Basic Rocks of Northeastern Maryland. — Leonard. i6j 
to be affected in their composition by the nature of the enclos- 
ing mass. 
RELATIONS OF THE DIFFERENT ROCK TYPES. 
Having considered somewhat in detail the different rock 
types represented in ihe area it remains to discuss the rela- 
tions existing between them. It has been shown that they are 
more or less closely associated in their geological occurrence 
and that many of the types are connected by intermediate varie- 
ties. The rocks grow successively more and more basic from 
tne south toward the north. If one starts from Rowlands- 
ville or from a point one mile north of Colora and travels north 
to the Pennsylvania line he passes over in succession biotite- 
granite, quartz-mica-hornblende-dioryte (tonalyte), dioryte 
and quartz-dioryte, hypersthene-gabbro and noryte and ser- 
pentine. The distance traversed in thus passing from the acid 
to the basic and ultra-basic types is sometimes two miles and 
never exceeds three miles. 
GEOLOGICAL RELATIONS OF THE DIORYTES AND GRANITES. 
The close relationship existing between the dioryte and 
granite is evident both from their association in the field and 
from the study of a large number of sections made from spec- 
imens collected at many points in the intermediate zone. As 
already stated, the diorytes form a belt varying in wndth from 
one to one and a half miles, bordering the granite on the north 
and lying between that and the gabbro. It was everywhere found 
impossible in the field to draw any sharp line of separation be- 
tween the granites and diorytes, and along quite a well defined 
intermediate belt there is a gradual passage from one to the 
other. This transition is even more apparent from the micro- 
scopical study of a large number of thin sections which show 
a change in the mineralogical composition of the rocks. It has 
already been stated that there are two well marked varieties of 
the dioryte, one a quartz-mica-hornblende-dioryte or tonalyte 
and the other a quartz-dioryte. The former, which is very 
granitic in appearance, is confined almost entirely to the south- 
ern portion of the area next to the granite, and forms a con- 
necting type between that rock and the quartz-dioryte, while 
the latter is found along the northern border. By an increase 
