256 RODDY— CONCRETIONS IN STREAMS. [May 7, 



Mineral Content of the Little Conestoga Waters. 



One would infer from the number of concretions growing in the 

 Little Conestoga and also from the thickness of each lamina in a 

 concretion that the mineral content of this stream's waters is high, 

 I have verified this by determining the salinity of the stream under 

 varying conditions. The salinity in a wet month was 330 parts in a 

 million, while in a dry month this rose to 365 parts in a million. 

 Streams in which I have found no trace of concretionary structures 

 have a much lower salinity, the Big Conestoga Creek for example 

 having a salinity of 190, the Pequea Creek 195, and the Susque- 

 hanna, in March, above the mouth of the Pequea and below the 

 mouth of the Big Conestoga, about 200 parts in a million. The 

 various springs flowing into the Little Conestoga have an average 

 salinity nearly as high as that of the Little Conestoga itself. 



The basin of the Little Conestoga is underlain with much more 

 soluble limestone than any of the other streams so far investigated. 

 This accounts for the high salinity of its waters and also for the 

 distribution of the concretions so far as we know that distribution. 

 Further search and study will certainly reveal that many streams 

 of the world contain concretionary structures and determine the 

 conditions of their distribution and formation. I trust the be- 

 ginning I have made in the investigation of stream concretions will 

 lead to a wide and thorough study of this interesting and important 

 biological as well as geological problem. 



The various facts tabulated on page 257 and correlated with the 

 fact that the blue green algse are about equally abundant in the various 

 streams mentioned in the table would seem to indicate that deposi- 

 tion of CaHo(COo)o is always going on in all the streams during 

 the growing season, but that when the salinity is low solution by the 

 stream waters balances deposition and no concretions are formed. 

 When, however, the salinity is high, solution can not take place and 

 laminated structures due to seasonal or other changes are formed 

 either in concretionary form or more rarely as reefs. This is put 

 forward as a working hypothesis, many more observations and 

 analyses are needed however before the various problems connected 

 with these formations can be fully solved. 



