THE FOSSILIFEROUS ROCKS. 



oolitic structure. Microscopic sections of these sandy lime- 

 stones (fig. 14) show numerous generally angular or oval grains 

 of silica or flint, each of which is commonly surrounded by a 

 thin coating of carbonate of lime, or sometimes by several such 

 coats, the whole being cemented together along with the shells 

 of Foraminifera and other minute fossils by a matrix of crystal- 

 line calcite. As compared with typical oolites, the concretions 

 in these limestones are usually much more irregular in shape, 

 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 min- 

 ute shells and fragments of 

 the skeletons of larger marine 

 animals. 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 is another lime-salt, which is of interest 

 to the palaeontologist. 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- 



* 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. 



Fig. 14. Slice of arenaceous and 

 oolitic limestone from the Carbonifer- 

 ous series of Shap, Westmoreland; mag- 

 nified. The section also exhibits Fora- 

 minifera and other minute fossils. (Orig- 

 inal.) 



