KOCKS: THEIB CONSTITUENTS AND KINDS. 79 



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

 " Egyptian marble," are dolomite, according to Hunt. 



Hydraulic Limestone. — A limestone containing 20 to 30 per cent of clay, and 

 affording a quicklime, the cement from which will " set " under water. It is often mag- 

 nesian. An analysis of a kind from Rondout, N. Y., afforded carbonic acid .34-20, lime 25-50, 

 magnesia 12-35, silica 15-37, alumina 9-13, sesquioxide of iron 2-25. In making ordinary 

 mortar, quartz sand is mixed with pure quicklime and water, and the chemical combina- 

 tion is mainly that between the water and lime, together with an absorption subsequently 

 of carbonic acid. Evaporation to dryness is necessary to hardening. With "hydraulic 

 cement," silica and alumina (that of the clay) are disseminated through the lime, and 

 hence these ingredients enter into chemical union with the lime and water, and make a 

 much firmer cement, and one which "sets" under water. Portland cement is made by 

 mixing 70 per cent of chalk with 30 of fine mud from the Thames. 



OoLTTE. — Limestone, either magnesian or not, consisting of minute concretionary 

 spherules ; looks like the petrified roe of fish, and hence the name, from the Greek ui6v, 



egg- 



Chalk. — A white, earthy limestone, easily leaving a trace on a board. Composi- 

 tion, the same as that of ordinary limestone. 



Makl. — 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. Marl is used as a fertilizer ; and beds of clay or sand that can be so used are 

 often in a popular way called marl. 



Shell Limestoxe, Coral Limestone. — A rock made out of shells or corals. 



Travertine. — A massive limestone, formed by deposition from calcareous waters, 

 and largely through the agency of fresh-water Algae, as at the Yellowstone Park (W. H. 

 Weed). But part is a deposit from solution. 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 

 Tihurtine. 



Crystalline Limestone. 



Stalagmite, Stalactite, Dripstone. — 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, and sometimes also dripstone. The waters, filtering down from the overlying 

 son, contain a little carbonic acid or some organic acid, and are thus enabled to dissolve 

 the limestone, which is deposited again on evaporation. The layers of successive deposi- 

 tion are usually distinct, giving the material a banded appearance. 



Granular Limestone, Calcyte (statuary marble). — Limestone having a crystal- 

 line granular texture, white to gray color, often clouded with other colors from impiirities. 

 It is a metamorpMc rock. Its impurities are often mica or talc, tremolite, rohite or gray 

 pyroxene or scapolite; sometimes serpentine, through combination with which it passes 

 into ophiolyte (p. 89); occasionally chondrodite, apatite, corundum. 



Varieties. — a. Statuary Marble; pure white and fine grained, b. Ornamental 

 and Architectural Marble ; coarse or fine, white, and mottled of various colors, and, when 

 good, free not only from iron in the form of pyrite, but also from iron or manganese in 

 the state of carbonate with the calcium, and also from all accessory minerals, even those 

 not liable to alteration, and especially those of greater hardness than the marble, which 

 would interfere with the polishing. c. Ver-d- antique, or ophiolyte. d. Micaceous. 

 e. Tremolitic ; contains bladed crystallizations of the white variety of hornblende called 

 tremolite. f. Grapjhitic ; contains graphite in iron-gray scales disseminated through it. 

 g. Chloritic; contains disseminated scales of chlorite, h. Chondroditic ; contains dis- 

 seminated chondrodite in large or small yellow to brown grains. 



Dolomyte. — Not distinguishable by the eye from granular limestone. The dolo- 



