THK NATURAL HISTORY SURVEY. 33 



stituents. These grains are especially numerous in a horn- 

 blende schist that occurs in quantities in various places. These 

 rocks are especially liable to disintegration, and their consti- 

 tuents are dissolved and segregated by the action of water. 



Rutile. Ti,O 4 . Tetragonal. 



The microscope reveals as occasional inclusions in quartz 

 and in biotite reddish needles of rutile. They are slender 

 acicular crystals often several hundredths of a millimeter in 

 length. Their cleavage is parallel to the primary and the sec- 

 ondary prism. Their fracture is subconchoidal to uneven. 

 They are brittle, their hardness is 6 and their specific gravity 

 4.2. Their lustre is metallic, their color reddish brown, and 

 their streak pale brown. Transparent to opaque. Insoluble 

 and infusible. 



Limonite. 2Fe 3 O 3 +3H a O. Orthorhombic. 



Bauerman and Wallerant call limonite an amorphous min- 

 eral. Dana says that it is not crystallized. Naumann says that 

 it is microcrystalline and cryptocrystalline. Klockmann says 

 the crystal form is unknown. Tschermak says that it is crys- 

 talline, but does not occur in crystals. Groth classifies it as 

 orthorhombic. Those authorities which call limonite amorphous 

 are evidently describing the rapidly precipitated iron oxide 

 such as is found in the bottom of ponds, mixed with other 

 substances, or the incrustation produced by rapid deposi- 

 tion of the hydrated iron oxide or the form known as iron rust. 

 These forms should not be taken to represent the mineral any 

 more than limestone should stand as representative of calcite, 



When the crystallography of the mineral is discussed, that 

 occurrence should be considered which represents the purest 

 and most homogeneous occurrence of the mineral. If we 

 consider this condition of limonite we find it in dense or fibrous 

 masses, composed of long thin acicular crystals, which to- 

 gether build up rounded mamillary, botryoidal, reniform, or 

 stalactitic groups. Groth's classification of them as ortho- 

 rhombic is undoubtedly the correct one. Hardness, 5; gravity, 

 3.6 to 4; lustre, silky, submetallic, greasy, dull to earthy; color, 

 various shades from black to brown; streak, yellowish brown. 

 Opaque. Yields water in closed tube and turns to hematite. 

 Soluble in hydrochloric acid. Found in all parts of the county 

 as coloring matter in the soil; can be seen as [an iridescent 



