PART 1. CHAPTER IV. 



53 



Concretionary Structure in Stratified Rocks, 



of strata of marlstone, like that observed in many ancient Euro- 

 pean formations, and like them containing freshwater shells.* 



It is probable that some of the heterogeneous materials which 

 rivers transport to the sea may at once set under water, like the 

 artificial mixture called pozzolana, which consists of fine volca- 

 nic sand charged with about 20 per cent, of iron, and the addi- 

 tion of a small quantity of lime. This substance hardens and 

 becomes a solid stone in water, and was used by the Romans in 

 constructing the foundations of buildings in the sea. 



Consolidation in these cases is brought about by the action of 

 chemical affinity on finely comminuted matter previously sus- 

 pended in water. After deposition similar particles seem to exert 

 a mutual attraction on each other, and congregate together in 

 particular spots, forming lumps, nodules, and concretions. Thus 

 in many argillaceous deposits there are calcareous balls, or sphe- 

 rical concretions, ranged in layers parallel to the general strati- 

 fication ; an arrangement which took place after the shale or 

 marl had been thrown down in successive laminae; for these 



laminae are often traced in the 

 concretions, remaining parallel to 

 those of the surrounding uncon- 

 solidated rock. (See Fig. 51.) 

 Such nodules of limestone have 

 often a shell or other foreign 

 body in the centre, f 

 Among the most remarkable examples of concretionary 

 structure are those described by Professor Sedgwick as abound- 

 ing in the magnesian limestone of the north of England. The 

 spherical balls are of various sizes, from that of a pea to a 

 diameter of several feet, and they have both a concentric and 

 radiated structure, while at the same time the laminae of original 

 deposition pass uninterruptedly through them. In some cliffs 

 this limestone resembles a great irregular pile of cannon-balls. 

 Some of the globular masses have their centre in one stratum, 

 while a portion of their exterior passes through to the stratum 

 above or below. Thus the larger spheroid in the annexed sec- 



Fig. 51. 



Catcareolis nodules in Lias. 



Fig, 52. 



Spheroidal concretions in magnesian 

 limestone. 



tion (Fig. 52.) passes from the stra- 

 tum b upwards into a. In this 

 instance we must suppose the depo- 

 sition of a series of minor layers, 

 first forming the stratum and 

 afterwards the incumbent stratum a; 

 then a movement of the particles 



* Principles of Geology, Index, " Superior, Lake." 

 t See De la Beche's Geological Researches, p. 95. 



