Ch. IV.] 



CONCEETIONARY STRUCTURE. 



37 



Fig. 56. 



Spheroidal concretions in magnesian 

 limestone. 



are often traced in the concretions, remaining parallel to those of the sur- 

 rounding unconsolidated rock. (See fig. 55.) Such nodules of lime- 

 j,.^ ^j. stone have often a shell or other foreign 



body in the centre.* 



Amono; the most remarkable exam- 

 pies of concretionary structure are those 

 described by Professor Sedgwick as 

 Calcareous nodules in Lias. abounding in the magnesiau limcstoue 



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 con- 

 centric and radiated structure, while at the same time the laminae of 

 original deposition pass uninterruptedly through them. In some chfFs 

 this limestone resembles a gi*eat 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 section (fig. 56) passes from the 



stratum b upwards into a. In this in- 

 stance w^e must suppose the deposition of 

 a series of minor layers, first forming the 

 stratum b, and afterwards the incumbent 

 stratum a ; then a movement of the par- 

 ticles took ]3lace, and the carbonates of 

 lime and magnesia separated from the 

 more impure and mixed matter, forming the still unconsolidated parts of 

 the stratum. Crystallization, beginning at the centre, must have gone 

 on forming concentric coats, around the original nucleus without inter- 

 fering with the laminated structure of the rock. 



When the particles of rocks have been thus rearranged by chemical 

 forces, it is sometimes difl&cult or impossible to ascertain whether certain 

 lines of division' are due to original deposition or to the subsequent ag- 

 gregation of similar particles. Thus suppose three strata of grit, A, B, 



C, are charged unequally with calcareous 

 matter, and that B is the most calcareous. 

 If consolidation takes place in B, the con- 

 cretionary action may spread upwards 

 into a part of A, where the carbonate of 

 lime is more abundant than in the rest ; so that a mass d, e, /, forming 

 a portion of the superior stratum, becomes united with B into one solid 

 mass of stone. The original line of division d, e, being thus effaced, the 

 line d, /, Avould generally be considered as the surface of the bed B, 

 though not strictly a true plane of stratification. 



Pressure and heat. — When sand and mud sink to the bottom of a 

 deep sea, the particles are not pressed down by the enormous weight of 

 the incumbent ocean ; for the water, w^hich becomes mingled with the 

 sand and mud, resists pressure with a force equal to that of the column 



Fig. 57. 



IN 11 



inj.j.ijj.L.[jj 



De la Beche, Geol. Researches, p. 95, and Geol. Observer (1851), p. 686. 



