. A little rocky island, that will be 

 referred to later, lies in the opening by which the water makes its 

 escape, at about one third of the distance across it from the northern 

 bank. It probably owes its existence to its exceptionally rocky 

 character ; for it is only 12 ft. in height, and has, no doubt, been 

 separated from the neighbouring bank by the undermining action 

 of the water. Below the geyser-rim the descent is broken up by a 

 most remarkable series of terrace-basins formed by the deposition 

 of silica as the water cooled in its passage to the lake. It is to the 

 character of these basins that the writer wishes to direct attention. 

 They number many hundreds, and are of various sizes and in dif- 

 ferent stages of formation. Some of them are sufficiently large and 

 deep for half a dozen persons to swim in at the same time ; others 

 only occupy a space of a few square feet, and in some instances are 

 filled up nearly to the brim by the deposited silica. Nearly all of 

 them, I believe, can be referred to one main type of formation, 

 however they may vary in appearance. 



In considering the formation of so great a number of terrace- 

 basins it must be remembered that the water issuing from the 

 geyser-funnel, saturated with silica, has had to flow down an in- 

 clined surface about three acres in extent, and during this passage 

 has cooled down from about boiling-point to something like 70° or 

 80° F. This latter temperature varies, of course, according to the 

 amount of water issuing from the geyser and the temperature of 

 the surrounding atmosphere. The colour of the water also varies 

 very considerably at different parts of the terrace, showing that 



