144 GEOLOGICAL HISTORY OE THE YELLOWSTONE PARK. 



present sinter was laid down. It however corroborates other meth- 

 ods and possesses the advantage of being a direct way. 



It may be well to add here that there exists the greatest contrast 

 between the deposits of the Mammoth Hot Springs and those found 

 upon the plateau. At the Mammoth Springs they are nearly ])\\ve 

 travertine, with only a trace of silica, analyses showing from 95 to 99 

 per cent of calcium carbonate. On the plateau, the deposits consist 

 for the most part of siliceous sinter, locally termed "geyserite." The 

 reason for the difference is this : At the Mammoth Hot Springs the 

 steam, although ascending from fissures in the igneous rock, comes in 

 contact with the waters found in the Mesozoic strata, which here form 

 the surface rocks. The Jura or Cretaceous limestones liave furuished 

 the lime held in solution and precipitated on the surface as travertine. 

 On the other hand, the mineral constituents of the plateau waters are 

 derived almost exclusively from the highly acidic lavas, which, as it 

 will be seen by reference to the analyses, carry but a small amount of 

 lime. 



Deposition of sinter from the hot waters of the geyser basins de- 

 pends in a great measure on the amount of silica held in solution, 

 which varies considerably at the different localities and may have 

 varied still more in past time. The silica, as determined by analyses, 

 ranges from .22 to .60 grammes per kilogramme of water, the former 

 being the amount found in the water of the caldron of the Excelsior 

 Geyser and the latter at the Coral Spring in the I^Torris Basin. Analysis 

 shows that from one-fifth to one-tliird of the mineral matter held in so- 

 lution consists of silica, the remaining constituents being readily solu 

 ble salts carried off by surface drainage. A few springs highly charged 

 with silica, like the Coral, deposit it on the cooling of the waters; but 

 such springs however are exceptional, and I do not recall a single in- 

 stance of a spring in the Upper Geyser Basin precipitating silica in this 

 way. At most springs and geysers it results only after evaporation, 

 and not from mere cooling of the water. It seems probable thai the 

 nature and amount of alkaline chlorides and carbonates present influ- 

 ence the separation of silica. Temperature ■ also may in some degree 

 infiueuce the deposition. My friend, Mr. Elwood Hofer, one of the best 

 guides to this region and a keen observer of nature, has called my 

 attention to an observation of his made in mid-winter, while on one of 

 his snow-shoe trips through the Park. He noticed that certain over- 

 flow jjools of spring water, upon being frozen, deposited a considerable 

 amount of mineral matter. He has sent me specimens of this material, 

 which, upon examination, proved to be identical with the silica depos- 

 ited from the Coi al Spring upon thecoolingof the water. Demijohns of 

 gej'ser water which have been standing for one or two years have failed 

 to precipitate any silica. Quite rei-ently, in experimenting n\)on these 

 M'aters in the laboratory, it was noticed that on reducing them nearly 

 to the freezing point no change took place, but upon freezing the waters 



