[Illustration:  Fig. 13.—­Slice of oolitic limestone from the Jurassic series (Coral Rag) of Weymouth; magnified. (Original.)]

“Oolitic” limestones, or “oolites,” as they are often called, are of interest both to the palaeontologist and geologist.  The peculiar structure to which they owe their name is that the rock is more or less entirely composed of spheroidal or oval grains, which vary in size from the head of a small pin or less up to the size of a pea, and which may be in almost immediate contact with one another, or may be cemented together by a 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 uncommonly in the Silurian, Devonian, and Carboniferous formations, and, indeed, in almost all rock-groups in which limestones are largely developed.  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. 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 crystalline 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 surrounding envelope of lime being very thin, and often exhibiting no concentric