Basic Rocks of Xortheastcrn Maryland. — Leonard. 141 
They are found best developed in Cecil county, where they lie 
between the granites on the south and the gabbros and norytes 
on the north. While the diorytes are found west of the Sus- 
quehanna river in Harford county, they do not there form so 
well defined a zone bordering the main eruptive mass. 
It is not possible to draw any sharp line of separation be- 
tween the granite and dioryte. On the south this rock passes 
by intermediate t3'pes into the biotite-granite of the Rowlands- 
ville area, which, as will be shown later, has very much the 
composition of a dioryte. Along the northern border the 
diorytes are intimately associated with the gabbros and norytes, 
but the transition from one to the other is not well marked, as 
in the case of the granites, and intermediate types between 
the pyroxene and hornblende rocks are rare. 
Two varieties of dioryte are present in the area, namely, a 
quartz-dioryte and a quartz-mica-hornblende-dioryte. The for- 
mer is confined for the m.ost part to the northern portion of the 
dioryte belt, next to the norytes ; the latter is found mostly in 
the southern portion where it passes into the biotite-granite. 
For the purpose of description it will be convenient to consider 
these separately, taking up first the most acid variety. 
Quartz-mica-hornblcnde-dioryte. This rock is found at 
many points between Rising Sun and the Susquehanna ; it is 
abundant along Stone run, extending south of that stream as 
far as the Harrisville road ; large boulders of it are numerous 
at the foundry, and at Harrisville, and outcrops occur at the 
junction of Stone run and Octoraro creek, at Porter Bridge, 
and along the road leading westward from the bridge to the 
Rowlandsville road. Across the Susquehanna river in Har- 
ford county good localities for this rock are near Darlington 
and one mile east of Thomas Run P. O., on the south side 
of Deer creek. 
This dioryte varies considerably in appearance according 
to the fineness or coarseness of the grain and the relative pro- 
portions of the light and dark constituents. As a rule it is a 
medium coarse-grained, granular rock of a grayish color, such 
as that occurring south of Stone run. It is very granitic in ap- 
pearance and might easily be mistaken at first sight for a horn- 
blende-granite. The dioryte of the Porter Bridge region is 
•darker in color, often fine-grained, and the hornblende usually 
