BLACK GRANITES. 59 
their polished surfaces are the physical properties of chief economic 
importance. 
Doctor Merrill" explains the cause of these contrasts very satis- 
factorily : 
The impact of the hammer breaks up the granules on the immediate surface, 
so that the light falling upon it is reflected, instead of absorbed, and the 
resultant effect upon the eye is that of whiteness. The darker color of a 
polished surface is due merely to the. fact that, through careful grinding, all 
these irregularities and reflecting surfaces are removed, the light penetrating 
the stone is absorbed, and the effect upon the eye is that of a more or less 
complete absence of light, or darkness. Obviously, then, the more transparent 
the feldspars and the greater the abundance of dark minerals, the greater will 
!»<■ the contrast between hammered and polished surfaces. This is a matter 
worthy of consideration in cases where it is wished, as in a monument, to have 
a polished die, surrounded by a margin of hammered work to give contrast. 
The ordinary granites, Avhile taking a high polish, do not afford 
such strong contrasts between hammered and polished surfaces as 
do the " black granites." In some black granites this seems clearly 
to be due to their larger percentage of the black minerals, but in 
others, as some of the quartz diorites, in which the black minerals do 
not exceed those in some gray granites, the cause of this marked con- 
trast must be sought in some optical property of the soda-lime feld- 
spar and in its relative abundance. 
"BLACK GRANITES" OF MAINE. 
CLASSIFICATION. 
The black granites at the quarries and prospects visited by the 
writer include : 
(1) Gabbros: Gabbro, hypersthene-olivine gabbro, norite, olivine 
norite. 
(2) An altered diabase porphyry. 
(3) Diorites : Quartz diorite, mica-quartz diorite. 
The appearance and the petrographic characteristics of the stone at 
each quarry will be stated in Part II of this bulletin, in the de- 
scriptions of the quarries and of their products, and a classification 
of black granites based upon economic principles will be found on 
page 75. These black granites vary considerably in shade and a little 
in color. The olivine norite of the Ileal quarry, near Belfast, is 
almost black, but under a side light shows small, brilliant dark-green 
areas of hypersthene. The Vinalhaven olivine norite is quite black 
and fine textured. The Addison hypersthene-olivine gabbro is black, 
with small, irregular white areas of feldspar. The South Berwick 
"Merrill, Geo. P., The physical, chemical, and economic properties of building slums 
Maryland Geol. Survey, vol. 2, 1898, p. 64. 
