NODULAR OR CONCRETIONARY STRUCTURE. 



189 



Fig 



the chalk, and the clay iron-stone nodules of the coal strata and hy- 

 draulic lime-balls, common in many clays, are familiar illustrations of 

 this structure. 



Cause. — Nodular concretions seem to occur whenever a more solu- 

 ble or more suspensible substance is diffused in small quantities through 

 a mass of entirely different and more fixed material. Thus, if strata of 

 sandstone or clay have small quantities of carbonate of lime or carbon- 

 ate of iron diffused through them, the diffused particles of lime or iron 

 will gradually, by a process little under- 

 stood, segregate themselves into more or 

 less spherical or nodular masses, in some 

 cases almost pure, but generally inclos- 

 ing a considerable quantity of the mate- 

 rial of the strata. In this manner lime- 

 balls and iron-ore balls and nodules, so 

 common in sandstones and clays, are 

 formed. In like manner, the flint nod- 

 ules of the chalk were formed by the 

 segregation of silica, originally diffused 

 in small quantities through the chalk-sediment. Very often some for- 

 eign substance forms the nucleus about which the segregation com- 

 mences. On breaking a nodule open, a shell or some other organism is 

 often found beautifully preserved. These nodules, therefore, are a 

 fruitful source of beautiful fossils. In most cases, probably in all 

 cases, the segregating substance must have been to some extent soluble 

 in water pervading, or suspensible in water percolating the stratum ; 

 and the reason why they are so frequently associated with fossils is that 

 decomposing organic matter renders many substances such as lime 

 carbonate, iron oxide and silica more soluble. Sometimes the nodules 



run together, forming a 

 more or less continuous 

 stratum (Fig. 173). In such 

 cases, the segregating mate- 

 rial is more impure. 



Forms of Nodules. — The 

 typical and most common 

 form is globular. This is 

 well seen in lime-balls and 

 iron-balls. Sometimes these 

 balls are solid, sometimes 

 they have irregular cracks 

 in the center (Fig. 169), 

 sometimes they have a radiated structure (Fig. 170), sometimes they 

 are hollow like a shell (this is common in iron-balls). They vary in size 



Fig. 170. 



-Dolomite containing Concretions, Sunderland 

 (after Jukes). 



