Thus, many marbles (such as Derbyshire marble) are simply "crinoidal limestones" (fig.

9); whilst various other British marbles exhibit innumerable organic remains under the microscope.

Black marbles owe their colour to the presence of very minute particles of carbonaceous matter, in some cases at any rate; and they may either be metamorphic, or they may be charged with minute fossils such as _Foraminifera_ (_e.g._, the black limestones of Ireland, and the black marble of Dent, in Yorkshire).
[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.