KINDS OF ROCKS. 75 



carbonate of iron 5-95 = 99-75. The pale-yellow veins in the Italian black marble, 

 called "Egyptian marble," are dolomite, according to T. S. Hunt; and a limestone at 

 Dudswell, Canada, is similar. 



(3.) Hydraulic Limestone. — An impure or earthy limestone 

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

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

 Rondout, N. Y., afforded Beck — Carbonic acid 34-20, lime 25-50, 

 magnesia 12-35, silica 15-37, alumina 9*13, sesquioxyd of iron 2-25. 



(4.) Oolytk, or Oolytic Limestone. — A rock consisting of 

 minute concretionary spherules, and looking like the petrified roe 

 of fish : the name is from the Greek <L6v, egg. It is sometimes mag- 

 nesian. 



(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 made out 

 of shells or corals. 



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

 talline 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 Ti- 

 burtine. 



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

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

 which are called stalactites, and incrustations on the floors, which are called stalagmite. 

 The waters, filtering down from the overlying soil, contain a little carbonic acid, and 

 are thus enabled to dissolve the limestone, which is deposited again' on evaporation. 

 The layers of successive deposition are usually distinct, giving the material a banded 

 appearance. 



2. Crystalline Limestone. 



Granular Limestone (p. 55) (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 ; sometimes 

 serpentine (through combination with which it passes into ophiolyte, 

 p. 73), occasionally chondrodite, apatite, corundum. 



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

 stone (p. 55). 



3. Consisting of Sulphate of Lime. 



Gypsum. — Sulphate of lime, as described on p. 56. The earthy 

 kinds often contain the crystallized mineral in spots or fissures ; and 



