Minnesota Plant Life. 



31 



America and, indeed, in the world, to observe their action is at 

 the Yellowstone National Park, where "reat masses of traver- 





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Fig. 8. — Portion of a board uliich had liucii standing in the tank shown in fig. 10. It is en- 

 crusted with lime .stone deposited by a colony of blue-green algae. After photograph by 

 Miss Jo.sephine E. Tilden. From the Botanical Gazette. 



tine and sinter are formed, covering acres around the hot springs 

 in which the blue-green rock-forming algae find a congenial 

 home. Travertine is a kind of limestone ; sinter is a kind of 



