SEPARATION OF SILICEOUS SINTER 477 



golden-yellow, orange, or red, and in the hottest waters pale flesh- 

 pink or even white. "These algas are often so thickly encrusted 

 by silica that the plant structure is not recognizable even under the 

 microscope, and their presence is often only to be distinguished by 

 the color." (Weed-55 :6j/.) The color varies with the temperature, 

 so that in the differently heated portions different colors obtain. The 

 order of color from progressive cooling is white, pale flesh-pink, 

 bright orange, yellowish-green, emerald. The most luxuriant growth 

 exists in the pools into which the waters flow, where leathery sheets 

 of tough gelatinous material with coralloid and vase-shaped form 

 abound. In some cases, the algje twelve to fifteen inches long unite 

 their tops into a solid roof, which may become the floor of a second 

 story of algous growth in a new basin. "The exact manner in 

 which the alg?e of these waters eliminate the silica from solution 

 is not known, but the process appears to be due to the vital growth 

 of the plant, for both the algae filaments and their slimy envelope 

 are formed of gelatinous silica. Upon the death of the algre which 

 have separated this jelly from the spring waters there is a loss of a 

 large part of its water and a change to a soft, cheesy, but more 

 permanent form. This dehydration is carried still farther if the 

 silica be removed from the water and dried, but if allowed to 

 remain in the cold water pools there is a further separation of silica, 

 possibly due to organic acids, formed by the decaying vegetation 

 reacting upon the silica salts of the water. This hardens the exist- 

 ing structures, in certain cases, and generally covers the pillars with 

 a frost-like coating of silica." (Weed-55 :dd_/.) 



It is in the inner dead layers of the algous mass that the gelatine 

 hardens first into silica, the outer layer continuing alive. Fibrous 

 varieties of sinter are formed in many of the Yellowstone and New 

 Zealand hot springs by the growth of the alg?e Calothrix, Mastigo- 

 ncuia, and Leptotlirix, the first two producing a "furry" sinter in 

 strata from a sixteenth of an inch to half an inch thick, the last 

 forms fibrous, straw-like masses. 



Mosses and diatoms have also been found active in separating 

 out the silica of the hot springs of the Yellowstone. The moss 

 (Hypnwn adHiicuiii var. grasilescens, Br. & Sch.) grows on the 

 lower parts of the slopes where the water is cooled to blood heat, 

 and has lost much of its lime and part of its silica. The silica is 

 abstracted from the water by the physiological activities of the 

 plant and, in turn, encloses and buries the moss. Diatoms are espe- 

 cially active in the tepid marshes, Denticula valida being the pre- 

 dominant type, though a number of other species occur. Extensive 

 beds of diatomaceous earth are formed by their siliceous skeletons. 



