654 GEOLOGY. 



ancient and plastic type. The fact that they flourish in waters so hot 

 and sometimes also so sulphurous as to be fatal to most plants, suggests 

 the possibility of their introduction during the very early volcanic 

 stages of the earth, while conditions were yet uncongenial for other 

 plants. 



The geologic work of the thermal algse is well shown in the beautiful 

 travertine and sinter deposits of the Yellowstone Park (Figs. 215 and 

 218). At the Mammoth Hot Springs the deposits are calcareous, while 

 at most of the other hot springs silicious deposits are formed, in both 

 cases partly, but not wholly, by the aid of algse. The beautiful yellows, 

 reds, browns, and greens of these springs are not mineral coloring, but 

 living plants.^ In the calcareous waters, the algffi are believed to cause 

 the deposition of calcium carbonate from calcium bicarbonate by con- 

 suming the second equivalent of carbon dioxide that rendered the car- 

 bonate soluble.^ In the silicious waters, the process of deposition is 

 not understood. Similar deposits by the aid of algae take place in the 

 geyser regions of Iceland and of New Zealand, in the hot springs of 

 Carlsbad, where they have been well studied by Cohn,^ and in most 

 other hot springs. The same, or very similar, forms of algse abound in 

 nearly all waters, fresh and salt, but the question whether they make 

 calcareous and silicious deposits in notable quantity appears not to 

 have received as yet the critical investigation its importance deserves, 

 except in a few special cases. It is clear, however, that in the cool 

 waters such deposits do not reach the conspicuous amounts that they 

 attain in the thermal springs. In the shallow waters of the ocean, 

 especially in the warmer regions, lime-secreting algse are abundant and 

 make large contributions to the lime deposits. 



Among the higher algse are the lime-secreting corallines or nuUi- 

 pores (Rhodophycese, red algse), once regarded as animals, which con- 

 tribute a notable part of the calcareous substance of coral reefs. They 

 are important geologic agents in the temperate and tropical seas, and 

 have been traced as far back in time as the early Paleozoic era. 



The Challenger reports ' describe two forms of minute calcareous 

 spherical organisms, Rhabdospheres and Coccospheres, as very abun- 



^Weed. Ninth Ann. Rept. U. S. Geol. Surv., 1887-88, pp. 613-76; also 

 Bradley M. Davis. Science, Vol. VI, 1897, pp. 145-57. 



2 Cohn. Abhandl. Schles. Gesell. Naturwiss., Heft II, 1862. 

 ' Deep Sea Deposits, p. 257. 



