144 J^^'^ American Geologist. September, vmi. 
entire grain, which is broken into pieces. These unite to form 
a mosaic of interlocking fragments which still preserve the out- 
line of the original grains. The quartz is filled with numerous 
very miinute fluid inclusions. Occasionally it contains in addi- 
tion many fine needles, probably of rutile, but they are too 
small for the satisfactory determination of their character. 
Biotitc occurs in plates or lamellae without crystal bound- 
aries. It is very strongly pleochroic, the rays parallel to the 
-cleavage (b and c) being dark brown, and those at right 
angles (fi) light yellow. Inclusions of apatite, epidote and 
magnetite are common in the biotite. 
As accessory constituents magnetite and apatite are almost 
always present, and titanite occurs occasionally. The epidote is 
never absent from these diorytes and though it may in some 
instances be primary it is usually a secondary product devel- 
oped during the alteration of the feldspar. It commonly oc- 
curs in well defined crystals, irregular grains or compact aggre- 
gates. 
Ouartz-dioryte. As already stated the ciuartz-diorytes oc- 
cur chiefly along the northern border of the dioryte belt next 
the norytes and gabbros. As one passes north across the area 
the dioritic rocks are seen to grow darker in color, and there is 
a decrease in the amount of biotite accompanied by a corre- 
sponding increase in the hornblende. The quartz-dioryte and 
the tonalyte are thus connected by intermediate types and grad- 
uate into each other. By a decrease in the amount -of quartz 
this rock passes into a true hornblende-dioryte. 
Among the many localities where the quartz-dioryte occurs 
the following may be mentioned ; north of Rising Sun along 
Stone run ; at McKinsey's mill on the same stream, where it 
outcrops near the bridge, and along the Susquehanna one mile 
below Conowingo. In Harford county it is found one half 
mile south of Bel Air, along- Deer creek, one and half miles 
southwest of Darlington and at numerous other points. 
The quartz-diorytes have a medium coarse-grained texture 
and are dark, almost black, in color. On a fresh surface they 
have a mottled appearance due to the black hornblende stand- 
ing out against the white feldspar and quartz. The minerals 
composing the quartz-dioryte are plagioclase, quartz, horn- 
blende, and occasionallv a little accessorv biotite. Other ac- 
