Whatever may be the age of the formation 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 sections prepared for the microscope, each of these little concretions is seen to consist of numerous concentric coats of carbonate of lime, which sometimes simply surround an imaginary centre, but which, more commonly, have been successively deposited round some foreign body, such as a little crystal of quartz, a cluster of sand-grains, or a minute shell.

In other cases, as in some of the beds of the Carboniferous 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 limestones (fig.