LIME PRECIPITATES OF SPRINGS 



343 



posure to the air. Much Hme is also precipitated upon certain liv- 

 ing mosses ( Hypnum, etc.) by what appears to be the physiological 

 activity of the plant itself (see p. 475). The calcium carbonate de- 

 posits of the hot springs of the Yellowstone, etc., are probably due 

 to organic activities. Nevertheless a considerable part of the cal- 

 careous tufa or travertine is a purely hydrogenic deposit. (Fig. 67.) 

 At the Baths of San Vignone in Tuscany the travertine is de- 

 posited at the rate of six inches a year, while at San Filippo, Sicily, 





Fig. 6~. The Mammoth Hot Springs of the Yellowstone National Park, 

 showing the formation of calcareous deposits by hot springs. 

 (After Hayden.) 



it is one foot in four months, the deposit having grown to a height 

 of at least 250 feet, and forming a hill a mile and a quarter long 

 and a third of a mile broad. (Lyell-34 : ./Oi".) The deposits at 

 Narni, Italy, increase proportionally to the rate of flow, little lime 

 being deposited in stagnant water. The amount also increases with 

 distance from source and consequent length of exposure to the air, 

 and with decrease in temperature. Percolating ground water often 

 deposits lime in the pores of clastic strata and so cements them. 

 This is shown l)v the lime-sand dunes of the Bermudas, \vhicli are 



