WHITE SINTEE TEEEACES OE EOTO-MAHAnA. 177 



posited silica ; and the water will then spread itself over a larger 

 area, whose depressions will also be filled up until an inclined sur- 

 face will be presented, over which the water will now, as in the di- 

 rections a b\ a b", a b'", a b"". The sheet of water becoming 

 rapidly thinner as it spreads itself out in the form, of a fan, the 

 cooling will be much more rapid, on account of the greater surface 

 exposed to the influence of the atmosphere. Consequently points 

 will soon be reached equidistant from a, such as c c' c' , where the 

 silica will begin to be rapidly deposited. Thus a rim will be 

 formed, necessarily in the shape of an arc of a circle, as c' c c', with 

 a as its centre. This rim once begun, the deposition of silica, as 

 we have already seen, goes on most readily on the outer wall of 

 the rim, as at b, fig. 1 ; and consequently a basin will be formed, to 

 be modified afterwards in various ways as is seen in figs. 1 & 3. 



If the water flows over the rim c' c c' with perfect evenness of 

 depth, another rim will be formed below concentric with the one 

 above, and with a as its centre. This, however, is rarely or never 

 seen. The water in flowing down the wall of the basin generally 

 becomes somewhat uneven in depth, owing to slight irregularities 

 on its surface ; and the consequence is that the sheet of water has 

 a tendency to divide itself into different currents, and thus to form 

 different basins. To take an exaggerated case, we may suppose 

 five different currents to start from c' c' c c' c' (fig. 6), and to form 

 five different rims, each in the form of an arc of a circle and inter- 

 secting one another. 



Discussion. 



Dr. Evans asked whether the deposit described consists of pure silica, 

 and what is the nature of the rock through which the water depositing 

 it percolates. It seemed to him to be hardly likely that the water 

 could be that of the lake circulating through the rock, as, if so, it 

 could hardly acquire so high a temperature or so large an amount of 

 silica. He also inquired whether there are any cavities in the rocks, 

 especially containing geodes, and whether there is any known law 

 as to the solubility of silica in water of different temperatures. 



Prof. Ramsay inquired whether this was a spouting geyser. 



Mr. Deew questioned whether the silica deposited could be redis- 

 solved in the manner required by the author's hypothesis ; and in- 

 quired why, supposing the author's hypothesis to be correct, the de- 

 posit from the geyser does not form a dome. 



Prof. Ramsay remarked upon the great depth to which the water 

 must probably descend in order to acquire the temperature necessary 

 for dissolving the quantity of silica described. 



Mr. Sollas asked if there were any data as to the relation of 

 pressure as well as temperature to the solubility of silica. 



Mr. J. P. Walkee referred the re-solution of the silica to the 

 action of superheated steam. 



Mr. Whitakee considered that the fact of these springs breaking 

 out on a hill-side would account for the absence of domes such as 



