678 REPORT OF NATIONAL MUSEUM, 1897. 



B — CHARACTERS RELATING TO COHESION AND ELASTICITY. 



CLEAVAGK. 



This is that tendency of a mineral to break in the direction of inini- 

 niuni cohesion, and that direction is always parallel to some Diane 

 which occnrs or may occur in the crystal. The cleavage is character- 

 ize 1 first, according to direction that is, when parallel to certain faces 

 or planes, as cubic, octahedral, dodecahedral, rhombohcdral, and also 

 basic, prismatic, macrodiagonal, brachydiagoual, and so on; second, 

 according to the ease with which it may be obtained, as perfect, imper- 

 fect, interrupted, or difficult. 



Examples of Cleavage. 



C«btc.— Galena, Desloge mine, Missouri. (Cat. No. 17213, U.S.N.M.) 

 Oc<a/(ef?ra/.— Fluorito, Muscalonge Lake, New York. (Cat. No. 44864, U.S.N.M.) 

 Dodecahedral. — Sphalerite, Picos del Europa, Spain. (Cat. No. 17099, U.S.N.M.) 

 lihombohedral. — Calcite, Chester County, Pennsylvania. (Cat. No. 4649(5, U.S.N.M.) 

 /Vis/na/ic— Diopside, Achuiatofsk, Siheria. (Cat. No. 49237, U.S.IV.M.) 

 Bravhijd\a(jonal. — Diaspore, Chester, Massachusetts. (Cat. No. 47367, U.S.N.M.) 

 Macrodiagonal. — Kyaiiite, Litchfield, Connecticut. (Cat. No. 49728, U.S.N.M.) 

 Clinodia<jonal. — Orthoclasc, Way's quarry, Newcastle County, Delaware. (Cat. No. 

 49703, U.S.N.M.) 



Basal. — Topaz, Takayama, Japan. (Cat. No. 47119, U.S.N.M.) 

 Eminent. — Muscovite, Chester County, Pennsylvania. (Cat. No. 83 178, U.S.N.M.) 

 Perfect. — Labradorite, Isle of Paul, Labrador. (Cat. No. 83605, U.S.N.M.) 

 Distinct.— Fyvite, Leadville, Colorado, (Cat. No. 81847, U.S.N.M.) 

 Interrupted. — Hypcrsthene, Franklin, New Jersey. (Cat. No. 83617, U.S.N.M.) 

 Difficult.— Qnartz, Brazil. (Cat. No. 44686, U.S.N.M.) 



GLIDINO, I'KESSURE, AND SEI'AKATION PLANES. 



When a crystal is subjected to pressure in certain directions a new 

 series of ])lanes may be developed, as follows: Gliding planes, the sur- 

 faces along which there has been a slipping accompanied by a rotation 

 of the molecules. Pressure planes or percussion figures, the divisional 

 planes diverging from a point of pressure which is applied at right 

 angles to a natural face. Separation planes, false cleavages parallel to 

 possible faces, and which result from the deposition of an impurity on 

 these faces during the growth of the crystal. Examples of these are 

 shown in the specimens of calcite, galena, mica, and quartz. 



I'KACTUKK. 



This is that surface obtained by breaking the mineral in a direction 

 other than that of the cleavage. It may be designated as conchoidal, 

 that is, breaking with concavities more or less deep; even, when the 

 fracture ai)i)roxiniates a plane surface; uneven, when the surface is 

 irregular; hackly or splintery, when the surface is jagged. 



