32 NEW YORK STATE MUSEUM 



others of similar color lack. There is a strong contrast in that 

 respect between Gouverneur marble, for example, and a noncrystal- 

 line granular limestone. The nature of the surface exposed to view 

 also must be taken into account; in the darker stones, a marked 

 difference usually exists between the rock face and the hammered 

 surfaces, the latter being much lighter. The appearance of a stone 

 in a small sample may fail to give the actual effect when seen at 

 some distance in the walls of a building. 



The granites and related silicate rocks ordinarily change very 

 little, even on long exposure to the weather. Their coloration is 

 lent by the inherent colors of the various minerals, rather than by 

 the presence of some accidental ingredient diffused through the 

 jnass. In consequence of their usually complex mineral composition, 

 they appear mottled or speckled on close view and only assume uni- 

 form tints when viewed from a distance. The coarser the texture, 

 the greater is the distance required to produce blending. Among the 

 ingredients of igneous rocks, quartz exercises little part in the 

 coloration, itself being colorless or at most grayish or whitish. 

 Feldspar is the minenal ;to which the granites, syenites' and amoritho- 

 sites owe their characteristic colors. In the granites, it is mainly 

 white, cream or light pink, but is sometimes deep red. Its effect 

 is toned down by the darker minerals, so that the brilliant white or 

 red becomes gray or dark red in the body of the rock. The feldspar 

 in syenite may be pink or gray, but is not infrequently blue or 

 green. The feldspar (labradorite) of anorthosite has a dark green 

 to almost black color in fresh condition, but shows nearly white 

 when crushed and subjected to slight alteration. In the diorites, 

 gahbros and diabases, the dark silicates, like biotite, amphibole and 

 pyroxene, share importance with the feldspar and consequently 

 these rocks possess rather somber tones. ' 



Strength, The resistance which rocks offer to stress when ap- 

 plied to their surface varies much with the class and type. It 

 depends upon many different factors which are mainly related to 

 the mineral composition and texture, but which are also influenced 

 by external conditions. Some of the relations between the physical 

 characters of rocks, particularly textures, and strength have already 

 been mentioned. 



The igneous rocks as a class are distinguished from the other 

 rocks by the fact that their strength is uniform, irrespective of the 

 direction in which the stress may be applied. This depends, of 

 course, uipon their homogeneous composition and texture. In the 

 sedimentary and metamorphic classes, the planes of bedding or 



