Basic Rocks of Nortlicastcni Maryland. — Leonard. 137 
herein described are representative of those occurring through- 
out the entire belt and possess a special interest on account of 
the close association of the basic eruptives with the granites 
which border them on the south. 
In addition to the larger mass there are three smaller ones. 
The "Stony Forest" area is the most extensive of these and is 
situated in Harford county, between five and six miles east of 
Bel Air. It has a length north and south of five miles and 
width of about three miles. Of the other two, which are not as 
large, one lies just north of Calvert (Brick Meeting House), 
extending a short distance into Pennsylvania, and the other 
about one-half mile northeast of Elkton, near the eastern border 
of the state. The latter, as shown by Chester, is a western pro- 
longation of a larger area in Delaware. 
AREAL DISTRIBUTION AND GENERAL DESCRIP- 
TION OF ROCK TYPES. 
The rocks embrace many different types distributed in a 
series of zones trending northeast-southwest. They include 
quartz-mica-hornblende-dioryte, quartz-dioryte, noryte,.gabbro, 
pyroxenyte, peridotyte, serpentine and diabase. Occurring 
along' the southern margin of the area next the granites are the 
quartz-mica-hornblende-diorytes, or tonalytes. Passing north- 
ward these are in general succeeded by coarse-grained horn- 
blende-diorytes, which commonly carrv considerable cjuartz in 
good sized grains. These hornblendic rocks are in turn fol- 
lowed by norytes and gabbros, which are bordered on the north 
by the serpentine mass. The latter lies along the edge of the 
area and extends across the line into Pennsylvania. Coarse 
pyroxenytes are found along the boundary between the noryte 
and serpentine and both non-feldspathic types occur in dykes 
within the gabbro and noryte. (See map, plate xv.) 
It is not possible to draw any sharp line of separation be- 
tween the granite and tonalyte, for these are connected by in- 
termediate types and thus graduate the one into the other. The 
tonalyte varies considerably in appearance according to the fine- 
ness or coarseness of the grain and the relative proportion of 
the light and dark constituents. As a rule it is a medium 
coarse-grained, granular rock of a grayish color. Dark patches 
or segregations of finer grained material rich in biotyte fre- 
