KINDS OF ROCKS. 69 



through decomposing agencies, mutual attrition in running water, and other methods 

 ipp. 648, 688, 758). 



Soil is earthy material, mixed with the results of vegetable and animal decomposi- 

 tion, whence it gets its dark color and also a chief part of its fertility. 



11. Tripolyte (Infusorial Earth). — Resembles clay or chalk, but is a little harsh 

 between the fingers, and scratches glass when rubbed on it. Consists chiefly of silice- 

 ous shells of Diatoms (p. 59). Forms thick deposits, and is often found in old swamps 

 beneath the peat. Used as a polishing powder; also mixed with nitro-glycerine to 

 make dynamite. Occurs sometimes slaty, as at Bilin, Prussia; and also hard, from 

 consolidation through infiltrating waters. Consists of silica in the opal or soluble state. 



2. Limestones, or Calcareous Rocks. m 



1. Not Crystalline. 



1. Massive Limestone. — Compact uncrystalline limestone of dull gray, bluish- 

 gray, brownish and black colors; in texture, varying from earth}- to compact semi- 

 crystalline. It consists essentially of calcite or carbonate of calcium (p. 55), but is often 

 impure with clay or sand. 



Most limestones have been made out of comminuted shells, corals, and other like ma- 

 terial; and when of dark colors or black, it is usually owing to some carbonaceous mat- 

 ters present derived from the decomposition 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 (CO,); 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 (p. 55). — Consists of carbonate of cal- 

 cium and magnesium, but not distinguishable in color or texture from ordinary lime- 

 stone. The amount of magnesian carbonate present varies from a few per cent, to that 

 in true dolomite. Much of the common limestone of the United States is magnesian. 



In some limestones the fossils are magnesian. while the rock is common limestone. 

 Thus, an Orthoceras in the Trenton limestone of Bytown, Canada (which is not mag- 

 nesian), afforded T. S. Hunt, Carbonate of calcium 56 00, carbonate of magnesium 37-80, 

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

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



3. Hydraulic Limestone. — A limestone containing some clay, and affording a 

 quicklime the cement from which will " set " under water. 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, sesquioxyd of iron 225. In making ordinary mortar, quartz sand is 

 mixed with pure quicklime and water, and the chemical combination is mainly that 

 between the water and lime, together with an absorption subsequently of carbonic acid. 

 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. 



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

 spherules, and looking like the petrified roe of fish: the name is from the Greek mov, 

 egg- 



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

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

 be so used are often in a popular way called marl. The " Green sand " of New Jersey 

 (p. 45")) is of this kind. 



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



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

 or streams. 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 Tiburtine. It occurs in the Yellow- 

 stone Park, along Gardiner's River. 



