I9IS.] RODDY— CONCRETIONS IN STREAMS. 255 



through evaporation or change of temperature. It does, however, 

 suggest that the secretion or precipitation is chemical and dependent 

 on a life process that produces conditions for chemical reaction 

 where the plants or animals are most abundant. 



(7) Conway MacMillan in Minnesota Plant Life says: 



" Some slime moulds have the power of incrusting their tiny fruit bodies 

 with lime which they extract from their soil or from rain water which falls 

 upon them. Such forms are often observed in Minnesota upon dead wood 

 or fallen leaves, generally, in moist shady places in the deep forest. Some 

 of the blue green algse have the power of encrusting themselves with lime 

 and in watering troughs and tanks there sometimes occurs a calcareous 

 formation reminding one of the deposit in old tea-kettles. Such a crust is 

 true limestone extracted from the water by the chemical activities of the algae." 



Upon a larger scale the blue green algae have been conclusively 

 shown by Weed to be important factors in travertine formation in 

 the hot springs and geysers of Yellowstone National Park. 



Dr. MacFarlane without knowing of my discovery in the Little 

 Conestoga Creek has expressed the opinion that these apparently 

 insignificant plants have throughout all the ages played and are still 

 playing in all waters an important part in the formation of lime- 

 stones and dolomites. 



(8) The fact that many more or less ancient rocks have been 

 demonstrated to be of algoid origin by various scientists and are 

 similar to the Little Conestoga concretions in their concretionary or 

 laminated structures or both is favorable to the view that algae are 

 just as important agencies in rock formations in the present geo- 

 logical epoch as in the past. The similarity of Cryptozoon pro- 

 liferum, Ozarkian oolitic formations, N ezvlandia frondosa, Camasia 

 spongiosa, Collenia compacta, Collenia undosa and other structural 

 forms in rock formations to the work of recent algse in hot spring 

 and geyser regions has been vividly shown by Walcott, Wieland, 

 B. M. Davis and others. Some, at least, of the above-named for- 

 mations can be strikingly duplicated in their structural peculiarities 

 by the Little Conestoga concretions and reef-like masses of Round 

 Lake, — the Potsdam-Hoyt formation of New York state being 

 especially like what would result were infiltrating waters, cementa- 

 tion, and other solidifying agents or processes to act for a long time 

 upon the great mass of flood deposited concretions of the Little 

 Conestoga in Kendig's Woods. 



