248 RODDY— CONCRETIONS IN STREAMS. [May 7, 



The smaller concretions are invariably ellipsoidal in shape (see 

 Fig. i), and quite symmetrical unless broken by flood action. The 

 larger sized concretions, though of the same general shape, are less 

 symmetrical. Those in the stream are nearly always more regu- 

 larly ellipsoidal than those of the deposits in flood plains and 

 stream bars. This is, no doubt, due to their weathering through 

 solution or to their having been broken by flood waters during their 

 transportation to their present positions. 



The concretions in the stream are quite firm in texture; those 

 in the deposits are less compact. Both are porous and roughly 

 coralline in general appearance and internal structure. 



In color they vary from bluish green to whitish. The growing 

 specimens in the stream are generally bluish green. All specimens 

 after exposure for some time to sun, air, and rain or to the action 

 of soil waters become grayish white. 



Composition and Hardness. — Though the composition varies 

 slightly from place to place yet all are limy deposits concentric 

 around a nucleus. The main constituents in the concentric layers 

 are calcium carbonate, silica and organic matter of vegetable origin. 

 Upon dissolving out the limy constituents with dilute hydrochloric 

 acid, a mat is often left of vegetable materials composed of the 

 matted stems or tissues and cells of low type plants such as mosses 

 and algse. 



Few of the specimens tested had a hardness as great as that 

 of common calcite, most of them being about two in the scale of 

 hardness. The weathered concretions are generally less coherent 

 than those now forming in the stream. 



The following table shows the main constituents of the con- 

 cretions : 



Constituents. A. B. 



Organic matter 10% to 15% i to 12% 



H2O 1% 1% 



Si02 12% 12% 



CaCOs 60% to 75% 70 to 80% 



Fe 1% 2% 



Al Trace Trace 



MgCOs Trace to 1% Trace to 1% 



A of growing specimens. 



B of specimens from flood plain deposit. 



