CHEMICAL COMPOSITION 357 



its efficiency, as will be seen later. Concretions also form around open- 

 ings, particularl}^ around root marks or tubules, by which more direct 

 connection is kept open with the outer atmosphere. This is particularly 

 true of what we have described further on as incretions. 



Chemical Composition and Conditions of Formation 



As already hinted, concretions are rarely pure or composed of any one 

 mineral, but almost always include more or less of the rocks in which 

 they grow. Almost any soluble mineral may take concretionary form 

 if only evaporation or cooling gradually reduces the solution below the 

 point of saturation. Of course, only those which are nearly or quite 

 insoluble under ordinar}^ circumstances will form permanent concre- 

 tions. The more common concretions are therefore those formed by 

 chemical reactions which produce insoluble minerals from soluble. 



Doubtless calcareous concretions — that is, those composed of calcium 

 carbonate — are the most abundant, for carbonate of lime is everywhere, 

 and carbonic acid, its solvent, is quite as common. Moreover, the con- 

 ditions are frequent for its evaporation and the formation of carbonate 

 of lime, which is nearly insoluble in common waters. 



In quite a similar way carbonate of iron, or siderite, forms concretions 

 of clay ironstone, which are very common in certain strata. Another 

 quite common concretionary mineral is iron pyrites. It is hard as steel, 

 and its concretions of much size are formidable hindrances to the well- 

 driller. Fortunately they are commonly thin and brittle. Ferric sul- 

 phate reduced by carbon in organic compounds becomes FeS.^, and 

 possibly also ferrous carbonate acted upon by hydrogen sulphide pro- 

 duces the same compound. 



Silica even may be gathered in concretions from an alkaline solution 

 or from a solution of alkaline silicate through reaction with carbonate 

 of lime. Several of these processes are not fully understood, and the 

 above statements are not to be taken as results of careful experimenta- 

 tion, but as generally expressed probable reactions, strengthened in some 

 cases by corroborating observations. 



Another cause of precipitation quite different from most of the others 

 is the easy combination of oxygen with a ferrous compound, like ferrous 

 carbonate and ferrous sulphate, and probably also organic salts, by 

 which an insoluble ferric hydroxide is formed. This is particularly in- 

 fluential in forming ferric concretions of the incretion and excretion 

 types. 



The following is a table of chemical reactions probabl}^ producing 

 concretions : 



