432 DESCRIPTIONS OF ROCKS. 



composition of the plants or animals of the waters in which they were 

 formed. 



When burnt, limestone (Ca0 3 C) becomes quicklime (CaO), through 

 loss of carbonic acid (C0 3 ) ; and, at the same time, all carbonaceous 

 materials are burnt out, and the color, when it is owing solely to these, 

 becomes white. 



2. 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. bb). 



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 Ortlwceras, in the Trenton limestone of 

 Bytown, Canada (which is not magnesian), afforded T. S. Hunt, Cal- 

 cium carbonate 56 00, magnesium carbonate 37 '80, iron carbonate 5 95 

 = 99-75. The pale-yellow veins in the Italian black marble, called 

 "Egyptian 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 Shell-marl. 



Marl is used as a fertilizer ; and other beds of clay or sand that can 

 be so used arc often in a popular way called marl. The "Green 

 sand " of Xew Jersey (p. 429) is of this kind. 



5. Travertine. — A massive limestone, formed by deposi- 

 tion from calcareous springs or streams. The rock abounds 

 on the river Anio, near Tivoli, and St. Peters at Rome is 

 constructed of it. The name is a corruption of Tiburtine. 

 It occurs in the Yellowstone Park, along Gardiner's Kiver. 



6. Stalagmite. — See page 216. 



2. Crystalline Limestone. 



1. Granular or Crystalline Limestone (Marble). — Limestone 

 having a crystalline-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 



