OF STRATIFIED ROCKS. 
37 
acid. Shells and corals are also formed of the same ele¬ 
ments, with the addition of animal matter. To obtain pure 
lime it is necessary to calcine these calcareous substances, 
that is to say, to expose them to heat of sufficient intensity 
to drive otf the carbonic acid, and other volatile matter. 
White chalk is sometimes pure carbonate of lime ; and this 
rock, although usually in a soft and earthy state, is occa¬ 
sionally sufficiently solid to be used for building, and even 
passes into a compact stone, or a stone of which the sepa¬ 
rate parts are so minute as not to be distinguishable from 
each other by the naked eye. 
Many limestones are made up entirely of minute frag¬ 
ments of shells and coral, or of calcareous sand cemented to¬ 
gether. These last might be called ‘‘calcareous sandstones;” 
but that term is more properly applied to a rock in which 
the grains are partly calcareous and partly siliceous, or to 
quartzose sandstones, having a cement of carbonate of lime. 
The variety of limestone called oolite is composed of nu¬ 
merous small egg-like grains, resembling the roe of a fish, 
each of which has usually a small fragment of sand as a nu¬ 
cleus, around which concentric layers of calcareous matter 
have accumulated. 
Any limestone which is sufficiently hard to take a fine pol¬ 
ish is called marble. Many of these are fossiliferous; but 
statuary marble, which is also called saccharoid limestone, as 
having a texture resembling that of loaf-sugar, is devoid of 
fossils, and is in many cases a member of the metamorphic 
series. 
Siliceoiis limestone is an intimate mixture of carbonate of 
lime and flint, and is harder in proportion as the flinty mat¬ 
ter predominates. 
The presence of carbonate of lime in a rock may be ascer¬ 
tained by applying to the surface a small drop of diluted 
sulphuric, nitric, or muriatic acid, or strong vinegar; for the 
lime, having a greater chemical affinity for any one of these 
acids than for the carbonic, unites immediately with them to 
form new compounds, thereby becoming a sulphate, nitrate, 
or muriate of lime. The carbonic acid, when thus liberated 
from its union with the lime, escapes in a gaseous form, and 
froths up or effervesces as it makes its way in small bubbles 
through the drop of liquid. This effervescence is brisk or 
feeble in proportion as the limestone is pure or impure, or, 
in other words, according to the quantity of foreign matter 
mixed with the carbonate of lime. Without the aid of this 
test, the most experienced eye can not always detect the 
presence of carbonate of lime in rocks. 
