Basic Rocks of NortJicastcni Maryland. — Leonard. 145 
cessory constituents shown under the microscope are magnetite 
and apatite together with a Httle titanite (sphene) and zircon. 
Epidote and chlorite are common secondary minerals. 
The feldspar is acid bytownite. Stauroscopic measure- 
ments showed extinction angles on oP (001) of — 18° to — 20° 
and on the a P ^ (010) of — 30°. The specific gravity as de- 
termined bv means of the Thoulet solution was in one specimen 
2.71, in another 2.y2. 
Quartz is occasionallv present in small amount and cannot 
be detected without the aid of the microscope. The rock thus 
passes into a true hornblende-dioryte with a little accessory 
quartz, and this type is very basic in composition as shown by 
the accompanying analysis. As a rule, however, the quartz 
occurs in good sized grains and is blue in color, resembling in 
all respects that found in the tonalytes. The hornblende does 
not differ from that already described under the quartz-mica- 
hornblende-diory tes . 
Metamorphic changes. The diorytes have undergone con- 
siderable alteration through the extensive epidotization of their 
feldspar. The various stages in this process can be traced 
from where there are only scattered crystals of epidote and the 
plagioclase is still clear and bright to where, as an opaque white 
mass, it is almost wholly replaced by a compact aggregate of 
separate epidote individuals. In no instance was complete re- 
placement observed to have taken place, although often only 
slight traces of the original feldspar remain. This is genetical- 
ly the same process as the saussuritization of the feldspar that 
has gone on so extensively in the gabbros and has resulted in 
the replacement of that mineral by zoisite. In the latter case no 
iron is required, while in the formation of epidote this element 
is necessary. It is doubtless derived from the hornblende, or 
in case of the dioryte containing biotite some of the iron may 
be supplied from the latter source. 
The following very complete analyses of both varieties of 
the dioryte were made by ]\Ir. W. F. Hillebrand of the United 
States Geological Survey. I is an analysis of a typical quartz- 
mica-hornblende-dioryte from near the foundry on Stone run. 
This rock, which is quite granitic in texture and appearance, is 
composed of labradorite, quartz, hornblende and biotite, togeth- 
er with a little magnetite, apatite, titanite (sphene) and ortho- 
