KINDS OF ROCKS. 85 



taming some clay, and affording a quicklime the cement made of 

 which will set under water. An analysis of a kind worked at Kon- 

 dout, N.Y., afforded Beck — Carbonic acid, 34.20, lime, 25.50, mag- 

 nesia, 12.35, silica, 15.37, alumina, 9.13, sesquioxyd of iron, 2.25. 



(4.) Oolite, or Oolitic Limestone. — A rock consisting of minute 

 concretionary spherules, and looking like the petrified roe of fish : 

 the name is from the Greek uov, egg. It is sometimes magnesian. 



(5.) Chalk. — A white, earthy limestone, easily leaving a trace on 

 a board. Composition, the same as that of ordinary limestone. 



(6.) Marl. — A clay containing a large proportion of carbonate 

 of lime, — sometimes 40 to 50 per cent. If the marl consists largely 

 of shells or fragments of shells, it is called shell-marl. 



(7.) Shell Limestone. — Coral limestone. — A rock consisting of shells 

 or corals. 



(8.) Birdseye Limestone. — A compact limestone having crystal- 

 line points disseminated through it. 



(9.) Travertine. — A massive but porous limestone, formed by deposition 

 from springs or streams holding carbonate of lime in solution in the state of 

 bicarbonate. The rock abounds on the river Anio, near Tivoli, and it is there 

 used as a building-material. St. Peter's, at Rome, is constructed of it. The 

 name is a corruption of Tiburtine. 



(10.) Stalagmite, Stalactite. — Depositions from waters trickling through 

 the roofs of limestone caverns, form pendent calcareous cones and cylinders 

 from the roofs, which are called stalactite, and incrustations on the floors, which 

 are called stalagmite. The layers of successive deposition are usually distinct, and 

 make the material appear banded. They are rarely transparent, usually translu- 

 cent to subtranslucent or opaque, and white, grayish, or faint yellowish in color. 



2. Crystalline Limestone. 



91. Granular Limestone ($ 69) (Statuary Marble). — Limestone 

 having a crystalline granular texture, white to gray color, often 

 clouded with other colors from impurities. The impurities are 

 often mica or talc, tremolite, white or gray pyroxene, or scapolite; some- 

 times serpentine, through combination with which it passes into 

 ophiolite (§ 86), chondrodite, apatite, corundum. 



Dolomite. — Not distinguishable by the eye from granular lime- 

 stone (§ 69). 



3. Consisting of Sulphate of Lime. 



92. Gypsum. — Sulphate of lime, as described in § 70. The earthy 

 kinds often contain the crystallized mineral in spots or fissures ; 

 and in many places it is associated with anhydrite, or sulphate 

 of lime containing no water (§ 70 [27]). The borate of magnesia, 



