420 ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 5 



of the lake in turn are modified by the river passing through it. 

 For this reason a long river passing through country with great 

 variations in the altitude and in the other factors mentioned above 

 can exhibit interesting and varying algal forms in different points 

 of its course. 



The algae growing in hot springs have become adapted to life 

 under high temperature conditions, and some of them can live 

 and grow at temperatures as high as 167° F. All these so-called 

 " strictly thermal algae " are blue-green and grow within the hot 

 springs as well as in the outflow from them. The best known and 

 most thorouglily investigated of the thousands of hot springs in 

 the western part of the United States are those of the Yellowstone 

 National Park. Practically all the thermal algae are species that 

 have become acclimated to hot springs and that are not found else- 

 where. The waters of the hot springs are highly charged with 

 soluble calcium and magnesium compounds, especially bicarbonates. 

 Much of the lime deposited there results from the evaporation and 

 cooling of the water. However, most of what the geologists call 

 " travertine ", or the material deposited, is due to the action of 

 the abundantly present algae or their chlorophyll in consuming or 

 decomposing the carbon dioxide that is present in the water and 

 thus reducing the amount of calcium bicarbonate that may be held 

 in solution. The precipitated lime is a byproduct of the photo- 

 synthesis of the little plants. The travertine may attain a thickness 

 of 2 to 4 millimeters in a week. The terraces of travertine thus 

 formed are usually brilliantly covered by the overlapping layers of 

 algae, which are bathed by intermittent discharges of highly 

 mineralized fizz water. 



Hot springs are not the only places where the algae aid in the 

 deposition of calcium carbonate. The stones of the Scottish lakes 

 in winter are covered with a rich brown coating of diatoms, which 

 in summer often disappears. In several lakes, its place is taken by 

 a crust of grayish lime deposited from the blue-green algae. The 

 same occurrence has been noted m the Swiss lakes. 



In 8 feet of water on the sandy bottom of a Michigan pond that 

 is separated from Lake Michigan by a sand bar, curious hollow 

 pebbles 1 to 3 inches in diameter with a stratified or concentrically 

 zoned structure were picked up. Upon decalcification, these pebbles 

 were found to be composed of a densely interwoven mass of bluish- 

 green filaments, species of the algae Schizothrlx, Stigonema^ and 

 Dichothrix. The colony of algae evidently starts at some point of 

 attachment such as a shell and then grows out radially in all direc- 

 tions, each filament independent of the others and all of them pre- 

 cipitating calcium carbonate tubules. The tubules are strong enough 



