WHITE SINTER TEREACES OE EOTO-MAKANA. 173 



is here exposed, in a thin sheet, to the chilling effects of the atmo- 

 sphere. The deposit, therefore, chiefly takes place in these places, 

 and the formation grows along the outer wall b and at the edge of 

 the rim a. The effect' of this is to raise the rim of the basin and 

 to bulge it outwards. Again, the water passing down a 6, imme- 

 diately on entering the lower basin comes into contact to some 

 extent with the cooler mass of water d underlying m, and the 

 deposit c is formed. Thus, as the lip of the basin grows upwards 

 and outwards at a and b, the basin thereby tending to become 

 deeper, the deposit c has a tendency to advance into the basin and 

 rill it up. Very numerous instances of this may be seen. 



The resultant effect on any particular basin is evidently to make 

 it grow upwards and outwards ; and the general effect on the whole 

 sinter formation is to thicken the deposit, somewhat irregularly 

 perhaps, on the slope of the hill. This thickening will take place 

 approximately along a normal to the general slope of the whole 

 terrace at that particular point. It may be mentioned that the 

 particular form of basin-structure described above is not found so 

 distinct either where the water is at its hottest or at its coldest on 

 the terraces — that is, in the highest and lowest portions. In the 

 former case this, no doubt, results from the excessive deposition of 

 flocculent silica c as compared with the upward growth of the rim 

 a, due to the intermingling of the highly saturated and heated 

 water that has just arrived with the cooler mass below. In the 

 case of the lowest basins, where the slope is much more gradual and 

 the deposition from the colder water much less in quantity, the 

 growth of the rim proceeds scarcely more rapidly than the growth 

 of the portion within it, which then assumes the character of a 

 very gently sloping terrace bounded by a narrow rim only 2 or 3 

 inches in height. It may be noticed that the semicircular character 

 of the rim, as well as the flocculent deposit in the basin, is very con- 

 siderably modified according as the water flowing into it comes from 

 one or more basins. In some cases as many as six or eight basins 

 empty their water at slightly different temperatures into one, and 

 the shape of this latter is modified accordingly. Where the water 

 flows from a basin with a high outer wall, as it sometimes does, the 

 tendency is for it to produce stalactitic forms in the lower, and the 

 ordinary miniature forms in the upper part of the wall of this 

 basin. 



As to the rate of deposition at various parts of the terrace, this 

 evidently depends very much on the temperature of the water ; for 

 leaves of plants, twigs, and other objects placed on the rims of the 

 upper basins become thickly incrusted with silica within two or 

 three days, whilst on the lowest portions of the terrace it takes as 

 many weeks to produce the same result. The effect of this superior 

 deposition from the hotter water is seen also in the greater breadth 

 of the basin-walls, the water cooling most as it flows down the 

 outer wall, and consequently depositing its silica more readily and 

 thereby increasing the thickness of the walls. It may also be seen 

 in the numerous flocculent masses of silica that stand up somewhat 



