306 REPORT ON NATIONAL MUSEUM, 1886. 



materials work readily because their grains are but loosely coherent, while 

 others of softer materials are quite tough and difficult to work owing 

 to the tenacity with which their particles adhere to one another. Ob- 

 viously a stone in which the grains adliere closely and strongly one to 

 another will be less absorbent and more durable under pressure than 

 one which is loose textured and friable. A rock is called flinty when 

 tine grained and closely compacted like flint; cart//y when partially de- 

 conjposed into earth or loam ; friahle when it falls easily into powder 

 or crumbles readily under the tool. Upon the state of aggregation and 

 the fineness of the grain is dependent very largely the kind of fracture 

 possessed by a rock. Finegrained, compact rocks like flint, obsidian, 

 and some limestones, break with concave and convex shell like surfaces, 

 formiug a conchoidal fracture ; such stoue are called plucky by the work- 

 men and they are otten quite difficult to dress on this account. Others 

 break with a rough and jagged surface called hacMy or splintery. When 

 as in free-working sandstone and granite the broken surface is quite 

 straight and free from inequalities they are referred to as having a 

 straiylit or right fracture. 



(4) RILT AND GRAIN. 



The rift of a rock is the direction parallel to its foliation or bedding 

 and along which it can usually be relied upon to split with greatest 

 ease. It is best represented in mica schist, gneiss, and other rocks of 

 sedimentary origin. It is a property, however, com mon to massive rocks, 

 though usually much less pronounced. The grain is always in a direc- 

 tion at right angles with the rift. 



These are two most important qualities in any stone that it is desired 

 to work into blocks of any regularity of shape. Without them the 

 production of rough blocks for street paviug or for finely finished work 

 would be possible only with greatly increased expense, and only the 

 very softest stones could be worked with any degree of economy. 

 With them the hardest rocks are sometimes most readily worked. Thus 

 the Sioux Falls (Dak.) quartzite, one of the hardest known rocks, is as 

 readily broken out into square blocks for paving as a granite or soft 

 sandstone. 



(5) COLOR, 



The color of a stone is as a rule dependent more upon its chemical 

 than its physical properties. As will be noted, however, the color of 

 the granites and similar rocks is sometimes varied in shades of light 

 and dark accordingly as the feldspar are clear and glassy and absorb 

 the light or white and opaque and reflect it. The chief coloring matter in 

 rocks is iron, which exists either in chemical combination with the vari- 

 ous minerals or in some of its simpler compounds such as the sulphide, 

 carbonate, or oxide disseminated in minute particles throughout the mass 

 of the rock. The oxides of iron impart a brownish or reddish hue, the 

 carbonate or sulphide a bluish or gray, A very light or nearly white 



