BLACK GRANITES. 57 
they contain a considerable amount of one of the pyroxenes, or 
hornblende or biotite, and magnetite, which accounts for the general 
darkness of their shade or their greenish color. 
OKI (JIN. 
The gabbros and diorites are more or less granitic in texture, 
as they crystallized under conditions resembling those which attended 
the formation of granite. But the diabase was in part erupted 
through narrow fissures, forming dikes or sheets, and at many places 
reached the surface, always crystallizing with comparative rapidity. 
Diabase, however, occurs in Vinalhaven, as stated by Dr. George 
Otis Smith, " in large bodies which have the form of neither dikes nor 
sheets, being, in fact, part of the same masses as the diorites and 
gabbros." 
MINERALOGICAL AND CHEMICAL COMPOSITION. 
Gabbro consists essentially of a lime-soda feldspar and one or both 
of the varieties of pyroxene known as diallage and hypersthene. 
The former is a foliated silicate of iron and lime with about 12 per 
cent of magnesia; the latter is a silicate of iron with about 24 per 
cent of magnesia, and each of these minerals crystallizes differently. 
When hypersthene alone is present the rock is called a norite; when 
both are present it is a hypersthene gabbro. When the mineral 
olivine (a greenish silicate of iron with 50 per cent of magnesia) is 
present also the name olivine may be prefixed to the rock name. 
The accessory minerals in gabbros are ilmenite (a titanate of iron), 
magnetite, pyrite, apatite, biotite, garnet, and, rarely, quartz and 
metallic iron. The secondary minerals — that is, those derived from 
the alteration of the primary ones — are hornblende, chlorite, epidote, 
zoisite, analcite, serpentine, a Avhite mica, and calcite. The percent- 
age of silica in gabbros varies a little on either side of 50. Iron 
oxides and lime average 9 per cent each; magnesia, per cent. 
Diorite consists essentially of feldspar (of the series containing 
lime and soda) and hornblende with biotite, or biotite alone. Quartz, 
augite, and potash feldspar may or may not be present. The acces- 
sory minerals are magnetite, pyrite, titanite, zircon, apatite, garnet, 
allanite. The secondary are epidote, chlorite, a white mica, and 
calcite. When quartz is present the rock is called a quartz diorite. 
When black mica or augite are the preponderating iron-magnesium 
silicates the rock becomes a mica diorite or an augite diorite. In 
diorites the silica ranges from about 49 to 63 per cent, but in quartz 
diorite it rises to about 69 per cent, which is the minimum in granite. 
The iron oxides range from 0.52 to 9.70 per cent, the magnesia from 
less than 1 to over 11 per cent, but usually from 2 to 7 per cent. 
