Basic Rocks of Northeastern Maryland. — Leonard. 175 
The peridotytes and pyroxenvtes were erupted at a later 
period than the norytes and gabbros through which they have 
broken. Some of the pyroxenytes, however, apparently fonn 
peripheral facies of the noryte and probably belong to the same 
age as the latter. The pegmatytes are more recent than the 
basic rocks and serpentines and since the latter have originated 
from non-feldspathic types the acid dikes are also younger 
than most of the peridotyte and pyroxenyte. The last rock to 
be erupted was probably the diabase composing the dike which 
cuts the noryte-gabbro mass, and was formed in late Triassic 
or early Jurassic time. 
The northeastern ^Maryland area taken as a whole seems to 
furnish an example of the occurrence of several rock types 
which represent the facies of a single magma and unite to form 
a geological unit. This view does not necessitate the supposi- 
tion that all the rocks were formed either at the same time or by 
a continuous eruption, but there may have been several periods 
of activity during which different types were produced, and as 
already stated, this was doubtless the case in this district. 
PLATE XVI. 
Fig. I. — Diorytc. 
From one mile northeast of Rising Sun. Section No. 71. Magni- 
fied 20 diameters. Ordinary light. The hornblende occurs mostly in 
lath-shaped individuals which form clusters scattered through the feld- 
spar. Considerable magnetite is also present. 
Fig. 2. — Quartc-Mica-Honiblendc-Dioryte (tonalyte). 
■* From one-half mile west of Porter Bridge. Section No. 260. 
Magnified 20 diameters. Ordinary light. A good sized biotite individ- 
ual shows near the center of the field and hornblende is seen near the 
edge. The feldspar is largely altered to epidote, which forms irregu- 
lar crystal aggregates. The clear white mineral is quartz. 
PLATE XVIL 
Fig. I. — Norytc. 
From one mile west of Oak Grove schoolhouse. Section No. 237. 
Magnified 20 diameters. Ordinary light. The white feldspar is not 
as cloudy as in many sections. The light colored mineral is hyper- 
sthene and the black is magnetite. 
Fig. 2. — Norytc poor in feldspar. 
From cross-roads three-quarters of a mile south of Rock Springs. 
Section No. 104. Magnified 20 diameters. Ordinary light. 
