W. H. Weed — Formation of Siliceous Sinter. 357 



Moss sinter. — Siliceous sinters formed by algous vegetation 

 are common at all the geyser basins of the Yellowstone Park, 

 but so far as known the only places where mosses produce de- 

 posits of siliceous sinter are the Terrace Springs and the 

 springs issuing from the slopes west of the Upper Geyser Basin. 

 At the latter place the hot waters of the Hillside Springs, flow- 

 ing down the steep slopes have deposited their carbonate of lime, 

 and lost much of their silica, by the growth of those brilliant 

 masses of red jelly, which can be seen when several miles dis- 

 tant, coloring the slopes with their brilliant tints. Near the foot 

 of the slope, the water now cooled down to 80° F., fills a series of 

 terraced basins suggesting those of the Mammoth Hot Spring, 

 but covered with a bright green mossy growth. These basins are 

 formed of a porous, buff-colored sinter composed of the stony 

 forms of the moss covering its surface, which has been deter- 

 mined by Prof. Chas. E. Barnes, of the University of Wiscon- 

 sin as Hypnum aduncum Hedw., var. gracilescens Br. and 

 Sch. Specimens obtained by the writer show the green and 

 living moss passing into the hard siliceous sinter — without 

 break or interruption. Chemical analysis shows this sinter to 

 have the composition of a typical geyserite ; it is undoubtedly 

 formed by the action of the moss — as the composition of the 

 water shows that no silica would be deposited under ordinary 

 circumstances. 



In physical character, the sinters resulting from algous vege- 

 tation differ from those formed by evaporation or other inor- 

 ganic causes by their greater lightness and opacity. They are 

 often soft and easily crushed, and sometimes soil the fingers ; 

 their structure is readily distinguished from that of other forms 

 of sinter. 



Chemical analyses by J. Edward Whitfield of three varieties 

 of siliceous sinter from the Upper Geyser Basin are given below. 

 The first is a geyserite — a beaded deposit formed by the spray 

 of the Splendid Geyser ; the second an algous sinter from the 

 Solitary Spring, the third the sinter formed by the Hyp?ium 

 from the waters of the Asta Spring. 



Geyserite. Algas Sinter. Moss Sinter. 



Si0 2 81-95 93-88 89'72 



A1 2 3 6-49 1-73 1-02 



Fe 2 3 -- tr. 



Na 2 2-56 0-28 



K 2 0-65 0-23 



CaO 0-56 0-25 2*01 



MgO 0-15 0-07 tr. 



S0 3 .. 0-16 



CI tr. 0-18 



H 2 7-50 3-37 7-34 



Total 100-02 100-33 100-09 



