I9IS.] RODDY— CONCRETIONS IN STREAMS. 253 



Evidences That the Active Agents of the Concretionary 

 Formations in the Little Conestoga are Blue Green Alg^. 



That the concretions described in the first part of this paper are 

 the result of Hfe processes of plants may be proved in a number of 

 different ways, (i) The color of all growing specimens in the 

 stream is the characteristic bluish green color of the Cyanophyceas, 

 while those exposed to rain and sunshine are grayish white. Care- 

 ful microscopic examination also of such growing specimens re- 

 veals a varied thallophytic flora mainly of the Cyanophycese. 

 Species of the genera Gleocapsa, Gleotheca, Aphanocapsa, Nostoc, 

 Oscillatoria and Rivularia have been identified. Associated with 

 these are several of the green algae (Chlorophycese). Many species 

 of the Diatomaceas and Desmidaceas which generally live in close 

 association with blue green algse have also been identified and have, 

 no doubt, contributed the siliceous matter which is disseminated 

 through the calcareous matrix. Among the diatoms, species of the 

 genus Navicula both in free forms as well as stalked forms on algae 

 are quite prominent. The CJiaras are also occasionally present, 

 contributing a small percentage of so-called marly material. Some 

 bacteria have also been found in association with the other plants 

 but the bacteria have probably had little to do with the calcareous 

 deposition, but may contribute the iron which I find present in every 

 concretion that I have analyzed. 



(2*) The arrangement and structure of the laminae also favors 

 the view that these concretionary accumulation are due to life 

 processes. That periodic accretion alternates with a period of 

 quiescence is shown plainly by the concentric laminations of nearly 

 uniform thickness. The open porous nature of each lamina within 

 and the more solid character without, like the concentric arrange- 

 ment, is due without doubt to the seasonal conditions of the region. 

 Since algae are essentially thermophilic plants, each winter destroys 

 many of them and stops the growth of most of the rest and thus 

 at the beginning of the plant year (spring) few and widely scat- 

 tered algae at first produce slow and scattered accretion of the limy 

 matter ; later the plants become more abundant and by summer they 

 are crowded over the surface of each mass. This distribution of 

 the algje seasonally would naturally have its effects upon the struc- 



