28 PRINCIPLES OF PALAZONTOLOGY. 
carbonate of magnesia, and are highly crystalline. The ordi- 
nary 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 con- 
sequence of what is called “ concretionary action.” Examina- 
tion 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 pecu- 
liarities 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 suffi- 
ciently 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 struc- 
ture obliterated. Many other marbles, however, differ from 
ordinary limestone simply in the matter of density. Thus, 
many marbles (such as Derbyshire marble) are simply ‘cri- 
noidal limestones” (fig. 9); whilst various other British 
marbles exhibit innumerable organic remains under the mi- 
croscope. 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 Aovaminifera (ag., 
the black limestones of Ireland, and the black marble of Dent, 
in Yorkshire). 
“ Oolitic” Limestones, or “ oolites,” as they are often called, 
are of interest both to the paleontologist 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 
