LITHOLOGY. 119 



'the nodule is more advanced in its formation, a con- 

 siderable portion of one ingredient of the rock having 

 been by chemical action removed and aggregated around 

 the concretion. Again it may be seen in its complete 

 state when all such ingredient has been segregated from 

 the surrounding matrix and from the enclosed nucleus 

 also, in some instances entirely altering their composi- 

 tion and appearance. And at a still later stage concre- 

 tions may be noticed either wholly or partially decom- 

 posed, or they may have been dissolved by water 

 percolating through the mass which has carried away 

 their substance in solution. The cast only then remains, 

 and this also may be modified by similar action and to 

 any extent enlarged, or it may even be again filled with 

 mineral matter by a process of crystallisation. 



These concretionary nodules occur in rocks of all ages, 

 and as a rule consist of some substance which has formed 

 a minor ingredient of the enclosing mass. Notable 

 examples are the flints of the Chalk and the iron-stones 

 in Carboniferous clay and of decomposed concretions, 

 the iron pyrites found in Chalk, many clays and other 

 deposits as a ball of rusty-looking powder. 



Concretions may be formed within concretions by 

 further segregation of one particular substance, as, for 

 instance, of sulphide of lead or of zinc in clay-iron ore. 

 As throwing light upon the composition and origin of 

 the rock in which they are found, and on account of 

 their commercial value, concretions are of considerable 

 importance, therefore note should always be made of 

 their occurrence.* 



Slickensides. The walls of a fault or of a joint may 



* Ante, p. 53. 



