wrKI) .i FORMATION OF SILICIOUS SINTER. 3(13 



and abundant as a thin orange-colored mat about the great Prismatic 



Spring, is a LeptotTmx and produces a thatch-like or straw-like form 



of sinter. , . 



A collection of these hot water algffi made by the writer is being 

 studied by Prof. W. G. Fallow, of Harvard University. 



The importance of these plant growths in building ap sinter deposits 

 „ iay be realized when it is stated that in the walls of the great Excelsior 

 geyser a section of 16 feet in thickness is exposed, of which over 12 

 feel is recognized as clearly and undoubtedly of algous formation, and 

 the remainder of cemented fragmentsof weathered sinter. Even where 

 the deposit does not reveal its origin by its structure, as is the case in 

 the glassy compact sinter whose thin layers compose the plat form about 

 the Giant geyser, it is probable, judging from present conditions, that 

 it is hut an algous sinter altered by the long continued action of steam 

 and percolating waters. 



When the varied conditions of lite and of evaporation are observed, 

 it becomes at one apparent that any attempt to estimate the age of a 

 geyser by the thickness of its deposits is a most difficult problem and 

 a wholly unreliable foundation for other than comparative statements. 

 Sinter formed by evaporation is produced very slowly under the most 

 favorable conditions at the Firehole Geyser basins: one-twentieth of an 

 inch a year is the maximum. Sinter formed by plant life may attain a 

 thickness of eighl inches a year in limited areas. 



In general the sinters produced in these two modes may be readily 

 distinguished by their physical structure. In chemical composition 

 they are so alike that they cannot be separated. 



Of less importance, because of its greater rarity, is the production 

 of a siliceous deposit by a moss, Hypnum admeum, observed at the 

 Upper Geyser basin and at the Madison or Terrace springs. The moss 

 grows only in the cooled waters that have already had considerable 

 silica extracted while hot by the algous growths mentioned, but the 

 moss growing in the cold waters is rapidly incrusted. in fact appears 

 to build its structure largely of silica, and the resulting deposits cover 

 several acres at each locality. 



Diatom beds are common throughout the Park, but the deposits now 

 forming are all in cool marshes supplied by hot spring waters. The 

 resulting diatom earth, beds of which are sometimes six feet thick, 

 generally contains more or less glassy silica separated from the waters 

 by decomposing vegetable matter. 



