MARYLAND GEOLOGICAL SURVEY 165

west of Baltimore and extends thence as far south as Laurel. The
gabbro is a rather finergrained aggregate of hypersthene, diallage,
plagioclase (bytownite) and magnetite, with varying amounts of apa-
tite and brown hornblende. The unaltered gabbros are usually mas-
sive, heavy and dark colored. With the alteration the color changes
through a pale buff to the characteristic deep reddish brown. By an
increase in magnesia the gabbros pass by transitions towards the peri-
dotites and pyroxenites; or in alumina, to highly feldspathic rocks; or
in silica, to others which have free silica forming blue grains.

The action of pressure which has caused the recrystallization of the
gneiss and marble is also well marked in the gabbros. It has causepl
the iron constituent, pyroxene, to change to another green mineral
called hornblende; and has in some cases left the rock as massive
as at first or in other cases rendered it schistose. The resulting rock
is called gabbro-diorite. The change has always been most complete
where the mass of gabbro is smallest, as in the narrow beds which
connect the larger areas. This change is well shown along the Belair
road near Baltimore.

The gabbro offers great resistance to the ordinary processes of de-
composition, and hence it is strewn abundantly all over the area, which
it occupies, in the form of boulders. It is at the same time so hard,
so heavy, and so jointed that it could not be quarried to any advantage
as a building stone. The loose blocks are much used for constructing
stone 'walls or foundations, and occasionally whole buildings are
erected of them.

THE PERIDOTITE AND PYROXENITE. —The second type of eruptive
rock which penetrates the gneiss complex near Baltimore is younger
than the gabbro, but it is genetically closely allied to it. These two
types are connected by many intermediate varieties; and the more
basic rocks, which break through the gabbros as well as through the
gneiss, may be regarded as having resulted from a gabbro-magma
which had become relatively poor in alumina, or in alumina and silica.
The absence of alumina would prevent the formation of feldspar, and
hence in the first case crystallization produced an aggregate of pyrox-
ene (bronzite and diallage) called pyroxenite (websterite); while in the