THE PROPERTIES OF BUILDING STONES, ETC. 163 
The gray granites are dark or light colored, depending upon 
the size of the individual grains and the amount and kind of 
the ferro-magnesian minerals present. The light-colored granites 
have a preponderance of white feldspar and quartz, with musco- 
vite as the main ferro-magnesian mineral. The dark gray gran- 
ites contain less feldspar and quartz, and a greater abundance of 
biotite, hornblende, pyroxene. 
Other igneous rocks such as Ee Paipradlorive: granite’ with its 
blue iridescent color, and rhyolite with its almost black color, 
are commonly met with. The iridescent color of the former is 
imparted by the abundant porphyritic indiyiduals of labradorite, 
of which the rock is largely composed. The black color of the 
latter is due largely to its semi-crystalline groundmass, which 
often abounds in fine crystals of hornblende. Serpentine is an 
abundant constituent of some rocks, and as such imparts to them a 
green color. The dull greenish-gray color so conspicuous among 
the basic rocks such as gabbro, diorite, and diabase, is imparted 
mainly by the minerals of the hornblende, pyroxene, amphibole, 
chlorite, and epidote groups. 
The color of arock when freshly quarried may be almost per- 
fectly white but a few years, or perhaps months, of exposure to 
the weather may change the color to a buff, or streak it with 
irregular patches of brown. Such color changes result chiefly 
from the presence of easily decomposed minerals within the stone 
itself. The yellow color of many limestones is due to the pres- 
ence of finely disseminated iron, as the carbonate or sulphide, 
which has altered to the oxide. If a stone contains either of 
these the color will change as a natural consequence of expo- 
sure to the atmosphere. The oxides of iron are more stable 
compounds than the sulphide or carbonate, and very seldom 
cause a change in color. 
A change in the color of the stone in a wall may be due to 
impurities in the mortar, cement, brick, or water used in the con- 
struction and not to the presence of easily decomposed minerals 
in the stone. The committee appointed to investigate the cause 
of the brown stains on the walls of the State Historical Library 
