BUILDING AND ORNAMENTAL STONES. 



373 



LIMESTONES COMPOSED LARGELY OF ORGANIC REMAINS. 



Fossiliferous limestones.— Many limestones are made up wholly or in 

 part of tbe fossil remains of marine animals, as is shown in the accom- 

 pany iug figure, which is drawn from a magnified section of a limestone 

 of the Cincinnati group from near Hamilton, Ohio. 



> % 



In some cases the remains are retained nearly perfect; again the 

 entire fossil may have been replaced by crystalline calcite. In other 

 instances stones are found which are made up only of casts of shells, 

 the original shell material having decayed and disappeared, as in the 

 Eocene limestone from North Carolina. Many of the most beautiful 

 marbles belong to the group of fossil limestones, as, for instance, the 

 red and white variegated Tennessee marbles. Crinoidal limestones are 

 made up of fossil crinoidal fragments. 



Shell limestones or shell sand-roclcs as they are called by some author- 

 ities, are made up of shells usually much broken, though sometimes 

 almost entire. The well-known coquina from Saint Augustine, Fla., is 

 a good illustration of this variety. Coral rock is of the same nature, 

 excepting that it is composed of fragments of corals. Chalk is a fine 

 white limestone composed mainly of the minute shells of foraminifera. 



MAGNESIAN LIMESTONES J ALSO CALLED DOLOMITIC LIMESTONFS. 



Under this head are included those limestones which contain 10 per 

 cent, and upwards of carbonate of magnesia. They may be finely or 

 coarsely crystalline ; light, porous, or compact j fossiliferous or non- 



