THE FOSSILIFEROUS ROCKS. 



29 



more or less abundant calcareous matrix. When the grains 

 are pretty nearly spherical and are in tolerably close contact, 

 the rock looks very like the roe of a fish, and the name of 

 " oolite " or " egg-stone " is in allusion to this. When the 

 grains are of the size of peas or upwards, the rock is often 

 called a " pisolite " (Lat. pisum, a pea). Limestones having 

 this peculiar structure are especially abundant in the Jurassic 

 formation, which is often called the " Oolitic series " for this 

 reason; but essentially similar limestones occur not uncom- 

 monly in the Silurian, Devonian, and Carboniferous forma- 

 tions, and, indeed, in almost all rock-groups in which limestones 

 are largely developed. ^Vhatever may be the age of the for- 

 mation in which they occur, and whatever may be the size of 

 their component "eggs," the structure of oolitic limestones is 

 fundamentally the same. All the ordinary oolitic limestones, 

 namely, consist of little spherical or ovoid " concretions," as 

 they are termed, cemented together by a larger or smaller 

 amount of crystalline carbonate of lime, together, in many 

 instances, with numerous organic remains of different kinds 

 (fig. 13). When examined in polished slabs, or in thin sec- 

 tions prepared for the micro- 

 scope, each of these little con- 

 cretions is seen to consist of 

 numerous concentric coats of 

 carbonate of lime, which some- 

 times simply surround an ima- 

 ginary centre, but which, more 

 commonly, have been suc- 

 cessively deposited round 

 some foreign body, such as a 

 little crystal of quartz, a clus- 

 ter of sand-grains, or a minute 

 shell. In other cases, as in 

 some of the beds of the Car- 

 boniferous limestone in the 

 North of England, where the 

 limestone is highly " arenaceous," there is a modification of the 

 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 

 oi 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, 



Fig. 13. — Slice of oolitic limestone 

 from the Jurassic series (Coral Rag) of 

 Weymouth ; magnified. (Original.) 



