58 THE GRANITES OF MAINE. 
Diabase consists essentially of a feldspar of the series containing 
lime, or soda and lime, together with a pyroxene or augite (alumina, 
lime, magnesia, iron), which, however, is frequently altered to horn- 
blende or other secondary minerals; also magnetite or ilmenite or 
both. Olivine may or may not be present, and some specimens con- 
tain a little quartz. The accessory minerals are orthoclase, biotite, 
pyrite, hypersthene, apatite. The secondary ones are hornblende, a 
white mica, chlorite, epidote, serpentine, calcite. The percentage of 
silica in diabase ranges from about 45 to nearly 57, of iron oxides 
from about 9 to 14, and of magnesia from 3 to 9. 
These " black granites," as will be seen by the foregoing descrip- 
tion, are distinguished chemically from the ordinary granites by 
their low percentage of silica (45 to 67 per cent), their high maxima 
of iron oxides (9 to 14 per cent), and of magnesia (9 to 11 per cent), 
and mineralogically by their dominant feldspar not being a potash 
feldspar, and generally also by their considerable content of the 
darker iron-magnesia minerals. 
TEXTURE. 
The general texture of the black granites corresponds in grade 
to that of the fine and medium .granites. In the diorites the arrange- 
ment and order of crystallization of the minerals always correspond 
to those of the granites, described on page 20. In some of the 
gabbros this is also true, but in others and in diabase the arrange- 
ment greatly differs. The feldspars are in needlelike crystals, be- 
tween which the pyroxene has afterwards crystallized. 
PHYSICAL PROPERTIES. 
Aside from their great toughness, the diorites and the Granitic 
gabbros probably differ but little in physical properties from gran- 
ites of the same grade of texture. By reason both of their peculiar 
texture and their mineralogical composition, the diabases and gabbros 
with " ophitic " texture, described on page 136, should differ con- 
siderably in physical properties from the granites. As these stones 
are rarely used in large buildings, owing to the difficulty of quarry- 
ing them either in blocks of sufficient size or at low enough cost, 
data as to their compressive strength and other useful physical 
properties are not available. 
The specific gravity of gabbro ranges from 2.66 to about 3, that 
of diabase from 2.7 to 2.98, and that of diorite averages 2.95. In 
these rocks it thus usually exceeds that of granite. 
As the black granites are used chiefly for monumental purposes, 
and particularly for inscriptions, their color, susceptibility to polish. 
and the amount of contrast between their cut or hammered and 
