432 DESCRIPTIONS OF ROCKS. 
ee of the plants or animals of the waters in which they were 
ormed. 
When burnt, limestone (CaO,C) becomes quicklime (CaO), through 
loss of carbonic acid (CO,); and, at the same time, all carbonaceous 
materials are burat out, and the color, when it is owing solely to these, 
becomes white. 
é. Magnesian Limestone. Dolomyte.—Carbonate of calcium 
and magnesium, but not distinguishable in color or texture 
from ordinary limestone. The amount of magnesium car- 
bonate afforded by analyses varies from a few per cent. to 
that of true dolomite (p. 45). 
Much of the common limestone of the United States is magnesian. 
That of St. Croix, Wisconsin, the ‘‘ Lower Magnesian,” afforded Owen 
42°43 per cent. of magnesium carbonate. 
In some limestones the fossils are magnesian, while the rock is 
common limestone. ‘Thus, an (7thoceras, in the Trenton limestone of 
Bytown, Canada (which is not magnesian), afforded T. 8. Hunt, Cal- 
cium carbonate 56:00, magnesium carlonate 37°80, iron carbonate 5 95 
=99°75. The pale-yellow veins in the Italian black marble, called 
‘‘Kgyptian marble,” and ‘‘ portor” (see above), are dolomite, accord- 
ing to Hunt ; and a limestone at Dudswell, Canada, is similar. 
3. Chalk.—A white, earthy limestone, easily leaving a 
trace on a board. Composition the same as that of ordi- 
nary limestone. | 
4, Marl—A clayey or earthy deposit containing a large 
proportion of calcium carbonate—sometimes 40 to 50 per 
cent. If the marl consists largely of shells or fragments 
of shells, it is called Shedl-marl. 
Marl is used as a fertilizer ; and other beds of clay or sand that can 
be so used are often in a popular way called marl. The ‘‘ Green 
sand” of New Jersey (p. 429) is of this kind. 
5. Travertine.—A massive limestone, formed by deposi- 
tion from caleareous springs or streams. ‘The rock abounds 
on the river Anio, near Tivoli, and St. Peter’s at Rome is 
constructed of it. The name is a corruption of Tiburtine. 
™t occurs in the Yellowstone Park, along Gardiner’s River. 
6. Stalagmite.—See page 216. 
2. CRYSTALLINE LIMESTONE. 
1, Granular or Crystalline Limestone (Marble).—Limestone 
having acrystalline-granular texture, white to gray color, but 
often of reddish and other tints from impurities. It is a 
metamorphic rock ; it was originally common limestone ; it 
became crystalline under the action of more or less heat ; in 
