162 SLODIES HOR SLO DEN ES: 
When iron occurs in sedimentary rocks, more especially 
sandstone, it often serves as a cement by which the original 
particles are bound together. However, it may also occur as an 
original constituent in the shape of finely disseminated particles. 
Carbonaceous matter in the form of graphite, or bitumen in the 
shape of petroleum occurs mainly in limestone and marble, often 
contributing to these rocks the blue or grayish-blue colors so 
commonly observed. 
Among sedimentary rocks the color varies widely, not only 
in the same quarry, but often in the same bed. Certain beds in 
a quarry may have a delightfully cheerful, uniform color, while 
those immediately above or below may be dull and somber. In 
many places the coloring matter is distributed through the beds 
in regular bands, but occasionally it is very curiously dissemi- 
nated, forming irregular, fantastic figures. White sandstone is 
often colored with large and small brown spots, while brown 
sandstone is sometimes similarly variegated with white spots. 
All stone which is distinctly mottled or irregularly colored is 
known as ‘‘variegated stone.” 
The color of an igneous rock is usually composite, as a result 
of the blending of the distinct colors of the mineral particles. 
The color, however, does not depend entirely upon the colors of 
the individual minerals, but in part upon the size and distribu- 
tion of the constituent particles. In some instances the individ- 
ual grains are sufficiently large to retain their own color, and 
the stone is spoken of as being mottled. 
With respect to color, granites are ordinarily classified as red 
and gray. Whether a granite belongs to the first or second class 
will depend mainly upon the red or white color of the feldspar. 
Many granites contain both red and white feldspar, but as long 
as the red variety is sufficiently abundant to impart a reddish 
tone to the rock, it is called red granite. The most brilliant red 
granites have a preponderance of medium-sized, deep red feld- 
spar individuals. As the feldspar individuals become finer 
grained and less deeply colored and biotite, amphibole, or 
pyroxene becomes more abundant, the color is subdued produc- 
ing dull red effects. 
