30 PRINCIPLES OF PALAZONTOLOGY. 
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 
Fig. 14.—Slice of arenaceous and Skeletons of larger marine ani- 
oolitic limestone from the Carbonifer- mals. The excess of lime in 
ous series of Shap, Westmoreland; mag- mu 
nified. The section also exhibits Fora- the sea-water was precipitated 
a and other minute fossils. (Ori- Poncee san d-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 is another lime-salt, which is of interest 
to the paleontologist. It does not occur largely in the strati- 
fied series, but it is found in considerable beds * in the 
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 (¢.¢., 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. 
