1915-] RODDY— CONCRETIONS IN STREAMS. 251 



are now regarded as of similar origin though differing much from 

 the Little Conestoga concretions in both form and structure. 



That concretions similar to those found in the Little Conestoga 

 occur in other streams is evident from observations made in Center 

 County, Pennsylvania, by Dr. Wieland, who, however, had not 

 recognized them as of Algoid origin until I called his attention to the 

 well known activity of some algae in precipitating calcium carbonate. 

 In a recent personal letter to me Dr. Wieland describes concretions 

 that he found in 1888 in a stream near Lemont, Center County, Pa. 

 He, however, says, " I just thought of them as very interesting 

 objects from the viewpoint that they showed once more how abun- 

 dant is COo whether derived from plants or other sources. In 

 short I knew too much and too little to make the least use of what 

 I found." 



Origin. — In 1854, W. Ketchell in the First Annual Report of the 

 Geological Survey of New Jersey refers to Chara as active agents 

 in the formation of fresh water marl. In 1864 Frederick Cohn 

 found that a number of aquatic plants, especially Chara Mosses and 

 Algae, caused the deposition of travertine at the waterfalls of TivoH. 

 The deposition he attributed to the activity of the plants in absorb- 

 ing carbon dioxide and so setting the lime carbonate free. That 

 is, these low type plants consume carbon dioxide and exhale oxygen. 

 When this is done in water containing calcium bicarbonate they 

 deprive that salt of its second molecule of carbonic acid and the 

 insoluble neutral carbonate of lime is precipitated. 



W. S. Blatchley and G. H. Ashley in their report on the lakes 

 of Indiana in 1900 also refer to the activity of plants in the pre- 

 cipitation of insoluble lime carbonate. But they also thought that 

 the dissolved lime brought into the lakes by streams and deposited 

 mechanically by evaporation was a more important agency than the 

 plants. 



In 1900 C. A. Davis discussed the origin of the marls of the 

 lakes of Michigan and came essentially to the same conclusion as 

 Cohn. He says : 



" But in water containing amounts of salts, especially of the calcium 

 bicarbonate, so small that they would not be precipitated if there were no free 

 carbon dioxide present in the water at all, the precipitation may be consid- 



