THE FOSSILIFEROUS ROCKS. 3! 



lime cannot be positively shown to be connected with the 

 previous operation of living beings, there is room for doubt 

 whether this salt is not in reality always primarily a product 

 of vital action. The phosphatic nodules of the Upper Green- 

 sand are erroneously called " coprolites," from the belief 

 originally entertained that they were the droppings or fossilised 

 excrements of extinct animals ; and though this is not the case, 

 there can be little doubt but that the phosphate of lime which 

 they contain is in this instance of organic origin.* It appears, 

 in fact, that decaying animal matter has a singular power of 

 determining the precipitation around it of mineral salts dis- 

 solved in water. Thus, when any animal bodies are undergo- 

 ing decay at the bottom of the sea, they have a tendency to 

 cause the precipitation from the surrounding water of any 

 mineral matters which may be dissolved in it ; and the organic 

 body thus becomes a centre round which the mineral matters 

 in question are deposited in the form of a "concretion" or 

 " nodule." The phosphatic nodules in question were formed 

 in a sea in which phosphate of lime, derived from the destruc- 

 tion of animal skeletons, was held largely in solution ; and a 

 precipitation of it took place round any body, such as a decay- 

 ing animal substance, which happened to be lying on the sea- 

 bottom, and which offered itself as a favourable nucleus. In 

 the same way we may explain the formation of the calcareous 

 nodules, known as "septaria" or "cement stones," which 

 occur so commonly in the London Clay and Kimmeridge 

 Clay, and in which the principal ingredient is carbonate of 

 lime. A similar origin is to be ascribed to the nodules of 

 clay iron-stone (impure carbonate of iron) which occur so 

 abundantly in the shales of the Carboniferous series and in 

 other argillaceous deposits ; and a parallel modern example is 

 to be found in the nodules of manganese, which were found 

 by Sir Wyville Thomson, in the Challenger, to be so numer- 

 ously scattered over the floor of the Pacific at great depths. 

 In accordance with this mode of origin, it is exceedingly 

 common to find in the centre of all these nodules, both old 

 and new, some organic body, such as a bone, a shell, or a 

 tooth, which acted as the original nucleus of precipitation, and 



* It has been maintained, indeed, that the phosphatic nodules so largely 

 worked for agricultural purposes, are in themselves actual organic bodies 

 or true fossils. In a few cases this admits of demonstration, as it can be 

 shown that the nodule is simply an organism (such as a sponge) infiltrated 

 with phosphate of lime (Sollas) ; but there are many other cases in which 

 no actual structure has yet been shown to exist, and as to the true origin 

 of which it would be hazardous to offer a positive opinion. 



