PRINCIPLES OF PALEONTOLOGY. 



often lengthened out and almost cylindrical, at other times 

 angular, the central nucleus being of large size, and the sur- 

 rounding envelope of lime be- 

 ing very thin, and often exhib- 

 iting no concentric structure. 

 In both these and the ordinary 

 oolites, the structure is funda- 

 mentally the same. Both have 

 been formed in a sea, probably 

 of no great depth, the waters 

 of which were charged with 

 carbonate of lime in solution, 

 whilst the bottom was formed 

 of sand intermixed with minute 

 shells and fragments of the 

 skeletons of larger marine ani- 

 mals. The excess of lime in 

 the sea-water was precipitated 

 round the sand-grains, or round 

 the smaller shells, as so many 

 nuclei, and this precipitation must often have taken place time 

 after time, so as to give rise to the concentric structure so char- 

 acteristic of oolitic concretions. Finally, the oolitic grains thus 

 produced were cemented together by a further precipitation of 

 crystalline carbonate of lime from the waters of the ocean. 



Phosphate of Lime v=, another lime-salt, which is of interest 

 to the palaeontologist. It does not occur largely in the strati- 

 series, but it is found in considerable beds * in the 



Fig. 14. Slice of arenaceous and 

 oolitic limestone from the Carbonifer- 

 ous series of Shap, Westmoreland ; mag- 

 nified. The section also exhibits Fnra- 

 ini>tifera and other minute fossils. (Ori- 

 ginal.) 



fied 



Laurentian formation, and less abundantly in some later rock- 

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

 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 Inver- 

 tebrates (e.g., Crustacea). It is, indeed, perhaps more dis- 

 tinctively than carbonate of lime, an organic compound ; and 

 though the formation of many known deposits of phosphate of 



* Apart from the occurrence of phosphate of lime in actual beds in the 

 stratified rocks, as in the Laurentian and Silurian series, this salt may also 

 occur 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. 



