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 structure. In both
these and the ordinary oolites, the structure is fundamentally
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