MARYLAND GEOLOGICAL SURVEY 167

subsequently have been greatly changed and recrystallized by the
earth movements which have taken place since their formation. The
exact time at which these rocks were intruded into the surrounding
masses is not definitely known. They are clearly older than the
youngest of the granites at Broad Branch and are younger than some
of the older more metamorphic granites and granite gneisses. It
seems reasonably probable that they were erupted just before or just
after the gabbros.

THE GRANITE. —The eruptive granites of central Maryland rival
the gabbros in extent and petrographical interest, while they greatly
surpass them in economic importance.

These rocks are, as a rule, granitites, of medium grain and remark-
ably compact and homogeneous texture. They sometimes carry a con-
siderable quantity of muscovite (Guilford), and are noticeable for the
large and constant proportion of allanite which they contain, this latter
mineral being surrounded by a parallel growth of isomorphous epidote.

Variations in the structure of the granites are due to the develop-
ment of porphyritic crystals, as at Ellicott City and along the road
from Meredith's Bridge on the Gunpowder river to Cockeysville.
Other structural facies are -due to secondary features, like foliation,
produced by dynamic agencies.

The granites are partly younger and partly older than the other
eruptive types, and are not connected with them, as those with each
other, by intermediate facies. They represent entirely distinct
epochs of eruptive activity. The evidence of their eruptive origin is
most satisfactory and conclusive. They form intrusive bosses with
diverging dikes and apophyses; they produce disturbance and crump-
ling in the rocks through which they break; they enclose fragments
of the older rocks—gneiss, marble, quartz-schist, gabbro and pyroxen-
ite; and finally they produce all the well-known phenomena of con-
tact-metamorphism, both in these fragments and in the rocks which
adjoin them.

The granites are extensively quarried for building and paving
stones at Port Deposit, Woodstock, Granite, Ellicott City and Guil-
ford, which represent great granite masses.