ilmenite, and hornblende. Their structure is ophitic in the finer
varieties, and to some extent in the coarser kinds as well. They are
holocrystalline in form and true glassy bases are rare, rendering the
term diabase more appropriate than basalt.

There is greater variety in texture, from fine aphanitic traps up to
coarse grained dolerites with feldspars one-third of an inch long. The
coarser varieties are easily quarried and are often used for building
stone under the name of granite.

These forms are retained to the present day with no material change
except that of immediate weathering, but to alterations of this kind
they are an easy prey, and yield the most characteristic forms. The
narrow dikes produce ridges between slight valleys of sandstone or
shale, the wide bodies produce broad flat hills or uplands. The rock
weathers into a fine gray and brown clay with numerous bowlders of
unaltered rock of a marked concentric shape.

While the diabase dikes are most prominent in the Newark rocks, they
are also found occasionally in the other terraces. In the Catoctin
Belt they appear irregularly in the granite and schist. Rare cases
also occur in the rocks of the Piedmont plain. The diabase of the
Newark areas is almost exclusively confined to the red sandstone, and
the dike at Leesburg cutting the limestone conglomerate is almost the
only occurrence of that combination.

The diabase occurs only as an intrusive rock in the vicinity of the
Catoctin Belt. Of this form of occurrence, however, there are two
types, dikes and massive sheets or masses. The dikes are parallel to
the strike of the inclosing sandstone as a rule, and appear to have