WHITE SINTER TERRACES OF ROTO-MAHAnA. 



171 



along with the cooling a continual precipitation of the silica in 

 solution is going on. In the geyser-basin itself, and also in a few 

 of the terrace-basins nearest to it, the water is of the most intense 

 sapphire-blue. On the middle terraces it is like turquoise ; and on 

 the lowest it is partially opaque with a faint blue tinge. The con- 

 trast between the hoarfrost-like deposit of which nearly the whole 

 terrace is formed, and the sapphire- and turquoise-coloured water in 

 the basins, is most striking. 



The precipitation and deposition of the silica being due to the 

 cooling of the water, the same cause which operates to form the rim 

 of the geyser-basin will also operate in a different degree after the 

 water has begun to now down the slope, and other rims more or less 

 complete in form will be produced. These are the lips of the 

 terrace-basins, or what appear from a distance to be the several 

 terraces of the formation. Their shape is of necessity more or less 

 circular ; for if water, saturated at a high temperature with silica, 

 issues from a gej^ser-pipe and spreads equally on all sides, becoming 

 cooler as it proceeds, a point is reached at which a deposition of 

 silica must take place in the form of a circular rim ; and this once 

 begun will of necessity continue until a distinct basin is produced; 

 for the cooling will be greatest at the upper edge and on the outer 

 wall of the rim, and it is there that the deposition will be greatest. 

 "When this basin has been built up until external obstacles on three 

 sides compel the water to escape by the fourth, another rim, and 

 if the slope is sufficient a series of rims, will be formed, which in 

 turn will give rise to others. All these basin-rims are very similar 

 in form, being approximately semicircular or, at least, arcs of circles. 

 The water flows over each rim in a sheet of extreme evenness of 

 depth, there being no difference perceptible by the eye over the 

 whole extent of the rim, which in some cases is from 60 to 80 ft. in 

 length. This strikes the visitor at first as somewhat remarkable ; 

 but a little consideration shows that it is a necessity of the case ; for 

 it' any part of the rim through accident or otherwise be injured, it 

 is evident that the water must flow through the opening thus made, 

 and the injury be repaired before the other parts of the rim can be 



Fig. 1. — Section of Sinter Basins at Roto-Mahana. 



augmented. It will perhaps be best, before describing the whole of 

 the terrace more generally, to give an accurate description of one 



n 2 



