400 EEPOET UNITED STATES GEOLOGICAL SUEVEY. 



overflow the slope O will receive an addition. At K and S, the bottom of 

 the basin and the sides of the tubes, the deposit will take place when the 

 water is low and they are empty. This occurs only at comparatively 

 few times. The rate, therefore, at O and S would be very different. 

 The rate given by Forbes is what occurs under the most favorable cir- 

 cumstances. Ordinarily it is much less, but to what degree it is im- 

 possible to tell. It must be remembered, too, that there is no proof 

 that the horizontal layers of sinter (or those that are comparatively 

 horizontal) extend in that position to the bottom of the tube. Indeed, 

 the layers added to the tube are laid on perpendicularly, i. e., on the sides 

 of the tube. Descloizeaux is inclined to give a great antiquity to even the 

 newest deposits of the geysers in Iceland. He thinks that the increase 

 within the historical period has been small, and in this oinuion he is 

 probably correct. 



Mr. Josiah Martin, in describing the Pink Terrace or Otakapuarangi 

 at Eotomahana Lake, in New Zealand, says the upper plateaus are in- 

 scribed with the names of tourists and the dates of their visits: "These 

 slabs are perfectly smooth, and inscriptions in pencil become indelible 

 under a coating of silica in a short time ; but upon examining dates of 

 twenty or thirty years since, the deposit of film was so thin as to be 

 scarcely perceptible except as a thin glaze."* 



Hochstetter, speaking of the TeTarata, says that whatever lies upon 

 the terraces becomes incrustated in a very short time. Miss C. F. Gordon- 

 Cumming says that at Eotomahana the deposit is rapid, and fern leaves 

 and sticks become thickly incrusted in a few days, as though they had 

 been crystallized by a confectioner.t 



These incrustations are found probably where the water is sujjplied 

 rapidly, and can evaporate under favorable circumstances. We have 

 no data as to the rapidity with which the formation of the siliceous de- 

 posits of the Yellowstone National Park occurs. That it varies at dif- 

 ferent places is evident, and without doubt the variations agree with 

 those noted in other regions, especially Iceland and New Zealand. 



On the xjlateau or terrace of the Fountain Geyser in the Lower Fire- 

 Hole Basin in 1878 we found numerous inscriptions and names on the 

 white deposits and in the shallow basins surrounding the Fountain Gey- 

 ser. These had been written with lead-pencil by members of General 

 Howard's command, which jDassed through the Lower Geyser Basin in 

 1876. When we saw them they were indelible, but the films of silica 

 rendering them so were very thin indeed. 



CAUSE OF THE DEPOSITS. 



The main cause of the deposits, both calcareous and siliceous, is the 

 evaporation of the water containing the carbonates and the silica. Cal- 

 careous or carbonated waters deposit also simply by cooling or by losing 

 carbonic acid upon coming to the surface. In the specimen of water that 

 we brought in from Cleopatra Spring (at Mammoth Hot Springs), after 

 standing for three years sealed, there was a slight deposit of amorphous 

 carbonates. In the specimen from Spring No. 17 (on the main terrace 

 at Mammoth Hot Springs), after standing the same time, a small deposit 

 of calcite was formed (see page 389). 



Bun sen found that the geyser water from Iceland did not deposit upon 

 cooling, and our experience was the same in 1871 with a specimen from 

 the Grand Geyser, which was kept in a bottle for six months. 



* The Geysers, Hot Springs, and Terraces of New Zealand, by Josiali Martin. Popular 

 Science Review, October, 1879, p. 381. 

 t At Home in Fiji, p. 209. 



