32 Minnesota Plant Life. 



silica or sand rock and it therefore appears that not only can 

 some blue-green algae extract lime salts from the water, but 

 other varieties can form quartz as well. No doubt very much 

 of the limestone and even of the granite that occurs in ledges 

 over the continent was begun in ancient warm seas by the action 

 of organisms similar to the blue-green algae, while at the Yel- 

 lowstone Park, or upon a smaller scale in one's watering trough, 

 the same rock-forming processes are still going on. 



Skin-algae. Still another variety of blue-green alga produces 

 broadly expanded membranes, or skins, along the bottom of 

 springs, and on pebbles in streams or lakes. In general the 

 plants of this group may be recognized by their color and they 

 are among the lowest of the algae. 



