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. pzswm, 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. Whatever 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 ULL 
some of the beds of the Car- Fig. 13.—Slice of oolitic limestone 
boniferous limestone in the LM ent aoe oe a 
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 
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, 
