. Elements of geology. Geology. 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 R
. Elements of geology. Geology. 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 strat
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