TwOCKS : THEIR CONSTITUENTS AND KINDS. 81 



Varieties. — a. Kaolin ; purest unctuous clay, white, and when impure, of other 

 colors. The white is used for making porcelain by mixing with pulverized feldspar and 

 quartz, also for giving weight and body to writing-paper, b. Potter's clay ; plastic, free 

 from iron, and therefore good for white pottery ; mostly unctuous ; usually containing 

 some free silica, c. Ferruginous, Brick-clay; containing iron in the state of oxide or 

 carbonate, and consequently burning red, as in making red brick ; generally in thin layers, 

 which are alternately good clay and fine sand. d. Containing iron in the state of a 

 silicate, and then failing to turn red on being burnt, as the clay of which the Milwaukee 

 brick are made. e. Alkaline and Vitrifiahle ; containing 25 to 5 per cent of potash or 

 potash and soda, owing to the presence of undecomposed feldspar, and then not refrac- 

 tory enough for pottery or fire-brick, f. Marly ; containing some calcium carbonate, 

 g. Carbonaceous, Black, AmpeUte ; from the presence of lignitic or coaly material, 

 h. Alum-bearing ; containing aluminous sulphates, owing to the decomposition of iron 

 sulphides present. 



Bork-flour is rock pulverized to extreme fineness, so as often to resemble clay although 

 containing very little of it. Feldspar in this fine state is present in much clay. Some 

 rock-flour consists mainly of pulverized quartz. 



Alluvium, Silt, Lcess. — Alluvium is the earthy deposit made by running streams 

 or lakes, especially during times of flood. It constitutes the flats adjoining, and is usually 

 in thin layers, varying in fineness or coarseness, being the result of successive depositions. 

 Silt is the same material deposited in bays or hai'bors, where it forms the muddy bottoms 

 and shores. Lcess is an earthy deposit, coarse or fine, following the courses of valleys, like 

 alluvium, but without division into thin layers ; fertile ordinarily from the amount of vege- 

 table matter present, and containing also land or fresh-water shells. 



Detritus (from the Latin for worn) is a general term applied to earth, sand, alluvium, 

 silt, gravel, because the material is derived, to a great extent, from the loear of rocks 

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



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

 whence it gets its dark color and also a chief ijart of its fertility. 



Till. — Unstratified or imperfectly stratified deposits of bowlders, gravel, and clay, 

 derived from a continental glacier. Also called bowlder clay. Usually firmly compacted, 

 owing to the presence of clay or rock-flour when not properly consolidated. 



Tripoltte (Infusorial Earth). — Resembles clay or chalk, but is a little harsh be- 

 tween the fingers, and scratches glass when rubbed on it. Consists chiefly of siliceous 

 shells of Diatoms. Forms thick deposits, and is often found in swamps beneath the 

 peat (see page 1-53). Occurs sometimes slaty, as at Bilin, Prussia; and also hard, from 

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



Crystalline Rocks. 



The descriptions of crystalline rocks are arranged under the following 

 heads : — 



I. Siliceous rocks, or those consisting mainly of Silica. 



II. Rocks having as a chief constituent one or more of the alkali-bearing 

 MINERALS, Feldspar, Mica, Leucite, Nephelite, Sodalite. — In the first three of the 

 following subdivisions, potash-feldspar is present as a distinctive feature ; m 4, leucite 

 also contains a potash-bearing mineral ; in 5 and 6 a soda-lime or a lime feldspar is 

 characteristic. 



1. Potash-Feldspar and Mica Series. 



2. Potash-Feldspar and Hornblende or Pyroxene Series. 



3. Potash-Feldspar and Nephelite Rocks, Hornblendic or not. 



4. Leucite Rocks, Pyroxenic or not. 



5. Soda-lime-Feldspar and Mica Series. 



Dana's maxual — 6 



