32 PRINCIPLES OF PALEONTOLOGY. 



groups, whilst it occurs abundantly in the form of nodules in 

 parts of the Cretaceous (Upper Greensand) and Tertrary deposits. 

 Phosphate of lime forms the larger proportion of the earthy 

 matters of the bones of Vertebrate animals, and also occurs in 

 less amount in the skeletons of certain of the Invertebrates (e.g., 

 Crustaca}. It is, indeed, perhaps more distinctively than 

 carbonate of lime, an organic compound ; and though the for- 

 mation of many known deposits of phosphate of lime cannot be 

 positively shown to be connected with the previous operation c 1* 

 living beings, there is room for doubt whether this salt is not 

 in reality always primarily a product of vital action. The phos- 

 phatic nodules of the Upper Greensand are erroneously called 

 " coprolites, " from the belief originally entertained that they 

 were the droppings or fossilized 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 dissolved in water. Thus, when any 

 animal bodies are undergoing decay at the bottom of the sea, 

 they have a tendency to cause the precipitation from the sur- 

 rounding water of any mineral matter which may be dissolved in 

 it; and the organic body thus becomes a center 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 destruction of animal skeletons, was held largely in solution ; 

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

 decaying animal substance, which happened to be lying on the 

 sea-bottom, and which offered itself as a favorable 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 (im- 

 pure 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 



* 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. 



