CHIEF DIVISIONS OF THE AQUEOUS ROCKS. 21 



Laurentiaji formation, and less abundantly in some later 

 rock -groups, whilst it occurs abundantly in the form of 

 nodules in the parts of the Cretaceous (Upper Greensand) 

 and- Tertiary 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., Lingula, among the Brachio- 

 pods ; Conularia and Theca, among the Pteropods ; and the 

 Crustacea generally). It is, indeed, perhaps more distinctively 

 than carbonate of lime, an organic compound ; and though 

 the formation of many known deposits of phosphate of 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 Greensand 

 are erroneously called " coprolites," from the belief originally 

 entertained that they were the 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. 1 The true 

 " coprolites " that is, the petrified excreta of fishes, reptiles, 

 and mammals are also largely composed of phosphate of 

 lime. 



The last lime-salt which need be mentioned is gypsum, or 

 sulphate of lime. This substance, apart from other modes 

 of occurrence, is not uncommonly found inter stratified with 

 the ordinary sedimentary rocks, in the form of more or less 

 irregular beds ; and in these cases it has a palseontological 



disseminated through the rock, when it can only be detected by chemical 

 analysis. It is interesting to note that Dr Hicks has recently proved the 

 occurrence of phosphate of lime in this disseminated form in rocks as old as 

 the Cambrian, and that in quantity quite equal to what is generally found to 

 be present in the later fossiliferous rocks. This affords a chemical proof that 

 animal life flourished abundantly in the Cambrian seas. 



1 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 phos- 

 phate of lime (Sollas) ; but there are many other examples 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. 



