THE FOSSILIFEROUS ROCKS. 31 
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. 
