WKKD. 



FORMATION OF TRAVERTINE. 301 



masses of gelatinous alga 1 , often several inches thick and forming mat- 

 like coverings in the sluggish overflow channels, show the action of 

 plant life most clearly. The successive layers of membrane-like 

 material carry minute little crystals and stellate accretions scattered 

 about in the plant tissue. These grow into small pellets that uniting 

 together produce firm layers. Thin lamina- of carbonate of lime also 

 form between the membranes, and a compact deposit of travertine 

 resnlts from a combination of the two. 



Siliceous sinter—The hot springs and geysers of the Yellowstone are 

 surrounded by large areas of siliceous sinter that often entirely cover 

 the floor of the geyser basins. About the spouting vents this material 

 has been built up into mounds and cones of unique forms ami great 

 beauty. The more quiet pools have built up more or less regular 

 mounds of white sinter which are in places as much as 20 feet in height 

 above the surrounding level. Besides these deposits, the alkaline 

 waters of the geyser regions have left deposits of silica wherever they 

 have flowed, and many square miles within the park are covered by 

 white and glistening deposit of this material. 



rutil the Yellowstone deposits were studied it was the generally 

 accepted theory that the geyser waters reached the surface heavily 

 charged with silica, which by relief of pressure, by cooling, and by 

 evaporation was precipitated out and deposited by the waters. Obser- 

 vation of the natural conditions under which the Yellowstone deposits 

 are forming, together with experiments and a study of the chemical 

 analyses of the geyser waters, showed that the silica brought to the 

 surface by the geyser waters was not separated out and deposited 

 by the first two causes, but that deposits are formed about the geysers 

 and the margins of springs by evaporation, producing atrue geyserite. 

 A new mode of deposition was then recognized, namely, the separation 

 of silica by plant life, by the alga- that are abundant in the hot waters 

 of the region. It is by this agency that by far the largest part of the 

 sinter deposits of the region have been formed. 



This algous vegetation is sure to be observed by every visitor to 

 the region. Its varied tints of pink, yellow, orange, red, brown, and 

 green adorn the slopes of geyser cones, flush the white silica of the 

 little basins with their tints, and mark the waterways with their bril- 

 liant colors. It is ever present where the temperature does not 

 exceed 186° F., often lining the great bowls of the cooler springs and 

 langa with leathery sheets of brown or green. Where a constant over- 

 flow prevails, the channel is often tilled by a vigorous growth in which 

 an alga' mat is formed having the consistency of a firm jelly, and most 

 beautifully colored. In whatever form it is found, and no matter how 

 brillantly tinted, this algous material if removed from the water and 

 dried in the hot sun of the region rapidly loses its color, shrinks in 



