Amongst the numerous varieties of limestone, a few are of such interest as to deserve a brief notice. _Magnesian limestone_ or _dolomite_, differs from ordinary limestone in containing a certain proportion of carbonate of magnesia along with the carbonate of lime. The typical dolomites contain a large proportion of carbonate of magnesia, and are highly crystalline. The ordinary magnesian limestones (such as those of Durham in the Permian series, and the Guelph Limestones of North America in the Silurian series) are generally of a yellowish, buff, or brown colour, with a crystalline or pearly aspect, effervescing with acid much less freely than ordinary limestone, exhibiting numerous cavities from which fossils have been dissolved out, and often assuming the most varied and singular forms in consequence of what is called "concretionary action." Examination with the microscope shows that these limestones are composed of an aggregate of minute but perfectly distinct crystals, but that minute organisms of different kinds, or fragments of larger fossils, are often present as well. Other magnesian limestones, again, exhibit no striking external peculiarities by which the presence of magnesia would be readily recognised, and though the base of the rock is crystalline, they are replete with the remains of organised beings. Thus many of the magnesian limestones of the Carboniferous series of the North of England are very like ordinary limestone to look at, though effervescing less freely with acids, and the microscope proves them to be charged with the remains of _Foraminifera_ and other minute organisms. _Marbles_ are of various kinds, all limestones which are sufficiently hard and compact to take a high polish going by this name. Statuary marble, and most of the celebrated foreign marbles, are "metamorphic" rocks, of a highly crystalline nature, and having all traces of their primitive organic structure obliterated. Many other marbles, however, differ from ordinary limestone simply in the matter of density. 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.)]